The Evolutionary Psychology FAQ
Last updated September 8, 2004.

This FAQ is written and maintained by Edward Hagen, formerly of the Center for Evolutionary Psychology, University of California, Santa Barbara, and now at the Institute for Theoretical Biology in Berlin. The FAQ assumes a basic knowledge of genes and natural selection. Its purpose is to outline the foundations of evolutionary psychology. These foundations are extremely robust (though not beyond criticism). The status of specific hypotheses (e.g., mate selection preferences, cheater detection modules) is more debatable, and will not be discussed in detail here. In addition, I address many of the common misconceptions about evolutionary psychology. This FAQ draws upon the work of many individuals. Comments and criticisms regarding it are welcome: A Russian translation of this FAQ is also available.

Frequently asked questions:
  • What is evolutionary psychology?
  • What is the EEA and why is it important? (general answer)
  • What is the EEA? (detailed answer)
  • Isn't it true that we can't know what happened in the distant past, so the EEA concept is useless?
  • Why is the EEA equated with the Pleistocene?
  • Why couldn't humans have evolved during the last 10,000 years?
  • What is an adaptation?
  • What is a psychological adaptation?
  • What about spandrels?
  • What is domain specificity and why is it necessary?
  • What is a module?
  • How can we identify psychological adaptations?
  • Why are adaptations not for the good of the species?
  • Why are genes selfish?
  • Do selfish genes mean selfish people?
  • Why is the heritability of adaptations generally zero?
  • How can evolutionary psychologists talk about adaptations without talking about specific genes?
  • Are there enough genes to build psychological adaptations?
  • What about 'plasticity'?
  • What about learning?
  • What about gene-environment interactions?
  • Are evolutionary psychologists primarily interested in what makes humans different from other animals?
  • Is evolutionary psychology just a politically correct version of sociobiology?
  • Is evolutionary psychology another form of genetic determinism?
  • Is evolutionary psychology racist?
  • Is evolutionary psychology sexist?
  • Is evolutionary psychology a form of Social Darwinism?
  • Is rape an adaptation?
  • If my 'genes made me do it', am I still responsible?
  • Do evolutionary psychologists think that everything is an adaptation?
  • Why do some people hate evolutionary psychology?
  • More thoughts on Evolutionary Psychology and political (in)correctness
  • What are your politics? (Translation: Doesn't evolutionary psychology have a crypto conservative political agenda?)
  • Does evolutionary psychology have any problems?
  • References and other reading
  • What do you do?
  • What is evolutionary psychology?

    In the three and a half centuries since William Harvey proved that the purpose of the heart is to pump blood, physiologists have revealed the functional organization of the body in blinding detail. Their discoveries demonstrate beyond question that the structure of the body serves survival and reproduction. Further, there is near unanimity among biologists that this functional structure is a product of natural selection. In our century, psychologists have developed powerful techniques that conclusively demonstrate that cognition, too, has structure. Evolutionary psychologists are betting that cognitive structure, like physiological structure, has been designed by natural selection to serve survival and reproduction.

    Evolutionary psychology focuses on the evolved properties of nervous systems, especially those of humans. Because virtually all tissue in living organisms is functionally organized, and because this organization is the product of evolution by natural selection, a major presumption of evolutionary psychology is that the brain, too, is functionally organized, and best understood in evolutionary perspective. It is clear that the body is composed of a very large number of parts, and that each part is highly specialized to perform a specific function in service of the survival and reproduction of the organism. Using the body as a model for the brain, it is a fair guess that the brain, too, is composed of one or more functional parts, each of which is also specialized to facilitate the survival and reproduction of the organism (we'll get to genes in a bit). Thus, according to evolutionary psychology, neural tissue is no different from any other tissue: it is functionally organized to serve survival and reproduction. This is the foundational assumption of evolutionary psychology. Because vision, hearing, smell, pain, and motor control are indisputable functions of the nervous system that clearly have utility for survival and reproduction, this assumption has a high degree of face validity. Further, these examples suggests that the brain may best be conceived not as an organ with a single function, but rather as composed of a large, and potentially vast number of functional parts. Evolutionary biologists refer to the functional components of organisms as 'adaptations'. Evolutionary psychologists often refer to brain functions as psychological adaptations, although they are not qualitatively different from other adaptations.

    The functional organization of the body has been elucidated primarily by the direct examination of morphology. A detailed analysis of the structure and composition of our organs and tissues has resulted in an excellent understanding of their purpose. Unfortunately, this has not been the case with the brain. The gross morphology of the brain appears to have little connection with its functional properties. Although we have a fair understanding of nerve cells--the primary constituents of neural tissue--the properties of the brain clearly come from higher order assemblages of such cells, not just the cells themselves. This is just as true of organs like the heart as it is of the brain. Because nerve cells can rapidly change state (e.g., their firing rate), because such state-changes involve little energy, and because they can be well insulated from their neighbors, it is possible for a nerve cell to be in one state, whereas some of its close neighbors may be in completely different states. This is in marked contrast to, say, muscle cells. If one muscle cell is involved in a contraction, then nearby cells almost certainly are as well. Neural tissue is quite different. Even the individual states of nerve cells in a network depend critically on the topology of the network itself. Further, assemblages that are actually distinct may have a complex three-dimensional distribution that can be very difficult to untangle. These properties of neural tissue make it exceedingly difficult to "see" the morphology of neural assemblages--with few exceptions, the network topology of virtually our entire brain is currently "invisible." It exists at a scale above the individual cell, but well below that which can be teased apart with any imaging technology currently available. Until recent decades, much of our immune system was similarly "invisible."

    Evolutionary psychology offers one way around this technological limitation. If researchers had a sound basis for proposing brain functions a priori, they could then seek indirect evidence that brains in fact have these functional properties. Philosophers and scientists had long wondered why living things are made up of an amazing array of beautifully designed mechanisms, an organization which non-living things completely lack. Why is it that entities that reproduce manifest overwhelming evidence of design, but entities that don't reproduce are utterly devoid of the same? As Darwin and Wallace first perceived, the association of reproduction and design is not accidental. Evolution by natural selection is currently accepted as the only process whereby entities can acquire functional properties. Functional organization is the consequence of the reproductive feedback that characterizes natural selection. If a population of reproducing entities (hereafter organisms) varies in some trait, if the variations can be passed on to offspring, and if, as a consequence of possessing a particular variant, an organism produces more offspring on average than organisms that lack that variant over evolutionary time, then the population will come to consist solely of organisms possessing the reproductively efficacious variant trait. In this way, populations of organisms will tend to acquire traits that facilitate reproduction and lose traits that hinder reproduction.

    We now know that what is passed on to offspring is a large DNA molecule that is further partitioned into numerous sections called genes. Because the structure of this DNA is intimately bound up with the structure of the organism, variations in the DNA are strongly associated with variations in the organism. Changes in DNA are referred to as mutations, and result from environmental hazards such as radiation, toxins, etc.

    Reproduction is an enormously complex process. At any given moment in the human body, there are thousands of process that, should they fail to complete successfully, would result in death within minutes. For this reason, any given random change in the body is likely to hinder survival and reproduction, not facilitate it. There are far more ways for a mechanism to fail than there are ways to improve it. How many times has a change occurred to your car so that it got much higher than the EPA estimated miles-per-gallon rather than much lower? Thus, the vast majority of DNA mutations result in changes to the body (also called the phenotype) that hinder reproduction. Occasionally, however, a mutation occurs that results in a change to the phenotype that facilitates reproduction. Because this mutation can be passed on to offspring, and because this mutation tends to result in more offspring, the mutation becomes more frequent in the population. Over time, this process will result in organisms that have a sophisticated repertoire of mechanisms that facilitate reproduction

    We now have the answer to the question posed above: what functions is the brain likely to perform? If brain tissue is organized like all other tissue, it will perform precisely those functions that facilitate reproduction. More accurately, because evolution by natural selection is an historical process, and because the future cannot be predicted, the brain and body will perform functions that facilitated reproduction (note the past tense). Whether they currently do so will depend on how closely the present resembles the past. If we can develop an accurate picture of a species' reproductive ecology--the set of physical transformations that had to occur over evolutionary time for individuals to reproduce--we can infer those properties the organism is likely to have in order to ensure that those transformations reliably took place. Evolutionary time, the time it takes for reproductively efficacious mutations to arise and spread in the population, is often taken to be roughly 1000-10,000 generations; for humans, that equals about 20,000-200,000 years.

    Over the last 200,000 years, humans regularly encountered spiders and snakes, creatures whose toxins would have significantly impeded the reproduction of individuals unlucky enough to get injected with them. Over the last 100 years, humans have regularly encountered automobiles, encounters that also can seriously impede reproduction (e.g., by getting run over). Because 200,000 years is long enough for humans to evolve protective mechanisms, but 100 years isn't, we can predict that humans may well possess an innate aversion to spiders and snakes, but not to automobiles--even though far more people are currently killed by cars than by spiders or snakes. Once we have firmly established that avoiding spiders and snakes would have reliably facilitated the reproduction of ancestral humans, we can then design experiments to determine whether humans in fact possess an innate, cognitive ability to detect and avoid these animals (more on how to do this below). A major lesson of evolutionary psychology is that if you want to understand the brain, look deeply at the environment of our ancestors as focused through the lens of reproduction. If the presumptions of evolutionary psychology are correct, the structure of our brains should closely reflect our ancestral reproductive ecology. Thus, evolutionary psychology provides a method for perceiving the functional organization of the brain by studying the world--currently a far more tractable problem than disentangling neural assemblages.

    What is the EEA and why is it important? (general answer)

    The Environment of Evolutionary Adaptedness. This phrase, first coined by John Bowlby of attachment theory fame, has been the source of much confusion and controversy. First of all, the EEA is NOT a specific time or place. Roughly, it is the environment to which a species is adapted. Animals that lived in different environments or made their livings in different ways faced different reproductive problems, and that's why all animals aren't the same. Fish faced different problems than did butterflies, and as a result they have different adaptations. The EEA for any specific organism is the set of reproductive problems faced by members of that species over evolutionary time. The EEA for a particular species of fish is likely to be completely different than the EEA for a particular species of butterfly, even if those species both evolved in the same locations over the same periods of time. Each of these species faced reproductive problems that the other didn't, and thus their EEA's are different. The EEA concept is very similar to the notion of 'niche' in evolutionary biology.

    I have used the past tense when referring to the solving of reproductive problems because adaptations evolved over a large number of generations and are therefore "tuned" to reliable aspects of past environments (see the next section). If the environment changes, then the adaptation may be "out of tune" with the present environment and fail to properly perform its reproductive function.

    The EEA concept is extremely important for understanding the functional properties of organisms, including the functional organization of the human brain. As outlined in the previous section, the functional properties of organisms arise by the process of evolution by natural selection. This means that the functions that organisms have are precisely those that solved long standing, recurrent reproductive problems. Reproductive problems are all the various things organisms had to do to survive and reproduce in a particular environment over evolutionary time--find food, find mates, avoid predators, combat pathogens, etc.

    This observation is particularly important for understanding the functional organization of the human brain. Because we cannot (yet) directly study the wiring of the brain (except in a very few cases), we need another 'window' or set of tools for perceiving brain functions. Darwin's theory provides this window. If we can specify all the reproductive problems faced by our ancestors (i.e., if we can specify the human EEA), we can specify all the potential functions that our bodies and brains could have, in principle. With respect to the brain in particular, if we can specify all the reproductive problems involving information processing, we can specify all the possible psychological mechanisms that could have evolved. Whether humans possess any particular psychological mechanism (i.e., an ability to solve a particular reproductive problem involving information processing), becomes an empirical question. Fortunately, it is much easier to find something if you have some idea what you are looking for. Studying the past is, at present, easier than studying brain wiring. The EEA concept therefore provides a much needed tool for determining, a priori, what kinds of functions, or mechanisms, the human brain is likely to have: the human brain solves the reproductive problems posed by past environments; it allows us to do all the things we needed to do to survive and reproduce in ancestral environments--find food, find mates, detect and avoid predators and other dangerous animals, etc. We can understand the functional organization of human bodies and brains precisely to the extent that we can understand the human EEA.

