Evolutionary psychology (EP) draws on theory from evolutionary biology in order to understand the design of the mind. EP's influence has grown a great deal over the past two decades, among both behavioral scientists and the general public. Still, many people still think of EP's core assumptions as being radical and extraordinary. In this post I want to focus on the most basic of these assumptions and point out why, from a biological perspective, they are actually not extraordinary at all.

EP sees the brain as being composed primarily of adaptations. An adaptation is a mechanism that solved some problem related to individual survival and reproduction in the ancestral past. As far as biologists know, the fundamental organizational principle of all kinds of organismal tissue—in every variety of living thing—is one of functionally-specialized adaptation. Consider, for example, the organs of your own torso. Your heart functions to pump blood, your intestines to extract nutrients from food, your liver to filter blood, and so on. Each adaptation is domain specific: it excels at its own function, but is generally much less useful for any other task. So the pancreas is good at producing hormones like insulin, but bad at gestating babies or processing oxygen; opposable thumbs are useful for grasping but useless for lactation or sight. You could make similar observations about any adaptation in any species you analyzed, from the flowers of cacti to the flagella of bacteria to the talons of eagles. Functionally specialized adaptation, in other words, is the universal evolutionary design principle.

All EP does is take what would seem to be the default position: it assumes that this universal evolutionary design principle applies to the human brain just as much as it does to any other kind of organismal tissue. Although this may seem like an extraordinary assumption to people who are unaccustomed to thinking about the brain as the product of evolution, it's an utterly ordinary assumption from the perspective of adaptationist evolutionary biology. What would be extraordinary, from this perspective, would be if this principle did not apply to the human brain. But it apparently does apply: for the past 20-25 years, EP researchers have been building a formidable body of evidence that their assumptions are correct. This evidence suggests that people have psychological adaptations that are specialized for, for example, selecting reproductive partners in various contexts, identifying and punishing cheaters in cooperative interactions, avoiding incest and achieving altruism with close kin, recognizing familiar human faces, choosing nutritive and non-toxic food, pursuing social status, avoiding disease, delivering care to one's children, and many other kinds of functions. And whereas each adaptation is excellent for solving problems in its particular domain, it tends to be much less useful in other domains. None of the adaptations from the domains described above, for example, would be particularly useful for escaping nonhuman predators, empathizing with friends, or avoiding falls from high places.

It's also important to note that contrary to a common critique, the adaptationist evolutionary biology on which EP is based does not assume that all organismal traits are adaptations. It actually assumes that many traits are non-adaptive, and is especially interested in determining which traits are by-products of adaptations (for example, male nipples probably have no function of their own but exist as by-products of selection for female nipples). A fundamental goal of adaptationist evolutionary biology—ever since the "adaptationist program" was formulated in the 1960s by biologist George Williams [1]—has always been to explicitly and clearly lay out the criteria that would allow scientists to distinguish adaptations from non-adaptive traits.

If EP's adaptationist expectations are strikingly mundane from an evolutionary biological perspective, why are they still seen as extraordinary by many, including many who would say that they accept wholeheartedly the idea that humans are the product of evolution? Why, in other words, would those who accept evolution be reluctant to also accept that the brain is organized according to the universal evolutionary design principle of functionally-specialized adaptation? Perhaps because they don't realize that when it comes to explaining function in biology, natural selection really is the only game in town. The only known force in the universe that can create biological function—that is, that can produce a goal-directed trait—is natural selection. So if you agree that the brain exists to accomplish any goals at all, then you've already committed yourself to the idea, at least by the rules of evolutionary biology, that the brain is composed of adaptations designed by natural selection. Of course, acknowledging that the brain evolved by natural selection is just a first step to understanding what the functions of mental adaptations actually are. But it's a step in the right direction.


1. Williams, G. C. (1966). Adaptation and Natural Selection. Princeton University Press.

Copyright Michael E. Price 2011. All rights reserved.

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