    What is the EEA? (detailed answer)

    In order to understand the precise definition of the EEA, we must understand the definition of a selection pressure. Many of the misconceptions about the EEA can be avoided by adhering closely to the precise definition of the EEA derived from the theory of natural selection. As noted above, the EEA is the set of all selection pressures faced by an organism's ancestors over 'recent' evolutionary time (i.e., over approximately the last 1000-10,000 generations). To understand what a selection pressure is, we must understand how a mutation spreads in a population. It must alter the phenotype in some way that enhances reproduction (ignoring drift and other similar processes for the moment). As emphasized elsewhere in this FAQ, reproduction is an enormously complex process; that it happens at all is a near miracle. Reproduction involves a vast number of physical processes that must proceed correctly if reproduction is to be successful. Given the design of an organism, given all the physical transformations that have to take place in order to reproduce, and given ALL the environmental conditions that the organism may encounter with some non-zero probability during its life, there is a (relatively) small set of *potential* transformations of the environment--where the term environment may include aspects of the organism itself--that will enhance rather than impede reproduction. These potential reproduction enhancing transformations are called selection pressures. Stated another way, selection pressures are those aspects of the environment that can have a notable impact on the reproduction of members of a particular species over evolutionary time. The EEA of any species is the set of all features of the environment that could have had some impact on the reproduction of members of this species over recent evolutionary time.

    For example, let's assume that an herbivore regularly ingests a particular plant toxin, and that this toxin has a detrimental effect on sperm quality. Let's also assume that there are enzymes that can neutralize this toxin, but that the herbivore cannot produce these enzymes. The fact that the plant toxin can be neutralized by an enzyme is an example of a *potential* transformation that could facilitate the reproduction of the herbivore (because it would result in improved sperm quality). Thus, the plant toxin is a selection pressure and is therefore an aspect of the EEA of the herbivore. Should a mutation arise that produces a toxin neutralizing enzyme, this mutation will spread in the population. After many generations, all herbivores of this particular species will now be able to neutralize this plant toxin. If the plant goes extinct, the herbivores will still be able to produce the detoxifying enzyme (for many generations, at least), and this particular toxin is still considered an aspect of the species' EEA. Alternatively, if no mutation ever arose to produce a detoxifying enzyme, this plant toxin was still a feature of the species EEA. It was a selection pressure, even though no adaptation evolved to neutralize it.

    On the other hand, if a different toxic plant also grew in the same area as the first toxic plant, but the herbivores never ate that plant, then the second plant and its toxin are not considered part of the species' EEA. The second plant and its toxin were never a selection pressure--they had no impact on the herbivores' reproduction, and no transformation of the second toxin would facilitate herbivore reproduction. So, in use, the EEA refers not only to transformations of the environment that were necessary for reproduction, but also transformations that could have *potentially* facilitated reproduction. It does *not* refer to aspects of the past that could not impact reproduction in any way.

    Notice that for a mutation to spread to fixation (i.e., to the entire population), it must transform the environment in a reproduction facilitating way for many generations (1000, say). This means that the mutation must interact, via the phenotype, with a recurrent aspect of the environment--an aspect that the organism and its descendants are likely to encounter with some significant probability over each of their lifetimes. For example, in the case of the plant toxin, it is not necessary that every individual herbivore regularly ingested the toxin; it is only necessary that, over evolutionary time, members of this species encountered the toxin frequently enough that those who could neutralize it would have had, on average, a reproductive advantage over those who couldn't.

    Once a mutation has reached fixation, it must continue to experience a selection pressure (stabilizing selection) in order to remain in the genome; otherwise it will tend to be eliminated by subsequent mutations. Eyes evolved long before humans appeared, but if sunlight was not a part of the human EEA, we would have lost our visual capabilities, as have certain species of cave-dwelling fish. In the case of the plant toxin, if that particular plant went extinct, the ability to produce the neutralizing enzyme would degrade over time due to random mutations of the gene that produced the enzyme--there were be no selection pressure against organisms that could no longer neutralize the toxin, because the toxin is no longer part of the environment. Thus, even if an adaptation evolved in an ancestor species (as eyes did in an ancient ancestor of humans), the selection pressures that maintained eyes over recent evolutionary time are considered part of the human EEA. Stabilizing selection pressures are part of the EEA.

    It is worth noting that the organism is part of its own EEA. For example, the heart creates a pressure differential in the circulatory system; this pressure differential then results in a nutrient rich liquid (blood) being circulated to other tissues. Thus blood, arteries, and veins were essential features of the EEA of the heart (and thus of all organisms with hearts, including humans).

    The definition of the EEA as the set of all selection pressures acting over recent evolutionary time has some notable implications. First of all, selection pressures are adaptation specific. The selection pressures acting on visual abilities are (in general) not the same as those acting on toxin neutralizing abilities. Thus, the evolutionary history of vision will (again, in general) not be the same as the evolutionary history of toxin neutralization. For example, one adaptation (like vision) may have a much longer evolutionary history than another (like the ability to neutralize a specific toxin). Another implication is that species can be adapted to a variety of mutually exclusive environmental conditions e.g., day and night, hot and cold, feast and famine, high population densities, low population densities, male biased sex ratios, female biased sex ratios, lots of predators, few predators, etc., so the EEA definitely does not refer to a fixed or static time or place.

    Perhaps the most important implication is the following: organisms possess functional traits because those traits were selected for over evolutionary time. This means that those traits reliably performed their functions in past environments, and may or may not properly perform them in current environments. Thus, the EEA refers to those aspects of past environments to which an organism is adapted. Any organism can possess adaptations which no longer serve any reproductive function, and may even impede reproduction.

    A couple of quick examples will illustrate many of the foregoing points. We have lungs because oxygen existed in our atmosphere *in the past,* not because oxygen exists in the immediate present. Should oxygen somehow disappear from the atmosphere, we would still have lungs, they just wouldn't work (and we would quickly go extinct). Fortunately, the current environment strongly resembles the EEA in this regard. As another example, Richard Coss has done work on physiological and psychological ground squirrel adaptations to rattlesnake predation. He shows quite convincingly that these ground squirrels retain protective adaptations even when they haven't faced rattlesnake predation pressure for very long periods, but that these adaptations are increasingly degraded the longer the squirrels have been in rattlesnake free environments.

    Some aspects of the modern environment do diverge quite radically from the human EEA. Two examples: 1) automobiles kill far more people today than do spiders or snakes, but people seem to be far more averse to spiders and snakes than they are to automobiles. Why? Because in the EEA, spiders and snakes were a serious threat, whereas automobiles didn't exist. Thus, it was possible for us to evolve an innate aversion to spiders and snakes, but not to automobiles. 2) Safe and highly effective birth control is a modern invention. For most of human history, women were pregnant or lactating for much of their adult lives. Interestingly, women in natural fertility populations (modern populations that don't have access to modern methods of birth control) appear to have a much lower rate of reproductive cancers than do women in populations with easy access to modern birth control. It has been argued that early and frequent pregnancies may prevent reproductive cancers. Because modern forms of birth control did not exist in the EEA, there was no selection pressure against the reproductive cancers that may accompany their use.

    If the current environment of a particular species fails to resemble the species' EEA in too many ways, then the species will go extinct. Since the human species is clearly not going extinct, the common complaint that evolutionary psychology views humans as currently living in an entirely novel environment is clearly false. Most aspects of the modern environment closely resemble the human EEA. Hearts, lungs, eyes, language, pain, locomotion, memory, the immune system, pregnancy, etc., all work as advertised--excellent evidence that the modern environment does not radically diverge from the EEA.

    Much research in evolutionary psychology proceeds as follows. First, identify a plausible selection pressure (often called a reproductive problem), like predation, for example. Second, hypothesize a cognitive solution to this problem, e.g., the ability to detect and avoid predators. Finally, devise and conduct experiments to see if humans in all populations have a specialized ability to detect predators and then avoid them. If they do, this implies that the human species has evolved psychological mechanisms for detecting and avoiding predators. Click here for more on this particular example.

    Isn't it true that we can't know what happened in the distant past, so the EEA concept is useless?

    No! It is a common misconception that the EEA refers to aspects of the past that differ from the present. In fact, the EEA refers the aspects of the past whether or not they correspond to aspects of the present. For any living species, most aspects of its EEA will correspond closely to aspects of its present environment, otherwise it would go extinct; if the present environment of any organism differs too much from its EEA, its functions will most probably fail to ensure survival and reproduction.

    There are many mundane facts about the past that we know to be true (which also happen to be true of the present): there was gravity, sunlight, oxygen, plants, animals, parasites, cliffs, rivers, lakes, predators, toxins, men, women, children, old people, parents, brothers and sisters, mates, rocks, sticks, trees, faces, etc., etc. We also know that women got pregnant and men didn't. This single fact about the EEA is the foundation for much research on mating strategies in both humans and other animals. Pregnancy involves numerous costs, and we therefore expect that females in many species will be more picky about mating than will males. This prediction has strong empirical support for both humans and other animals.

    There are also several aspects of the human EEA that differ from most present human environments. We also know that population densities were much lower than today, that most societies were very probably kin-based, that child mortality rates were very probably much higher than today. However, humans still took more than a dozen years to reach sexual maturity, and fathers were less certain of paternity than mothers were of maternity (as they are today, absent a genetic test). These latter facts form the foundation for considerable research into differential parental investment. Human offspring require enormous investment to reach sexual maturity. If half of one's children were likely to die, parents needed to be able to target their investment towards the healthiest, most viable offspring. Similarly, males should have targeted their investment at offspring that were likely to be their own. Both of these hypotheses have found considerable support among both humans and other animals.

    The EEA concept is an essential and logically necessary aspect of the theory of natural selection. We have lungs because there was an oxygen atmosphere in the *past.* Should our atmosphere suddenly disappear we would still have lungs, but they would be useless. If we could truly say *nothing* about the past, we would have to abandon the concept of adaptation. Fortunately, archaeologists, paleontologists, paleoanthropologists, historians, detectives, and cosmologists all make a living studying the past, so the problem obviously isn't insurmountable.

    Why is the EEA equated with the Pleistocene?

    The EEA is the set of selection pressures faced by a population over evolutionary time. The Pleistocene is a specific period of time beginning about 1.8 million years ago, and ending about 11,000 years ago. So, the two are not equal. However, the set of selection pressures that resulted in the evolution of the human body, including the brain, are almost certainly the selection pressures that acted on humans during the Pleistocene. As noted above, 'evolutionary time' for any species is roughly 1000-10,000 generations. Assuming a human generation to be about 20 years, that translates to 20,000 to 200,000 years. The period of time called the Pleistocene includes this, but is about 10 times longer, so that is a comfortable amount of time for complex adaptations to have evolved. Also, our genus, Homo, emerged in Africa around 2 million years ago, and by 1.8 million years, Homo had spread to Asia--the first hominid to leave Africa. At the end of the Pleistocene, humans invented agriculture, which resulted in a rapid abandonment of hunting and gathering, the means by which humans had survived for the preceding 2 million years. Within a few thousand years after the end of the Pleistocene, some humans were living in cities, a novel form of settlement. In short, the amount of cultural change experienced by humans over the last 10,000 years has been tremendous, possibly exposing humans to novel selection pressures, or eliminating previously important selection pressures. So, the Pleistocene--which (almost) encompasses the origins of our genus, but excludes the recent period of dramatic change--is conveniently identified as the epoch which shaped human physiology and psychology. It is important to note that many of our adaptations--perhaps most--evolved before the Pleistocene. Human anatomy is almost identical to primate anatomy, and, indeed, mammalian anatomy, all of which took their present form well before the onset of the Pleistocene. The reason we can still roughly equate the Pleistocene with the period of time which shaped human adaptations is that, if an adaptation which evolved prior to the Pleistocene, like vision, were not under stabilizing selection during the Pleistocene, that adaptation would have been lost during the 2 million years of the Pleistocene. Stabilizing selection maintains adaptations that have already evolved. For example, assume that the sun blinked out 2 million years ago (but, implausibly, that there were no other changes to the environment and most species did not go extinct). Humans and all other animals with vision would have lost their visual capabilities. Mutations would inevitably have occurred in the genes underlying our visual system, degrading our visual abilities. Since there wasn't any light, however, this degradation would have been inconsequential, and these mutations would not have been selected out of the population. After 2 million years, the visual system would be completely gone (this has actually happened for some species of cave-dwelling fish). Consequently, we can include sunlight as part of our EEA, and our visual system as a product of stabilizing selection for vision during the Pleistocene.

    Why couldn't humans have evolved during the last 10,000 years?

    They could, but not much. Evolutionary psychologists downplay the possibility of significant cognitive evolution in the 10,000 or so years since the advent of agriculture (a period of time known as the Holocene) for reasons of both science and political correctness. Scientifically, 10,000 years (500 generations) is not much time for natural selection to act, and it certainly is not enough time to evolve new, complex adaptations—sophisticated mechanisms coded for by numerous genes.

    It is possible, however, that humans could have evolved minor cognitive adaptations during the Holocene. Just as some populations whose subsistence relied on herds of domesticated animals evolved to digest lactose as adults, populations could have evolved simple cognitive adaptations that their hunter-gatherer ancestors did not possess. For this to occur, there would have had to have been environmental conditions that were (1) new, (2) constant over most of the Holocene, (3) relevant to reproduction, and (4) required novel cognitive abilities. Many of the changes experienced by humans over the Holocene, however, have been so rapid that natural selection just couldn’t keep up. Further, we know that very little has changed physiologically in the last 10,000 years—Australian aborigines were more or less isolated from other populations for perhaps 40,000 years, yet are essentially identical physiologically to other human populations—so probably very little has changed psychologically.

    Politically, EPs are understandably desperate to avoid any association with past racist attempts to essentialize population differences that are best explained by culture. If it were possible that human cognition had undergone significant evolution during the Holocene, then it would be theoretically possible to ascribe significant differences in behavior between different populations to genes, and that would be EP’s worst nightmare.

    If we had as thorough an understanding of our psychological adaptations as we do our physiological adaptations, then perhaps we might be able to identify some simple psychological adaptations based on one or two genes that evolved during the Holocene; these adaptations might be population specific, or they might be pan-human. But we understand almost nothing about our evolved cognitive abilities. Imagine studying skin color without knowing what skin is. That would be a complete waste of time! EP rightly emphasizes a current focus on pan-human, complex cognitive adaptations that, like the rest of the body’s adaptations, were selected for during the two million years of the Pleistocene.

    What is an adaptation?

    Adaptations solve reproductive problems, that is, adaptations have functions. The lungs are an adaptation, and their function is to transfer gases to and from blood. Muscles are an adaptation, and their function is to apply force to various parts of the body. The function of intestines is (in part) to extract nutrients from food. In general, all members of a species of the same sex and developmental stage share the same functional organization (i.e., have the same adaptations). All humans have bones, muscles, hearts, eyes, etc. While some problem solving abilities (functions) of the nervous system are obvious (e.g., vision), many are not; the goal of evolutionary psychology is to identify all functions of the nervous system.

    Functional organization implies specialization. Oxygenating blood is a different problem than circulating blood, and it would be difficult if not impossible for a single functional unit to effectively solve both problems. Efficiently transferring oxygen to blood requires very high surface areas of gas permeable tissue that would be quite unsuitable for pumping fluids. The nervous system is clearly specialized as well. Generating the relatively low bit rate serial streams that characterize speech is a very different problem from parallel processing the vast amount of data generated by the retinas. The nervous system appears to be composed of multiple, specialized, functional units. This begs the question, how many specialized functional units are there? A few moments of reflection reveals that there are least several: vision, hearing, speech, motor control, sensation, smell, memory. Are there only a few more than these, or many more?

    Finally, functional organization appears in the world only as a product of natural selection. Thus, the domains about which functions are organized are the domains that are relevant to fitness. Organisms possess precisely those functions that facilitated survival and reproduction in past environments. Any distinction in the world that could be exploited by an organism to increase its fitness represents a selection pressure. The set of such selection pressures that has acted on the human lineage is often referred to as the Environment of Evolutionary Adaptedness (EEA). Such selection pressures may or may not have resulted in the evolution of adaptations that exploit these distinctions to the benefit of the organism (and thus the genes that code for such adaptations). A complete specification of all selection pressures that have acted on a population over evolutionary time places an upper bound on the number of adaptations that can evolve. In the absence of constraints, one adaptation would evolve for each distinct selection pressure. The view outlined here implies that any organism can be partitioned into a finite number of functional components, and that such a partition in some sense constitutes a complete specification of the organism. This model of life is quite tractable, and has resulted in tremendous advances in biology.

    What is a psychological adaptation?

    A psychological adaptation is a functional component of the nervous system that solves a particular reproductive problem. Information processing is the highly abstract domain upon which psychological adaptations are thought to operate. That is, the reproductive problems solved by the nervous system are thought to be best characterized as information processing problems. Computer algorithms, broadly construed, are usually thought to provide the best model currently available for the information processing abilities of psychological adaptations. This model of animal psychology derives, in part, from the following observations:

    The information content of a physical system is the number of distinct and detectable states that that system can assume. Thus, a light switch can be either off or on--two distinct states that can be abstractly represented by 0 and 1. Since two is the minimum number of discrete states possible for any system, the minimum unit of information--the bit--represents two states (e.g., 0 or 1). Notice that flipping a light switch on or off is a physical transformation of the switch that requires *energy.* This is true of any change in the information state of any system. All 'information processing' involves energy dissipating transformations of physical systems. Because all adaptations (e.g., hearts, lungs, etc.) effect transformations of physical systems that involve a change in the informational state of that system, all adaptations can be thought of as 'processing information'. THERE IS NO FUNDAMENTAL OR QUALITATIVE DIFFERENCE BETWEEN INFORMATION PROCESSING ADAPTATIONS (i.e. psychological adaptations) AND ANY OTHER TYPE OF ADAPTATION!

    All adaptations effect physical transformations of target systems (e.g., the lens focuses light, muscles move bones) that can be construed as changing the informational state of the target system. So, in principle, the information processing model could be applied to all adaptations. However, there are quantitative differences that usefully distinguish information processing adaptations from other adaptations:

    1. High information content--the system can assume a large number of distinct and detectable states. For example, hearts can assume only a limited number of different states (e.g., beating fast, beating slow), whereas the retina can assume an astronomically large number of different states (e.g., all the possible combinations of activation levels of the 125 million rods and 6.5 million cones in each eye)

    2. State transformations only require small amounts of energy. Again, heart muscle requires a significant amount of energy to contract compared to the amount of energy necessary to activate a cone on the retina.

    3. State transformations can occur very rapidly. The frequency of contractions of heart muscle is slow compared to the potential frequency of state changes in the cones of the retina.

    The structure of animal senses suggest that the information processing model is apt. Animals devote a considerable fraction of tissue to sensors. Skin contains an extremely high density of tactile receptor cells that can individually change state in response to touch, temperature, and tissue damage. For example, the human hand has 17,000 such cells per square inch. Further, the energy required to register a sensation is relatively low. Finally, in addition to their high spatial density, receptor cells also possess the ability to represent changes with a high degree of temporal resolution. These cells are connected to the brain by nerve fibers that can communicate state changes in about 1/50 of a second. Thus, the properties of tactile sensors matches our definition of information processing adaptations quite well. If we then consider that animals also possess other high bandwidth sensors like eyes, ears, taste, and smell, and that each of these can assume a *vast* number of possible states in response to environmental conditions, we are forced to conclude that animals are organized to collect astronomical quantities of information, information which must then undergo further processing in order result in reproduction facilitating actions on the part of the animal. These, then, are the functions of psychological adaptations: collect information on the environment (including the organism itself), process this information to extract reproductively salient conclusions about the environment (e.g., there is a predator staring at me), and initiate reproduction facilitating transformations of appropriate target systems (e.g., turn around and run).

    What about spandrels?

    Evolutionary adaptation is a special and onerous concept that should not be used unnecessarily, and an effect should not be called a function unless it is clearly produced by design and not by chance. When recognized, adaptation should be attributed to no higher a level of organization than is demanded by the evidence. George C. Williams, opening words of Adaptation and Natural Selection, 1966.

    Repeating an argument made earlier by George Williams (1966), a couple of Harvard guys (Gould and Lewontin 1979) got a lot of mileage out of the observation that many organism 'traits' are not adaptations, but simply incidental byproducts. This is obviously true because the vague term 'trait' can refer to any conceivable aspect of an organism, like the grumbling of its stomach or snoring. Unlike the Harvard guys, Williams offered a way to separate the wheat from the chaff: adaptations must exhibit evidence of design.

    Williams' criterion is critical. Without it, it is possible to assign every molecule, cell, and tissue in the body to a spandrel. Consider this thought experiment. A CAT scan produces a detailed 2D image of a cross-section of the body, like slicing open an orange and photographing the freshly revealed surface. By taking a large number of 2D scans perpendicularly along the length of the body and inputting the stack of images into a computer, one can build up an amazing 3D view of the body's internal anatomy, just as one could build up a 3D view of the internal structure of an entire orange by slicing it into a large number of thin sections, photographing each one, and scanning the stack of 2D photographs into 3D software.

    Imagine that a team of scientists who know nothing of anatomy gets hold of a large stack of CAT scans of an entire human body, revealing all its tissues in detailed cross-sectional images. The scientists begin analyzing the body using the 2D images, not realizing that the individual scans can be composited into a single, 3D model. Instead, each scientist gets her own 2D image to analyze independently from the others. Each scientist develops sophisticated statistical models of the patterns of light and dark on her image, scribbling down elegant equations describing the image's shapes and curves. The statistics and equations developed by the team are a rigorous, factual description of the entire body, but it is a description that is empty. The patterns of tissues revealed by the CAT scans are, if considered alone, spandrels of the true, underlying functional organization that the team has failed to recognize. Ask the wrong questions, and virtually all normal body tissues will be part of a spandrel. Ask the right questions, and most normal body tissues will be recognized as playing a vital, functional role in the survival and reproduction of the organism.

    But wait! Isn't it somehow scientifically dangerous (e.g., Gould 1997), or at least embarrassing, to over hypothesize adaptive functions for traits that might not be adaptations? Nope. Such mistakes are no more scientifically dangerous than the opposite—under attributing function. Consider this example. Lumps of tissue at the back of the throat often become infected and therefore are (or were) frequently removed by surgery. Which scientific response do you prefer?: (1) Mock any suggestion that the lumps (tonsils) might serve an important function by loudly insisting that not all traits have adaptive functions; or (2) generate and test as many functional hypotheses as you can think of to make sure that by removing the tonsils no lasting harm is done to the patient?

    Just as anatomists have made mistakes, EPs will sometimes over attribute function to psychological phenomena that aren't really adaptations (my work, Hagen 1999, 2003, could be a prime example) and sometimes they will fail to recognize genuine functions. On one level I find it bizarre that Gould, Lewontin, or anyone else could possibly fear the "dangers and fallacies" (Gould 1997) of what is in fact routine science with an outstanding record—proposing and testing functional hypotheses for organism structure. On another level, however, I understand Gould and Lewontin's distress. EP has rudely broken into the cathedral of the mind, spray-painting 'sex', 'violence', and 'competition' across their beloved spandrels.

    Make no mistake, many spandrels, which EP terms byproducts, are enormously important in their own right. Symons (1979), a founder of EP, argued, for example, that the capacity of women to orgasm is a byproduct of a male adaptation for orgasm. It is an unstated premise of EP, however, that, by failing to recognize the evolved functional organization of the brain, psychology and the rest of the human behavioral sciences, like our team of misguided scientists, are condemned to study nothing but spandrels.


    Gould SJ (1997) The exaptive excellence of spandrels as a term and prototype. Proceedings of the National Academy of Sciences 94:10750-10755.

    What is domain specificity and why is it necessary?

    Domain specificity is an important property of physiological adaptations, and is presumed to be an important property of psychological adaptations as well. Domain specificity means that adaptations evolve to solve problems in particular domains, and therefore are less well suited to solve problems in other domains. A domain is a selection pressure or (equivalently) a reproductive problem. It is a physical transformation that, completed successfully, would facilitate the reproduction of the organism (or, more properly, genes coding for phenotypic traits that effect the transformation). Examples of domains include oxygenating blood, killing harmful bacteria, focusing light, extracting nutrients from food, filtering or neutralizing toxins, regenerating damaged tissue, etc. In other words, any of the myriad physical processes necessary for reproduction. In order to be successful, reproduction requires that a vast number of physical transformations complete efficiently and effectively. Adaptations are the structures that effect or evoke the necessary transformations. Lungs oxygenate blood, the immune system kills bacteria, the lens focuses light, the intestines extract nutrients from food, etc.

    Reliably and efficiently evoking a specific transformation requires a highly specialized structure. For example, extracting large quantities of oxygen from the atmosphere is a difficult task. Compared to solids and liquids, gases are quite dilute. Further, oxygen comprises only 20% of our atmosphere, and the earth's atmospheric pressure is relatively low. In order to transfer substantial quantities of oxygen into our (liquid) blood stream, it is necessary to efficiently expose blood to large quantities of air. In addition, it is important to keep blood inside the body while keeping air on the outside of the body. One solution would be to have a thin, very high surface area membrane that is continually 'washed' with air on one side, while a thin layer of blood circulates on the other side. This is exactly the type of structure that has evolved to solve this problem. The surface area of our lungs is about equal to that of a tennis court, and blood flows across one side in a single-celled layer--i.e., in the thinnest layer that is theoretically possible. Although lungs are ideal for oxygenating blood, they would be completely ineffective for circulating blood. Delicate lung tissue possesses none of the properties necessary to pump large quantities of liquids. The dense muscles of the heart are far more effective for this task. Thus, lung tissue has evolved to be highly specific to the domain of gas transfer, whereas heart tissue has evolved to be highly specific to the domain of pumping liquids. Specializing to solve a reproductive problem in one domain generally precludes an ability to solve reproductive problems in other domains. This appears to be a general property of mechanisms: mechanisms that do one thing well will not do other things well. Because reproduction involves a large number of distinct transformations of the world, organisms will be comprised of a large number of domain specific adaptations to effect these transformations.

    Evolutionary psychology is betting that psychological adaptations have to be just as domain specific as physiological adaptations. Information processing appears to be an excellent model for the general class of problems solved by psychological mechanisms. In the field of information processing, no one has invented a computer program that solves all problems. Each information processing problem requires specialized software to solve that problem. Spreadsheets are different from word processors are different from video games. Similarly, vision is different from hearing is different from pain is different from smell is different from sexual desire is different from navigation. In short, transformations of information are just as specialized as any other physical transformation and require equally specialized mechanisms to complete the task. Thus, psychological adaptations are as likely to be as domain specific as any other adaptation.

    What is a module?

    A module is a psychological adaptation (see above). Note that this usage of the term 'module' differs from several of the definitions of 'module' used by cognitive psychologists. For those familiar with the 'modularity' debate, I will make one brief comment: Fodor distinguishes between cognitive modularity with, and without, information encapsulation (Fodor 2000, p. 56-58). If, when performing the computations, modules only have access to information stored in the module itself, and cannot access information in other modules, the module is said to be informationally encapsulated. As a concept, information encapsulation is so unhelpful that one wonders whether its importation from computer science into cognitive science was botched. Why, except when processing speed or perhaps robustness is exceptionally important, should modules not have access to data in other modules? Most modules should communicate readily with numerous (though by no means all) other modules when performing their functions, including querying the databases of select modules.

    The original computer science concept of encapsulation, in contrast, is powerful: encapsulated modules access and modify data in numerous other modules when performing their functions, but only do so via well-defined interfaces. This means, roughly, that modules communicate in standardized ways, and that access to a module’s data and functionality is regulated by the module itself. As long as the interface between modules stays the same, programmers can tinker with modules’ implementations without disrupting other modules. In computer science, it is a module’s functionality that is encapsulated, not its data per se. (The standardized way in which nerve cells communicate is a low-level example of encapsulation in the brain. Whether natural selection could have evolved this useful architecture at higher, neural network levels in the brain is an open question, but it would clearly allow individual modules to evolve without interfering with other modules.)

    How can we identify psychological adaptations?

    We can identify psychological adaptations using the same criteria we use to identify any other adaptation: EVIDENCE OF DESIGN. We know the lung is an adaptation because 1) the many features of the lung--numerous chambers of gas-permeable tissue each surrounded by a network of blood vessels and each connected to the windpipe--correspond precisely to the nature of the problem: transferring oxygen to the blood, 2) solving this problem (oxygenating blood) would have greatly facilitated reproduction in ancestral environments (as it does in modern environments), and 3) natural selection is the only source of functional organization. QED, the lung is, without doubt, an adaptation. We can use these same criteria to identify psychological adaptations.

    In order to effectively solve reproductive problems, mechanisms (a.k.a. adaptations, functions, modules, organs) must possess a suite of specific structural features that effect the required transformations of the world. These features are called 'design features.' The gas-permeable chambers, surrounding blood vessels, and connections to the windpipe constitute some of the design features of the lung. By precisely specifying the nature of a particular reproductive problem (a process sometimes referred to as a task analysis), we can, a priori, describe the design features that a putative adaptation must possess if it is to effectively solve the problem. This is as true of information processing problems as it is of any other type of reproductive problem. For the case of psychological adaptations, we must first identify an information processing problem faced by humans in the EEA. We must then determine the features that any psychological adaptation designed to solve this problem must possess. Finally, we seek both direct and indirect evidence that the nervous system possesses the required design features--that is, that it can solve the reproductive problem. The more design features that are necessary to solve the problem, and the more such features that the nervous system appears to possess, the higher the probability that a psychological adaptation to solve the reproductive problem in fact exists. Thus, the determination that a psychological adaptation exists is an inherently probabilistic enterprise.

    A hypothetical example: suppose we can make the case that navigating in unfamiliar terrain would have been a reproductively important ability for ancestral humans. We would then specify all the information processing features relevant to solving this problem: measuring the angle of the sun, keeping accurate track of time, integrating velocity vectors, etc. We could then conduct numerous experiments to determine whether humans in fact possess each of these abilities, and, if so, whether they are properly integrated so that humans can effectively navigate in unfamiliar terrain. The more such abilities we can identify in humans, the greater the probability that humans possess a psychological adaptation for navigating in unfamiliar terrain.

    Why are adaptations not for the good of the species?

    Adaptations evolve through the differential reproduction of alternative alleles within a population or species. Thus, organisms acquire properties which allow them to out-reproduce members of their own species, not members of other species. It is theoretically possible for the differential survival of gene pools (species) to result in the evolution of organism features which would promote species survival at a personal reproductive cost to individual members of the species; it is extremely unlikely, however, that this process is responsible for the incredible array of complex functionality evinced by sexually reproducing, diploid species (Williams 1966). The length of time between speciation or extinction events is vastly longer than the length of time between generations. Consequently, differential reproduction of alleles within species can produce complex functionality much faster than can differential reproduction between species. An allele that provided a benefit to the species at an expense to the individual would be driven to extinction long before it could have a measurably positive impact on the survival of the species. (There are other forms of group selection, however, that are worth considering; see, e.g., Sober and Wilson Unto Others).

    Why are genes selfish?

    Richard Dawkins wrote a very popular book called the Selfish Gene that explained, for a popular audience, many of the exciting new theories and discoveries being made in evolutionary biology in the 1960’s and 70’s. The metaphor Dawkins chose, the selfish gene, was an extremely powerful metaphor, so powerful that it has often overshadowed the science itself! The controversies that swirl around EP are often tightly bound up with Dawkins’ metaphor. If our genes are selfish, are we all, deep down, unalterably selfish ourselves? Why did Dawkins chose this metaphor, what does it really mean, and what are its implications for EP and human nature?

    Simplifying greatly for the sake of the argument, there is a molecule, called a nucleotide, that comes in four different types, A, C, G, & T. Large numbers of these nucleotides can be linked together in a linear strand to make a much bigger molecule called DNA. Schematically, DNA looks like this: ACGTGCCT…etc. Human DNA consists of about 3 billion nucleotides chained together. Simplifying greatly again, small sections of the DNA strand called ‘genes’, which are usually several hundred to several thousand nucleotides long, are able to create a different type of long, linear molecule called a protein. Proteins are a kind of plastic—technically, a polymer—that, like the DNA chain, are made up of a small number of different molecular building blocks called amino acids (there are 20 different amino acid building blocks ). Just as the different chemical structure of the plastic in a plastic bag versus the chemical structure of the plastic in dental floss versus the chemical structure of the plastic in bullet-proof vests gives these different materials very different properties, the exact sequence of the different amino acids in a particular protein determines its biological properties. Different amino acid sequences give different proteins very different properties.

    To summarize, the sequence of nucleotides in small sections of our DNA (called genes) determines the sequence of amino acids of proteins created by the genes, and these amino acid sequences determine the proteins’ biological properties. Although scientists are still debating the exact number, DNA contains somewhere between 30,000 and 60,000 different genes, and can therefore create between 30,000 and 60,000 different proteins, each with unique properties. As I mentioned, proteins are a kind of plastic, so our DNA functions, in part, to create a large number of different plastics with different properties. These highly specialized plastics with very special biological properties are what our bodies are made of. DNA is a kind of lumber yard that provides, among other things, a large number of plastic building materials for making organisms.

    Simplifying once more (this time by ignoring sex), individuals pass on an exact copy of their DNA chain to their offspring: if my body is made up of a particular set of plastics, because my offspring has an exact copy of my DNA, my offspring’s body will be make up of exactly the same set of plastics, and so it will be exactly like me. Occasionally, however, one of the molecules in the DNA chain (i.e., one of the A, C, G, or T nucleotides), can become mutated (altered) by cosmic radiation, environmental toxins, etc.; these mutagens turn one type of nucleotide into another type of nucleotide (e.g., an A turns into G). If I pass on this mutated DNA, where only one of the 3 billion nucleotides is different from my own, then my offspring will be made of proteins that are almost exactly like mine, except for the protein which was made by the mutated section of DNA (the mutated gene), which will be a different protein with different properties. My offspring will not be exactly like me—he will be said to have a different phenotype (body type).

    Imagine that the gene that was mutated was the gene that made the plastic forming the lens of the eye. This plastic has a very special property: it is almost completely transparent. Most proteins, like those forming your skin, muscles, hair, etc., are not transparent. Because my offspring has a mutated form of the lens gene, there are now two types of genes in the population that make the lens protein: the normal version, possessed by most individuals (which I will call Tnormal, for normal transparency) and the mutated version possessed by my offspring. Different forms of the same gene are called alleles. Because it is far easier to make something worse than it is to make it better, most of the time when the gene producing the lens protein is mutated (and this happens very rarely), the altered lens protein produced by the mutated gene will not be as transparent as the original version (so I will label this allele Tlow, for low transparency). My offspring will therefore not be able to see as well as other members of his species who have the Tnormal allele, and thus normal versions of the lens protein.

    Let’s assume that my offspring’s lens protein is only slightly less transparent than the normal version. He will be able to live his life and have offspring. The population will therefore contain a mix of lens alleles; most of the population will have Tnormal, but some will have Tlow. Because those with the Tlow allele will not be able to see quite as well as other members of their species possessing Tnormal, on average they will not have as many offspring. Perhaps they notice prey slightly less often, and thus not have quite as much food, or perhaps they fail to notice predators slightly more often, and will therefore be killed and eaten at a slightly higher frequency. Over many, many generations, the fraction of individuals with the Tlow allele will decrease relative to those with the Tnormal allele simply because individuals possessing the Tlow allele produce, on average, fewer offspring. We say that the Tlow allele producing the less transparent lens protein is selected against, and that the frequency of this allele decreases with time. Notice that, if the total population size remains constant, that the decrease in frequency of the Tlow allele results in an increase in the frequency of the Tnormal allele.

    Imagine another mutation of the normal lens allele, creating a third allele (Tsuper) that produces a super transparent lens protein. The population now contains three alleles, Tlow, Tnormal, and Tsuper. Individuals possessing the Tsuper allele detect prey, on average, at slightly higher frequencies, and thus have more food, and they more frequently detect predators and therefore are eaten slightly less frequently. As a consequence, on average, they have more offspring than individuals possessing Tnormal. Over many, many generations, the frequency of the Tsuper allele will increase in the population, whereas the frequencies of the Tnormal and Tlow alleles will decrease, and perhaps disappear altogether. This is called evolution by natural selection: the frequencies of the three alleles have changed as a consequence of their reproductive effects. Over time, a population will acquire alleles that produce proteins that better solve critical reproductive problems, and lose alleles that produce proteins that less effectively solve these problems. There is widespread agreement that evolution by natural selection is responsible for the origins of the sophisticated organs and tissues like hearts, lung, livers, etc., that enable organisms to reproduce.

    Because, in a population of a given size, the increase in the frequency of Tsuper must decrease the frequencies of the other alleles, biologists began saying that different alleles were ‘competing’. (Usually, but not always, alleles increase their frequency by causing individuals possessing them to produce, on average, more offspring.) Dawkins, highlighting the iron-clad logic that alleles increase their frequency in the population if they cause more copies of themselves to be made relative to other alleles, and that by increasing their own frequency, they decrease the frequency of the alternative (competing) alleles, termed genes ‘selfish’. Alleles increase their frequency at the expense of other alleles.

    Do selfish genes mean selfish people?

    Not necessarily. Describing genes as selfish is an analogy that has nothing to do with our folk notion of selfishness. Adaptations evolve via the differential reproduction of alleles (different versions of the same gene). This means that one version of a gene (allele A) at a particular locus causes organisms bearing that version to have a different phenotype (body structure) than organisms bearing a different version of the gene (allele B) at the same locus. If organisms with phenotype A produce more offspring than those with phenotype B, allele A will increase in frequency in the population. Allele A is said to have 'out-competed' allele B. Thus, allele A is a 'selfish gene'--it increased its frequency at the expense of allele B. But, every adaptation in the body evolved in this manner! That means that the genes coding for your hair are just as 'selfish' as the genes coding for your fingernails, which are just as 'selfish' as the genes coding for your kneecaps! The same goes for psychological adaptations: the genes coding for vision are just as 'selfish' as the genes coding for memory, which are just as 'selfish' as the genes coding for muscle control.

    There is a narrow range of psychological adaptations whose properties do correspond to our folk notion of selfishness. When critical resources are limited, organisms which are able to obtain adequate supplies of these resources will out-reproduce those that don't. Obtaining such resources will often involve direct conflict between organisms, such as fighting for food or mates. Genes that code for fighting abilities that would allow organisms possessing those genes to out-compete other organisms for scarce resources will increase in frequency. So, the fact that some resources are limited means that strategies like aggression are likely to evolve in many species. Psychological adaptations for aggression correspond to our folk notions of 'selfishness', but it should be noted that these adaptations evolved by the same process as every other adaptation. The genes underlying these adaptations are no more 'selfish' than are the genes underlying any other adaptation.

    Why is the heritability of adaptations generally zero?

    One often reads in the paper that researchers have discovered the gene for homosexuality or shyness, or, more generally, that some trait is 'heritable'. Heritability means that variation in some trait (like the presence or absence of homosexuality) is correlated with genetic variation (e.g., the presence or absence of some gene). It is a common mistake to assume that if a trait is heritable, it is adaptive. The opposite is generally the case!

    Adaptations evolve by the differential reproduction of alternative alleles. Novel alleles arise by mutation. If the phenotypes associated with these novel alleles interact with aspects of the environment in such a way as to gain a reproductive advantage over phenotypes associated with other alleles at the same locus, the novel allele will (with some exceptions discussed below) go to fixation--that is, its frequency in the population will be 100%. Heritability is the proportion of variance in a phenotypic trait that is accounted for by genetic variance. As we have just seen, however, genes that confer a reproductive advantage generally go to fixation. Because their frequency in the population is 100%, the genetic variance at the loci of these genes is zero, so any variance in the corresponding phenotypic traits cannot be attributed to the non-existent genetic variance. Even though such traits are genetically specified, their heritability is zero! Everyone has the same genes.

    An obvious exception would occur if the population were sampled before an allele went to fixation. If half the population had an allele, and having the allele meant having the trait, then the heritability of the trait at that moment in time would be unity; most genes under positive selection, however, go to fixation relatively quickly. Another exception occurs when there is strong selection for a trait in one geographical area, but no selection, or even counter-selection, in other regions--the sickle-cell variant of the beta globin gene is a classic example. Because heterozygotes have a reproductive advantage in malaria-infested regions, the sickle-cell variant is maintained at high frequencies in those regions; because homozygotes are reproductively disadvantaged, the trait, prior to recent migrations, was virtually absent elsewhere. These dynamics prevent the sickle-cell allele from going to fixation, so the heritability of sickle-cell is high.

    The sickle-cell allele is an example of an adaptive genetic polymorphism: an allele that is maintained at a much higher frequency in the population than can be accounted for by mutation and drift, yet has not gone to fixation (and therefore coexists with other alleles at the same locus--thus the term 'genetic polymorphism'). Population or geographically specific selection pressures are one avenue for the maintenance of adaptive genetic polymorphisms; frequency dependent selection is another. An allele might be able to increase in frequency when it is rare simple because individuals possessing the gene are 'different'--this might happen if, say, members of the opposite sex preferred 'difference'. As its frequency increases, however, individuals possessing the allele are no longer 'different', so they are no longer as preferred as mates, and the allele will not go to fixation. Instead, it will exist in equilibrium with other alleles at the same locus. This type of frequency dependent selection (so called because the selection pressure on an allele depends on the frequency of the allele in the population) can result in adaptive genetic polymorphisms.

    Most adaptations of interest, however, are not like sickle-cell--they are not coded for by single genes, they are not population or geographically specific, nor are they frequency dependent. Adaptations like hearts and lungs are coded for by an enormous number of genes, and are universal in the human species. We will refer to adaptations coded for by single (or relatively few) genes as simple, and those coded for by many genes as complex. Complex adaptations must be universal, and their heritability (construed as the presence or absence of the adaptation) must be essentially zero. Complex adaptations evolve 'piece-by-piece'. The alleles coding for fundamental elements of what could eventually become a complex adaptation must arise before there will be a selection pressure favoring alleles that code for additional 'components' of the adaptation. An organism must first evolve light-sensitive tissue before there will be a selection pressure to evolve a lens, for example. Because complex adaptations are coded for by many genes, because the genes that code for the rudimentary functionality must evolve before there was selection pressure for the evolution of advanced functionality, and because genes under positive selection go to fixation relatively rapidly, almost all complex adaptations (and the genes that code for them) must be universal. Further, the probability that any individual would lack all (or even many) of the genes needed to code for a complex adaptation is essentially zero, so the heritability of the presence or absence of a complex adaptation is also zero. There is overwhelming physiological evidence for these claims. Although there are some simple adaptive physiological differences between populations (skin color, for example), the vast majority of tissues and organs are functionally identical in all humans. (If a complex organ was evolving in one population but not others, that population would almost certainly become reproductively isolated, branching into a new species).

    Aside from the simple, geographically specific adaptations noted above, or genetic diseases (which also consist of one or a few genes), what kinds of traits are heritable? In general, traits that are not under selection can have substantial heritable components, because if they were under positive or negative selection, the genes underlying those traits would go to fixation or be eliminated (respectively), eliminating the heritability; it is this fact that implies that if the trait in question is highly heritable, then it is probably not an adaptation. This is not to say, however, that there no aspect of a complex adaptation can be heritable. Although vision is not heritable, if a particular visual performance parameter were measured precisely enough, a small degree of heritability in that parameter could no doubt be detected. These minor differences would usually be genetic noise, however, not adaptive differences.

    How can evolutionary psychologists talk about adaptations without talking about specific genes?

    Although adaptations evolved to facilitate the reproduction of the genes that code for the adaptation, not the reproduction of the organism bearing those genes, this important distinction leads to no difference whatsoever in the analysis of large categories of adaptations. In most cases, the reproduction of the organism is required for genetic replication. This is why Darwin was able to propose adaptationist explanations that still stand. Darwin--and scientists to this day--can, for the most part, avoid the currently intractable problem of the precise relationship between genes and complex adaptations and instead focus on the eminently tractable problem of the reproductive functions of the body and brain. Scientists can confidently address the functions of hearts, lungs, blood, and uteruses using evidence of design without knowing anything about the genes that code for these organs. The question becomes, not "did these traits facilitate the reproduction of specific genes," but rather, "did these traits facilitate the reproduction of the organism in the EEA?" Similarly, we can address the functions of the brain without knowing anything about the genes that underlie these functions. In fact, who can doubt that vision, hearing, smell, and pain--phenomena that rely critically on the brain--served crucial functions that facilitated the reproduction of the organism and its close relatives over evolutionary time?

    If Darwin had known about genes, he would have been able to (among other things) modify the definition of adaptation to include functions that promoted the reproduction not only of the organism, but also of relatives of the organism (since they are likely to share some of the organism's genes). This modification allows evolutionary researchers to analyze an extremely large set of adaptations without ever having to refer to specific genes.

    Are there enough genes to build psychological adaptations?

    "People don't have enough genes to program all the behaviors some evolutionary psychologists, for example, believe that genes control."

    "Evolutionary psychology is dead but doesn't seem to know it yet."

    Paul Ehrlich, presumably referring to the announcement that the human genome contains only 30,000 genes (an estimate that is in flux)

    Some critics of evolutionary psychology claim that there simply aren’t enough genes to code for a large number of innate cognitive adaptations (Ehrlich and Feldman 2003). Curiously, they don’t suggest that there aren’t enough genes to build the thousands of anatomical adaptations that have already been discovered, they haven’t suggested the theory of natural selection is wrong, nor have they called for an immediate halt to the billions of dollars of research aimed at furthering the functional understanding of cells, tissues and organs, research that, if the critics were right, would be useless given that there aren’t enough genes to build all those adaptations. Current estimates are that humans have 20,000-60,000 genes. If genes and adaptations corresponded in a one-to-one fashion, then, if it took an average of 100 genes to code for an adaptation, there could only be 200-600 adaptations, a number we have already long surpassed in our investigation of anatomy and physiology.

    Adaptations, however, are not the simple product of genes. Rather, they are the product of gene interactions. Although the processes by which genetic information directs the development of cells, tissues, and organs are still largely unknown, it is well known that both genes and non-gene regions of DNA control the protein production of other genes, and that multiple proteins combine to produce an adaptation. These simple facts fundamentally alter the math. Imagine an organism with four genes, A, B, C, and D. In the naïve view, this organism could have at most four adaptations, one coded for by each gene. But if genes interact, then this organism could have as many as fifteen adaptations—not only those produced by A, B, C, and D, but also those produced by all possible combinations of A, B, C, and D (AB, AC, AD, ABC, ABCD, BC, BD, etc.). For an organism with ‘only’ 30,000 genes, the number of gene combinations explodes. The total number of two-gene combinations, for example, is almost half a billion. To produce an adaptation, however, often many more than two genes interact. The total number of 25-gene combinations is around 1086 (in comparison, the universe probably contains around 1080 particles). An organism obviously need make use of only a minute fraction of such gene combinations to produce an incredibly rich, functionally organized phenotype with enormous numbers of adaptations. (Some have claimed that gene interactions are themselves an impediment to the evolution of adaptations. Although this can be true over the short-term, it isn’t over the long term, e.g., Hammerstein, P. 1996. Darwinian adaptation, population genetics and the streetcar theory of evolution. J. Math. Biol. 34: 511-532).

    What about 'plasticity'?

    Behavior is often referred to as 'plastic,' a relatively vague and unhelpful term which usually means that behavior changes. The real question is why and how behavior can change in such seemingly useful ways. The descriptor 'plastic' contributes NOTHING to an understanding of either the why or the how of behavioral responses to environmental conditions. Useful behavioral change must come from a structured psychology that is generating the behavior. Even describing real plastics (i.e., various types of organic polymers) as 'plastic' reveals nothing about the nature of their 'plasticity'. The plasticity of plastic is a consequence of very specific and hierarchical microscopic properties of the polymer chains, including the types of chemical bonds found on the polymer backbone, the length of the chains, and the number and nature of links between polymer chains, just to mention a few. Similarly, the 'plastic' (i.e., changeable) nature of behavior results from very specific and hierarchical properties of the nervous system generating the behavior, and it is the latter which are of interest. At best, the term 'plastic' vaguely *describes* a property of behavior (that it can change in response to environmental change); it does NOT *explain* it. It is long past time to junk the term 'plasticity'.

    What about learning?

    Learning is extremely widespread--most organisms, including plants and bacteria, probably have the capability to learn, at least to some degree. Learning is often viewed as an explanation which competes with evolved psychology. If certain behaviors or ideas can be learned, it is claimed, then there is no need for an evolutionary explanation. This is wrong on two counts. First, learning requires specialized structures. Dirt doesn't learn. Rocks don't learn. The ability of any organism to learn requires that it has evolved adaptations which permit learning. Learning adaptations must exhibit a number of sophisticated properties. The quantity of information that any organism could, in principle, learn, is unimaginably vast. Learning capacity is finite, however--if an organism is going to use the information it has learned in any useful way, its learning adaptations require extraordinarily effectively 'filters' so that only 'useful' information is learned. Learning also requires that the organism has tissues which can change state. That is, upon exposure to useful information, these tissues can record it. Finally, the organism must be able to retrieve and use information that has been learned, but do so only when the information is needed. Learning is a very sophisticated ability that would be impossible without specialized adaptations.

    Second, imagine that an organism had evolved a perfect learning adaptation. This organism could learn everything. To qualify as an organism, however, it must still reproduce. Recall that reproduction is an enormously complex process, involving countless energy consuming transformations of the world. Learning also requires numerous energy consuming transformations of the world (the world, in this case, is the organism's brain). The energy needed for learning is not trivial, and it is unlikely that organisms would evolve the ability to learn if the energy costs of learning were not compensated by enhanced reproduction. Would a perfect learning mechanism, one that could and did learn everything, enhance reproduction? Very probably not. A perfect learning mechanism would pose the same problem noted above: only a tiny fraction of information is useful for reproduction. Because an organism with a perfect learning mechanism still needs to reproduce--presumably using what it has learned--it would need to identify precisely that information which was needed to reproduce. A difficult problem to say the least.

    Perhaps an organism could evolve a learning adaptation which learned exactly the information that it needed to reproduce, and nothing more. That is, it had the ability to learn everything about its own reproductive ecology. Is this possible? Such an organism would obviously need to observe members of its own species reproduce--learning everything it needed to know--or it would need to obtain this information from some other source, perhaps other members of the species who would pass on this critical knowledge. Learning everything by observation is very probably too costly in time and effort. Individuals just wouldn't be able to observe everything they needed to observe. Consider a heavily researched topic in evolutionary psychology: male mate preferences. Evolutionary psychologists have proposed that human males prefer, all else equal, to establish long-term mateships with younger (sexually mature) females. The reason this preference evolved is that males who married younger women had more offspring on average, over evolutionary time. Considerable research has demonstrated that a sexual preference for younger women is a male universal (teenage males tend to prefer women who are slightly older than themselves, and thus fertile, but these women are still young). How could an organism learn this type of information? It would have to very carefully observe lifetime reproductive outcomes for many different mateships, controlling for other variables like health, access to resources, etc. This is obviously impossible. Life is just too short to learn this kind of information, yet men have precisely the preferences predicted by evolutionary theory. These preferences must have evolved.

    Obtaining all the information one needs to reproduce from other members of one's species is problematic as well. Evolutionary theory strongly implies that individuals will have conflicts of interest. Obtaining all the information one needs to reproduce from potential competitors is, in itself, a very poor reproductive strategy. There will be strong selection pressures on the providers of such information to manipulate the provided information in ways that benefit themselves reproductively, quite likely at the reproductive expense of the receivers. It is extremely unlikely that learning adaptations could evolve which were completely dependent on other individuals to provide the information necessary for reproduction.

    None of this is meant to devalue learning. Learning is a critical strategy for many, if not most, organisms. The environment changes rapidly, and learning is an excellent way to adjust one's reproductive strategies to increase their probability of success. Evolutionary theory indicates, however, that learning is almost certainly structured by natural selection to focus on domains that are critical to reproduction. For example, some animals are dangerous, and others not. Because an organism may find itself in an environment with animals not encountered by its ancestors, it will need to learn which animals are dangerous, and which are not. Adaptations for learning about animals may well be innate, whereas information about which particular animals are dangerous is almost certainly learned.

    What about gene-environment interactions?

    To say that the phenotype is mutually determined by genes and environment is simply to make the trivial observation that the phenotype is a physical system; thus its dynamics, like the dynamics of all physical systems, is subject to the influence of other, coupled physical systems.

    Chemistry happens.

    It is false, however, to infer that this process is, IN GENERAL, adaptive. If Murphy's law has any force, most environmental perturbations on developmental processes will disrupt the normal development of the target adaptation. I would therefore expect that many developmental mechanisms exist specifically to buffer environmental variation, thus ensuring proper development of complex adaptations. Stated another way, I suspect that the body is designed to ensure that developing systems only 'see' the environmental variation they are supposed to see; much, if not most, of the time, this will involve shielding developing systems from variation, not exposing them to it.

    The exception to the latter, of course, is environmental variation that is critical for the development and performance of the adaptation. In these cases, specific development mechanisms have almost certainly evolved to sample the variation, and to then 'tweak' the target adaptation to enhance its performance under these conditions. In some cases, the 'tweaking' will be quite dramatic, such as acquiring a native language or learning social norms.

    Here's an analogy:

    You've just won the lottery, and want to have a tricked out SUV custom built for you so you can cruise the world's deserts. Do you:

    a) request that the manufacturer build the SUV in a sandstorm so that it will optimally perform in desert conditions,


    b) request that the manufacturer add specially designed equipment, such as tires with the proper treads and a bigger radiator, so that it will optimally perform in desert conditions.

    The first is analogous to what I fear many people are thinking when they invoke 'gene-environment' interactions; the latter is analogous to what I think is actually happening most of the time.

    Are evolutionary psychologists primarily interested in what makes humans different from other animals?

    No. Imagine if physiologists were only interested in what made humans different from other animals. They would then only study bipedalism, hairlessness, and a few other traits. They would not study hearts, lungs, livers, bones, etc. Similarly, evolutionary psychologists are interested in the functional structure of human cognition whether or not we share this structure with other animals. Indeed, evolutionary psychologists have put considerable effort into studying the psychology of mating, parenting, and aggression--each of which is very important to the study of most other animals. Human mothering psychology probably shares numerous features with the mothering psychology of other primates and mammals, for example.

    Is evolutionary psychology just a politically correct version of sociobiology?

    Yes and no. Although evolutionary psychology does adopt most of the theoretical framework of sociobiology, it is actually both more and less general. Evolutionary psychology is the study of animal nervous systems from an evolutionary perspective. As such, it includes numerous aspects of cognition that have nothing to do with sociality per se, such as vision, navigation, memory, toxin avoidance, foraging, etc. By contrast, sociobiology is the biology of sociality in plants, animals, and other organisms. Although sociobiology often focuses on social behavior, it may also focus on aspects of sociality that are not products of the nervous system, like large peacock tails (which probably evolved to stimulate the nervous system of the opposite sex, however). Thus, neither evolutionary psychology nor sociobiology contains the other as a subfield. However, social cognition and behavior do indeed constitute an important subset of evolutionary psychology, and many evolutionary psychology studies employ theories such as kin selection, reciprocal altruism, and sexual selection that form the core of sociobiology. If anything, sociobiology is a subfield of evolutionary psychology.

    Another difference between evolutionary psychology and sociobiology is that evolutionary psychology has strongly advocated an explicit focus on psychological adaptations (i.e., the functional organization of the brain), and a de-emphasis on adaptive behavior. This is because it is possible for different adaptations to generate similar behaviors (the fox and rabbit are both engaging in the same behavior--running--but for very different reasons), and it is possible for a single adaptation to generate different behaviors (predator avoidance can involve both running and remaining motionless). It is also possible for psychological adaptations to trigger, but not result in any overt behavior. For example, a predator may consider pursuing a prey animal, without actually doing so. Or, one individual may find another sexually attractive without attempting to mate with them. Finally, it is also possible for adaptations to no longer serve their purpose (e.g., squirrels that engage in snake avoidance behavior in environments that no longer have any snakes). Therefore, the proper focus is on adaptations and the behaviors they generate, not on 'adaptive behavior.'

    As for political correctness, evolutionary psychology strongly argues that there are very likely to be innate cognitive differences between the sexes, and has provided considerable evidence for this view in both humans and other animals (a politically incorrect position?), but also argues that there are very unlikely to be any significant cognitive differences between various human populations (generally viewed as a politically correct position). See Is Evolutionary Psychology Racist... below for more on this. Nonetheless, many evolutionary psychologists have made a special effort to advertise the politically correct implications of the theory, most likely in order to facilitate its acceptance.

    Is evolutionary psychology another form of genetic determinism?

    Yes and no. Definitely no if one is referring to behavioral genetics. All organisms, including humans, can be conceived as an integrated set of functional parts. Hearts, lungs, eyes, blood, bones, muscles, veins, kidneys, livers, skin, intestines, gonads, and the immune system all perform specific functions. Modern medicine is founded on this functional view of the body. In the parlance of evolutionary biology, these functional parts are called adaptations. Adaptations arise through a process of evolution by natural selection. Evolutionary psychology argues that natural selection acts on nerve tissue (or, if you prefer, on the genes that code for nerve tissue) the same way it does on every other type of tissue. Therefore the brain should be organized just like the rest of the body, as an integrated set of interacting adaptations. In fact, there is no fundamental distinction between physiological adaptations and psychological adaptations. Although the process whereby genetic information directs the development of bodily functions is still largely opaque, there are very compelling empirical and theoretical reasons to believe that there are genes for arms, legs, lungs, etc. Because all humans (with very rare exceptions) have arms, legs, lungs, etc., that are built the same way and have the same features, we can surmise that we all share essentially the same genes for these limbs and organs. The universal architecture of the body is genetically determined in this sense. Since psychological adaptations like vision and pain are no different from other adaptations in this regard, they, too, are genetically determined human universals.

    However, this is not what is usually meant by 'genetically determined.' Often, researchers propose a genetic basis for criminality, alcoholism, anti-social behavior, schizophrenia, heart-disease, what-have-you. In other words, a genetic basis is postulated for individual *differences,* not similarities. In general, evolutionary psychology is not concerned with individual genetic differences. Genetic differences between individuals are known to be quite minor compared to our genetic commonalties. As will be explained in more detail in the next section, the genetic basis for the functional organization of our bodies and brains must be shared by all humans. In the same way that physiologists want to know how the body works, evolutionary psychologists want to understand how the brain works. Although there are no doubt minor differences in heart morphology that have a genetic basis, all hearts are built and function in exactly the same way. Similarly, psychological adaptations, should they exist, must also be built and function in the same way across individuals, although there will, no doubt, be minor differences attributable to underlying genetic differences. An evolutionary psychological approach to individual differences that does not rely on genetic differences will be detailed in a future version of the FAQ.

    Is evolutionary psychology racist?

    No! Evolutionary psychology is the study of the functional organization of the brain, and this organization must be pan-human. Because humans are a single species, everyone on the planet has essentially the same functional organization of both the body and the brain. Why? The functional properties of organisms, including the functional properties of the nervous system (like vision, smell, locomotion, etc.) arose by natural selection. Natural selection builds adaptations like vision one piece at a time. The first pieces must be ubiquitous in the species before later pieces can evolve. For example, a species must have evolved a light-sensitive patch of tissue before there will be a selection pressure to evolve a lens to focus light onto that patch. If the genes coding for a light sensitive patch experience a reproductive advantage, they will very quickly spread in the population. After a relatively small number of generations, *every* individual in the population will possess genes for a light-sensitive patch. At this point, there will be a selection pressure to evolve a lens. Should a gene arise in the population that produces a lens (e.g., by a fortuitous mutation), and if having a lens provides a reproductive advantage through improvements in vision, this gene will quickly spread through the population as well. Virtually every individual will therefore possess an identical genetic blueprint for vision--both the gene for the light-sensitive patch, and the gene for a lens (in real life, many genes will be needed to code for both retinas and lenses). In general, the genetic blueprints for complex adaptations must be essentially identical in every individual in a sexually reproducing species (the one exception--sex differences--will be discussed elsewhere).

    Sexual reproduction provides additional evidence for this view. Our genetic blueprint is partitioned into 23 pairs of chromosomes. Each parent contributes only one chromosome of each pair to their offspring, the other coming from their mate. If the genetic blueprints of the parents differed in any significant way, that is, if one parental genome contained instructions for building a complex adaptation but the other parent lacked such genetic instructions, it is very unlikely that their offspring, whose genome derives equally from both parents, would be viable. Imagine taking half the blueprints for a four cylinder engine with a carburetor, combining them with half the blueprints for an eight cylinder engine with fuel injection, and attempting to build an engine from the plan that results. The odds that this engine would work at all are essentially nil. The fact that individuals from one population can mate with individuals from any other population and produce offspring that are completely normal means that humans everywhere share genetic blueprints that are essentially identical.

    Notwithstanding the above, just as it is possible for different populations to possess *minor* innate physical differences, it is also *theoretically* possible for different populations to possess *minor* innate cognitive differences , although no such differences are known to exist, nor have any plausible possibilities been put forth. People living near the equator have darker skin than those living farther north. This is certainly an adaptation. It arose because protection from ultraviolet radiation is a constant selection pressure near the equator. Since skin color relies on only a few genes, the shuffling of these genes that results from sexual reproduction does not interfere with the normal development of offspring. To make the case for simple, innate cognitive differences between populations, a specific and constant selection pressure would have to be identified that applied to one population for a large number of generations but did not apply to other populations (and in fact was selected against). Further, this selection pressure would have to have plausibly resulted in cognitive differences rather than physiological differences. Finally, it would have to be shown that members of one population actually possessed a specific cognitive ability not possessed by other populations. None of these requirements have been met, so innate, minor cognitive differences between populations remains merely a theoretical possibility.

    Is evolutionary psychology sexist?

    As noted elsewhere in this FAQ, there are no fundamental differences between physiological adaptations and psychological adaptations. Male and female bodies are identical in most ways, but profoundly different in some. Male and female hearts are (I presume) essentially identical, but testicles are very different from ovaries. The same pattern is likely to be true of the brain. Male and female cognitive abilities are likely to be identical in most respects, but to differ fundamentally in certain domains, especially mating. Evolutionary theory predicts, therefore, that there will be some innate differences between males and females, that these differences very probably include cognitive differences, and, perhaps, that little can be done to erase these differences.

    If you consider these implications to be sexist, then evolutionary psychology is sexist. Nothing in evolutionary theory privileges males over females, however, nor does evolutionary theory prescribe social 'roles' for either sex. Are ovaries superior to testicles? The question is meaningless. Are male mate preferences superior to female mate preferences? The question is equally meaningless. Evolutionary psychology focuses on the properties of individuals. Because social roles are properties of particular groups at particular points in time, evolutionary psychology has little to say about them. Stated another way, evolutionary psychologists can formulate hypotheses about individual preferences, but cannot predict much regarding the social arrangements that will result when individuals with different preferences negotiate a social contract. It is also important to remember that most social roles (e.g., jobs) in the modern world draw upon a vast array of physical and cognitive abilities. Though it is conceivable that superior female physical and cognitive abilities in certain domains may (very speculatively) enhance their performance for particular aspects of a given job, whereas superior male physical and cognitive abilities may enhance their performance in other aspects of the same job, in the end, overall performance is likely to be quite similar, with the distribution of female and male abilities broadly overlapping.

    Is evolutionary psychology a form of Social Darwinism?

    No! As explained above in the section on racism, the evolutionary psychology theoretical framework strongly suggests that all individuals possess essentially identical adaptations, cognitive or otherwise. However, social hierarchies appear to be ubiquitous in both human and non-human social groups. How do they arise if all individuals possess the same capabilities? These capabilities can be degraded or enhanced by age, sex, access to social and material resources, injury, disease, birth defects, etc.--the slings and arrows of outrageous fortune. The fact that social hierarchies exist, and that evolutionary theory may help explain why, in no way justifies their existence, nor does it validate any particular ranking of individuals. Evolutionary psychology is not a moral framework! It is a framework for understanding human nature.

    Is rape an adaptation?

    No one knows, nor is there currently enough evidence to decide the question either way. A better question is whether or not a rape adaptation in humans is conceivable. Here, I think the answer is clearly yes. That rape might be an adaptation is a reasonable hypothesis to pursue, and the proper framework is intersexual conflict. Nature is rife with violent conflict--conflict between members of different species (such as predators and prey), conflict between members of the same species (such as males competing for females), and conflict between males and females (such as the killing of offspring by unrelated males during harem takeovers). Further, many organisms clearly possess adaptations to successfully engage in violent strategies (e.g., fangs and claws). There is no principled reason why animal nervous systems could not be specialized for coercive mating, including rape. In humans, the benefits of rape for males may have outweighed the costs during the EEA in the following circumstances:

    High status males may be have been able to coerce matings with little fear of reprisal.

    Low status women (e.g., orphans) may have been particularly vulnerable to being raped because males need not have feared reprisals from the woman's family.

    During war, raping enemy women may have had few negative repercussions.

    Men who were low status, who were likely to remain low status, and who had few opportunities to invest in kin may have realized reproductive benefits that outweighed the considerable costs (e.g., reprisal by the woman's family).

    Whether human males possess psychological adaptations for rape will only be answered by careful studies seeking evidence for such cognitive specializations. To not seek such evidence is like failing to search a suspect for a concealed weapon. It is extremely likely that human males, like males of many other species, have both physiological as well as psychological adaptations for successfully engaging in violent strategies. Rape may well be one such strategy. However--and this is important--adaptations provide organisms with special abilities. Rape is a behavior. It could easily result (for example) from the ability of individuals to use physical aggression to achieve any one of a number of goals, including sex; it may not require any cognitive specializations whatsoever. In order for a rape adaptation to evolve, there would have to have been cognitive problems involved in successfully raping someone in the EEA that were specific to rape, and did not generally occur in other aggressive encounters. It is not entirely obvious what these problems might have been. Perhaps identifying circumstances that were propitious for rape, as outlined above, would be one example.

    More generally, the human sciences may be forced to consider that individuals are innately capable of doing bad things.

    If my 'genes made me do it', am I still responsible?

    Yep. Sorry. If you tell the judge that your genes made you do it, she can tell you that her genes are making her throw you in jail. It is likely that humans possess a sophisticated suite of cognitive adaptations for negotiating social contracts. One aspect of social contract psychology no doubt involves evaluating the costs and benefits of violating the social contract, another involves detecting such violators (the famous cheater detection module), and yet another surely involves strategies for punishing such violators. In other words, each of us possesses the cognitive ability to break the law as well as uphold it. Not a very radical idea, actually. In fact, legal systems tend to be organized around just this principle. Laws are designed to prevent people from doing things that they might construe as being in their interest but which would impose costs on everyone else: theft, assault, neglect of important but onerous responsibilities, etc. Banks recognize that it is an enduring feature of human nature to take valuable things that belong to others, especially if they are strangers. But, just because it might be human nature to steal doesn't mean it's OK to do so. That's why banks spend a lot of money on vaults with massive steel doors, timed locks, survey cameras, and armed guards. If you're caught robbing a bank, you will pay a hefty social cost (e.g., jail time).

    Do evolutionary psychologists think that everything is an adaptation?

    Evolutionary adaptation is a special and onerous concept that should not be used unnecessarily, and an effect should not be called a function unless it is clearly produced by design and not by chance. When recognized, adaptation should be attributed to no higher a level of organization than is demanded by the evidence. George C. Williams, opening words of Adaptation and Natural Selection 1966

    No. From an adaptationist perspective, there are four types of phenomena: adaptations, byproducts of adaptations, malfunctions of adaptations, and noise (this typology can be refined, although I won't do so here). Each of the first three types of phenomena are the subject of serious research efforts in evolutionary psychology. For example, Martin Daly and Margo Wilson primarily offer byproduct hypotheses for homicide in their landmark book Homicide (A. de Gruyter, 1988), a work that has been cited over 450 times according to ISI. Daly and Wilson are highly respected researchers in evolutionary psychology. They are considered pioneering researchers in the field, and currently edit the principle evolutionary psychology journal, Evolution and Human Behavior. In Homicide, Daly and Wilson argue that many adult homicides are byproducts of male status striving. They also discover that stepparents, by almost two orders of magnitude, are the single biggest risk factor for child abuse and infanticide. They do NOT argue that infanticide is an adaptation expressed in stepparents. Rather, they argue that the high rates of abuse and infanticide are a byproduct of a lack of parental solicitude. That is, that parenting requires an enormous degree of care and attention, but that the suite of parental care adaptations may fail to fully activate in stepparents due to a lack of genetic relatedness to the offspring. The resulting injuries and deaths are byproducts of a RELATIVE degree of decreased effort, and are not the products of adaptive behavior.

    Simon Baron-Cohen is an influential researcher exploring autism from an evolutionary psychological perspective (a quick search on a psychological research journal database yielded over 100 articles written by him). Far from arguing that autism is an adaptation, Baron-Cohen argues persuasively that it results from a malfunction of a 'theory-of-mind' module, a psychological adaptation designed to allow humans to infer the mental states of others. This theory has been quite influential, and has stimulated a significant amount of research. Similar approaches are being applied to schizophrenia by Christopher and Uta Frith, among others.

    It is true that many functional hypotheses have been offered for a variety of psychological phenomena, and it is also true that most of these hypotheses are probably wrong. However, hypotheses outnumber established theories in just about any field you care to name, and evolutionary psychologists are no less discriminating than other scholars. Every field has its 'brass ring.' For particle physicists, it is a new particle. For entomologists, it is a new insect taxon. For evolutionary psychologists, the brass ring often involves identifying a new psychological adaptation. Successfully identifying a new psychological adaptation will substantially increase a researcher's reputation. That's why so many functional hypotheses have been proposed. By the same token, other researchers will not easily accept new hypotheses--to accept a shaky idea would diminish their reputation. Just as particle physicists subject claims of new particles to intense scrutiny, so do evolutionary psychologists subject claims of new adaptations to intense scrutiny (I know, I've experienced this first hand). In my experience, the critiques of functional hypotheses by evolutionary psychologists are far more incisive than those of researchers with less experience in the field. This isn't surprising, really, but may be news to critics of evolutionary psychology.

    Why do some people hate evolutionary psychology?

    In my experience, most knee-jerk criticisms of evolutionary psychology are motivated by the following (incorrect) syllogism:

    I [the critic] want political change. Political change requires changing people. Evolutionary psychologists argue that people have innate and unchangeable natures. Evolutionary psychologists are therefore opposed to social or political change, and are merely attempting to scientifically justify the status quo. More generally, all scholars, particularly 'scientific' social scientists, need to acknowledge the ideological underpinnings of their work.
    The irony, of course, is that such critics are assuming that humans are unavoidably political by nature. Further, these critics also assume that not only are people essentially political, but that they act to promote their own self interest. How sociobiological! I doubt any evolutionary psychologist would disagree!

    Biological determinism is somehow seen as antithetical to social or political change. If evolutionary psychology actually predicted that social or political change was impossible, then it would be wrong on its face. There has obviously been a tremendous amount of social and political change over the course of human history. There is no real mystery, here, of course.

    Consider a hypothetical population of organisms whose 'natures' are completely genetically specified and unchangeable and, just to keep things simple, whose natures are identical. Suppose, further, that these organisms have a number of (identical) preferences, desires, what-have-you (all unchangeable), but, because resources are limited (say), they often find that social circumstances are at odds with their preferences and not all individuals can fulfill their desires. In other words, these creatures are often in conflict with one another. Finally, suppose that these organisms have the ability to negotiate with one another by offering and withholding benefits, and perhaps by imposing costs. It is not hard to see that even if individuals' natures are unchangeable, social outcomes are not! Because our hypothetical organisms are able to negotiate, they are able to form social arrangements that are (potentially) equitable. They can come to agreements that fairly divide resources, etc., and punish individuals who violate these agreements. When circumstances change, new agreements can be forged. Because circumstances will change, social change is inevitable.

    Human nature is, of course, vastly more complex than that of our hypothetical creatures. Even if humans had identical, innate, psychological architectures, there would, for all practical purposes, still be an enormous degree of individual diversity, diversity which multiplies the possibilities for negotiating social change. Let's assume for the moment that the brain had only two mechanisms, one which could detect temperature (hot or cold), and another which could detect illumination (light or dark). The brain could then be in one of four states: 1) it's hot and light out, 2) it's hot and dark out, 3) it's cold and light out, and 4) it's cold and dark out. If the brain had only ten mechanisms, each of which could be in only one of two states (and each of which was independent of the others), the brain could be in about 1000 states; if there were only 20 such mechanisms, then the brain could be in about a million states. Because the evolutionary psychological model of the brain posits a very large number of innately specified modules or mechanisms (perhaps hundreds or thousands), and because each is presumed to be exquisitely attuned to reproductively salient environmental stimuli (including, e.g., memories), and could therefore be in far more than two states (e.g., our visual system registers far more than a mere binary light or dark), the brain could potentially be in any one of an astronomically large number of different states, even if many of these modules were not independent of one another. The evolutionary psychological model of the brain has too much diversity, not too little.

    Further, to claim that humans would be somehow constrained by innate psychological mechanisms (should they exist) is an odd way to put things. Are we 'constrained' by our visual system? Would we somehow perceive the true nature of reality if we didn't have eyes? No, we wouldn't be able to see anything! Our visual system enables us to see, it doesn't constrain us. The more psychological adaptations we have, the more capabilities we have. Hammers are good for pounding in nails, but not so good for screwing in screws. Would we say that owning a hammer constrains us from screwing in screws? No. That is nonsensical.

    But what about learning? Why do we need innate mechanisms if we can learn? The answer is that our ability to learn comes from specialized neural machinery that provides us with that capability. If we didn't have psychological adaptations specialized for learning, we wouldn't be able to learn anything (see the section on learning above).

    Regarding the political nature of human discourse, evolutionary psychologists are keenly interested in the cognitive abilities that underlie the rich political behavior of people everywhere. The considerable research on 'cheater detection modules' represents the first baby steps in this direction. Further, the 'politically incorrect' assertions of evolutionary psychologists (e.g., that youth is a component of female mate value) are based on considerable empirical evidence. Critics are welcome to challenge the evidence or provide testable alternative explanations for it. As for the unavoidable personal biases that supposedly color all research, the only real solution is to encourage students with diverse backgrounds and experiences to study evolutionary psychology. It is naive to assume that individuals can easily perceive their own biases. Far better to develop a diverse community of researchers that engage with, and critique, each other's work. Evolutionary psychologists (as well as other scholars) have an intellectual obligation to encourage and train students from as great a range of ages, classes, ethnicities, and personal backgrounds as possible.

    More thoughts on Evolutionary Psychology and political (in)correctness

    In 1632, Galileo's Dialogue concerning the Two Chief World Systems, Ptolemaic & Copernican was published in Florence. The Dialogue effectively argued that Copernican theory was the factually superior theory of cosmology. Because the major moral/political power of the day, the Catholic Church, had grounded its authority in a Ptolemaic (i.e., Aristotelian) view of the physical world, Galileo's Dialogue was obviously quite threatening, and Galileo was summoned before the Inquisition in 1633. Galileo was found to be vehemently suspect of heresy, forced to formally abjure, and was condemned to life imprisonment.

    Today, apparently, a number of thinkers have, like the Catholic Church, also grounded their moral and political views in certain scientific assumptions about the world. In this case, these are scientific assumptions about human nature (mainly that there isn't one). Consequently, any body of theory and research which calls these assumptions into question will be seen as quite threatening. The problem, of course, is not with those who claim on theoretical and empirical grounds that there is a human nature, it is with those who have succumbed to the temptation to ground their theology in assumptions about humans which are scientifically testable. This is especially unwise because the science of human behavior and psychology is extraordinarily undeveloped at this point in time. There are, in effect, no solid facts or proven theories about our behavior, thoughts, and feelings. Any set of assumptions will undoubtedly be challenged by future research. Research which calls into question assumptions underlying popular moral and political views will then unfortunately find itself attacked on extra-scientific grounds. Such research will, in effect, be viewed as heresy. The solution? Don't base your moral or political views on supposed 'facts' about human nature! Example: If you believe, as I do, that 'racial' discrimination is wrong, you might be tempted (as I have) to claim that discrimination is morally wrong because it is scientifically wrong. That is, one mustn't discriminate on the basis of race because there are, in fact, no real differences between people of different races (and besides, 'race' isn't even a valid scientific category). But, what if scientists discover that there are in fact differences between the 'races'. Would that mean that discrimination is now OK? NO!!!!!! It would be a horrible step backwards if social proscriptions against discrimination were somehow called into question simply because scientists discovered population differences. DON'T BASE IMPORTANT SOCIAL VALUES ON SCIENTIFICALLY TESTABLE ASSUMPTIONS ABOUT HUMAN NATURE (even if one of those assumptions is that there isn't any human nature). If you do, these social values will inevitably be undermined by future scientific research. Just as most 17th century theories about the physical world were wrong, most 20th century theories about human nature are assuredly wrong, and thus should not, and cannot be used as a foundation for a moral framework.

    What are your politics? (Translation: Doesn't evolutionary psychology have a crypto conservative political agenda?)

    Sigh. It shouldn't matter, but of course it does. In case anyone is interested (and I hope no one is), I'm a liberal Democrat. I support a laundry list of leftist issues like women's rights, minority rights, immigrant rights, gay rights, affirmative action, strong environmental protections, etc., etc.

    Does evolutionary psychology have any problems?

    Yes. Here are what I see as a few of the major problems currently faced by evolutionary psychology:

    1. Evolutionary psychology is attempting to elucidate the functional organization of the brain even though researchers currently cannot, with very few exceptions, directly study complex neural circuits. This is like attempting to discover the functions of the lungs, heart, etc., without being able to conduct dissections. Although psychological evidence indisputably reveals that cognition has structure, it is less clear that it does so with sufficient resolution to provide convincing evidence of functional design. Can the current state of the art in cognitive psychology successfully cleave human nature at its joints? Maybe, maybe not. Despite these reservations, it is worth noting that virtually every research university in the world has a psychology department. Grounding psychology in an explicit framework of evolved function cannot help but improve attempts to unveil the workings of the brain. It is far easier to find something if you have some idea of what it is you are looking for.

    2. The domains of cognition proposed by evolutionary psychologists are often pretty ad hoc. Traditionally, cognitive psychologists have assumed that cognitive abilities are relatively abstract: categorization, signal detection, recognition, memory, logic, inference, etc. Evolutionary psychology proposes a radically orthogonal set of 'ecologically valid' domains and reasoning abilities: predator detection, toxin avoidance, incest avoidance, mate selection, mating strategies, social exchange, and so on. These latter domains and abilities are derived largely from behavioral ecology. Although mate selection surely involves computations that are fundamentally different from predator detection, it is not so clear that the organization of the brain just happens to match the theoretical divisions of behavioral ecology. The concept of 'object' is obviously quite abstract, yet it is equally obvious that it is an essential concept for reasoning about mates, predators, kin, etc. The same goes for other 'abstract' abilities like categorization and signal detection. Ecologically valid reasoning about domains such as kinship may require cognitive abilities organized at higher levels of abstraction like 'recognition.' On the other hand, numerous experiments show that reasoning can be greatly facilitated when problems are stated in ecologically valid terms. Negating if-p-then-q statements becomes transparently easy when the content of such statements involves social exchange, for example. The theoretical integration of more abstract, informationally valid domains with less abstract, ecologically valid domains remains a central problem for evolutionary psychology.

    3. Evolutionary psychology (and adaptationism in general) has devoted considerable theoretical attention to the issue of design, the first link in the causal chain leading from phenotype structure to reproductive outcome, but has lumped every other link into the category 'reproductive problem.' This failure to theorize about successive links can lead to spectacular failures of the 'design' approach. Three examples: 1) evidence of design clearly identifies bipedalism as an adaptation, but what 'problem' it solved is not at all obvious, nor does the 'evidence of design' philosophy provide much guidance (though more detailed functional analyses of bipedalism are further constraining the set of possible solutions). 2) Language shows clear evidence of design, and there are several plausible reproductive advantages to having language, so why don't many other animals have language? 3) It can be very difficult to determine whether simple traits are adaptations simply because there is insufficient evidence of design. Menopause may be an adaptation, but it has too few 'features' to say based on evidence of design alone (some 'features' of menopause, like bone loss, seem to indicate that it is not an adaptation). Very simple traits will not always yield to a 'design analysis,' simply because there isn't enough to grab onto.

    4. Evolutionary psychology is founded on a model of ancestral human reproductive ecology (the EEA), yet the current version of this model is woefully out of date. Life history theory, the sub-discipline of biology devoted to understanding the fundamental aspects of the reproductive ecologies of plant and animals, has made enormous strides in the last decade or so. Little of this work has entered the 'mainstream' of human evolutionary psychology. Part of the problem is that the units of analysis for life history theorists (e.g., body size, mortality rates, taxonomic categories) are quite different than those used by adaptationists (e.g., strategies, design elements). Yet life history arguments are central to much work in evolutionary psychology (e.g., parental investment). Evolutionary psychologists need to get up to speed on the current state of the art in life history theory.

    Hunter-gatherer theory is a related issue. Evolutionary psychology uses an odd mix of Kalahari and tropical Amazonian ethnography for its basic model of the EEA. Although much (if not most) work by evolutionary psychologists relies on indisputable features of the EEA such as women got pregnant and men didn't, it is time for evolutionary psychology to start talking more seriously with archaeologists and paleoanthropologists. We know a lot more about the past than we did even 10 years ago, and some of what we thought we knew has now been called into question.

    5. Convergent evolution vs. phylogenetic inertia. In contrast to early approaches to the evolution of human behavior that emphasized chimp or gorilla models, evolutionary psychology relies heavily on convergent evolution type arguments. The emphasis is on functional design, with little attention paid to traits derived by descent from recent and not-so-recent ancestors. Birds are as likely to be used as models as are baboons or bonobos. Functional arguments also typically pay little attention to phylogenetic constraints. Although it is not exactly clear what kinds of constraints human ancestry might place on human cognition, it surely places some. A synthesis of primate cognitive ethology and human evolutionary psychology that takes into account both the convergent evolution of similar psychologies in response to similar ecological problems, as well as phylogenetic history, has significant potential (as most primatologists would argue, I think).

    6. Finally, even the best work in evolutionary psychology remains incomplete. Two examples: 1) evolutionary psychologists have made several predictions about mate preferences, and these predictions have been verified in a broad range of cross-cultural contexts. However, the empirical data have not been subjected to many alternative interpretations. It is possible that they can be accounted for by other theories, and it will be difficult to be fully convinced that the evolutionary interpretation is correct until it withstands challenges from competing paradigms. The record on this account, however, is quite good so far. Competing theories such as the "social role", "structural powerlessness" and "economic inequality of the sexes" hypotheses have been tested in a number of studies and have received little, if any, support. 2) The cheater detection hypothesis, on the other hand, has withstood a blizzard of competing hypotheses, but it has been confirmed in only a very limited number of cross-cultural contexts: Europe, and one Amazonian group. Adaptations must be universal, and the variation seen in even the limited cross-cultural cheater detection studies suggests that further studies are warranted.

    References and other reading

    The theoretical foundations of evolutionary psychology are identical to the theoretical foundations of adaptationism. The following is a very brief annotated list of references and recommended reading:

    Williams, George C. (1966). Adaptation and Natural Selection. Princeton University Press. This is the founding document for adaptationism in general and evolutionary psychology in particular. It identifies 'adaptation' as a principle unit of analysis, and 'evidence of design' as the best evidence for adaptation. Evolutionary psychology can be thought of as Williams applied to the brain.

    Symons, Donald (1979). The Evolution of Human Sexuality. Oxford University Press. The first in-depth exploration of an evolutionary psychological hypothesis containing all the necessary arguments: reproductive problems (e.g., male and female mating strategies in light of the relative costs of pregnancy), ancestral environments (e.g., lack of effective birth control in the EEA), and evidence for psychological mechanisms to solve the aforementioned problems (e.g., male and female mate preferences).

    Barkow J.H., Cosmides, L., & Tooby J., eds. (1992). The Adapted Mind: Evolutionary Psychology and the Generation of Culture. Oxford University Press. This edited volume contains key papers explaining how to apply adaptationist arguments to the nervous system, how to account for learning and culture within this framework, and several examples of evolutionary psychology applied to specific problems.

    Beyond this, I refer the reader to the reading list at the Center for Evolutionary Psychology.

    What do you do?

    If you are interested in what I'm working on, here are some links:

    Evolutionary theory

    Darwinian medicine: depression, addiction, and child growth and development:

    Other stuff:

    Copyright 1999-2002 Edward H. Hagen