Evolutionary Theory and Psychology
In commemoration of the 200th anniversary of Charles Darwin’s birth and the 150th anniversary of the publication of his seminal work On the Origin of Species, this edition of Psychological Science Agenda includes a special section on evolutionary theory and psychology. Scientists and philosophers were invited to submit personal reflections on the significance and influence of Darwin’s theory and of current views of evolution within contemporary psychology. PSA thanks the authors for their provocative contributions.
Darwin’s Influence on Modern Psychological Science
By David M. Buss
Evolutionary Psychology and the Evolution of Psychology
By Daniel Kruger
Darwinizing the Social Sciences
By Robert Kurzban
Darwinian Psychology: Where the Present Meets the Past
By Debra Lieberman and Martie Haselton
Psychology’s Best Discovery Heuristic
By Edouard Machery
Survival of the Fittest?
By Gary Marcus
An Open Letter to Comparative Psychologists
By Daniel J. Povinelli, Derek C. Penn, and Keith J. Holyoak
Evolution of Human Sex Differences
By Wendy Wood and Alice H. Eagly
David M. Buss is Professor of Psychology at the University of Texas at Austin.
At the end of his classic treatise in 1859, On the Origin of Species, Darwin envisioned that in the distant future, the field of psychology would be based on a new foundation—that of evolutionary theory. A century and a half later, it’s clear that his vision proved prescient (Buss, 2009).
Evolutionary psychology is not a distinct branch of psychology, but rather a theoretical lens that is currently informing all branches of psychology. It is based on a series of logically consistent and well-confirmed premises: (1) that evolutionary processes have sculpted not merely the body, but also the brain, the psychological mechanisms it houses, and the behavior it produces; (2) many of those mechanisms are best conceptualized as psychological adaptations designed to solve problems that historically contributed to survival and reproduction, broadly conceived; (3) psychological adaptations, along with byproducts of those adaptations, are activated in modern environments that differ in some important ways from ancestral environments; (4) critically, the notion that psychological mechanisms have adaptive functions is a necessary, not an optional, ingredient for a comprehensive psychological science.
Darwin provided two key theories that guide much of modern psychological research—natural selection and sexual selection. These theories have great heuristic value, guiding psychologists to classes of adaptive problems linked with survival (e.g., threats from other species such as snakes and spiders; threats from other humans) and reproduction (e.g., mate selection, sexual rivalry, adaptations to ovulation). Advances in modern evolutionary theory heralded by inclusive fitness theory and the “gene’s-eye” perspective guide researchers to phenomena Darwin could not have envisioned, such as inherent and predictable forms of within-family conflict and sexual conflict between males and females.
Over the past decade, evolutionary psychology has increasingly informed each sub-discipline within psychology. In perception and sensation, it has led to the discovery of phenomena such as the auditory looming bias and the visual descent illusion. In cognitive psychology, based on a fusion of signal detection theory and the asymmetric evolutionary costs of cognitive errors, it has led to error management theory and the discovery of functional cognitive biases that are, strange as it may seem, “designed” to err in adaptive ways. Evolutionary social psychology has produced a wealth of discoveries, ranging from adaptations for altruism to the dark sides of social conflict. Evolutionary developmental psychology has explored the ways in which critical ontogentic events, such as father absence versus father presence, influence the subsequent development of sexual strategies.
Evolutionary clinical psychology provides a non-arbitrary definition of psychological disorder--when an evolved mechanism fails to function as it was designed to function. It also sheds light on common afflictions such as depression, anxiety disorders, eating disorders, and sexual disorders. And it provides a framework for examining how mismatches between ancestral and modern environments can create psychological disorders. Personality psychology, historically refractory to evolutionary analysis, is finally beginning to discover adaptive individual differences.
Hybrid disciplines too make use of the tools of evolutionary psychology. Cognitive and social neuroscientists, for example, use modern technologies such as fMRI to test hypotheses about social exclusion adaptations, emotions such as sexual jealousy, and kin recognition mechanisms.
More generally, evolutionary psychology breaks down the barriers between the traditional sub-disciplines of psychology. A proper description of psychological adaptations must include identifying perceptual input, cognitive processing, and developmental emergence. Many mechanisms evolved to solve social adaptive problems, such as when social anxiety functions to motivate behavior that prevents an individual from losing status within a group. And all adaptations can malfunction, as when social anxiety becomes paralyzing rather than functional, making clinical psychology relevant. The key point is that organizing psychology around adaptive problems and evolved psychological solutions, rather than around the somewhat arbitrary sub-fields such as cognitive, social, and developmental, dissolves historically restrictive branch boundaries. Evolutionary psychology provides a metatheory for psychological science that unites these fields, and justifies why the seemingly disparate branches of psychology truly belong within the covers of introductory psychology books and within the same departments of psychology.
Buss, D. M. (2009). The great struggles of life: Darwin and the emergence of evolutionary psychology. American Psychologist, 64, 140-148.
Daniel Kruger is Research Assistant Professor at the Prevention Research Center of Michigan, in the School of Public Health at the University of Michigan.
The framework of evolutionary theory will be increasingly adopted as the foundation for a cumulative understanding of psychological science. As the unifying theory of the life sciences, evolution by natural and sexual selection offers an unparalleled ability to integrate currently disparate research areas (Wilson, 1998), creating a powerful framework for understanding the complex patterns of causality in psychological and behavioral phenomena. The evolutionary perspective will grow from its perceived status as a special interest area into an organizing principle that pervades every corner of every field, as well as serve as a bridge across levels of analysis.
The incorporation of evolutionary theory into psychology has waxed and waned in the 150 years since Darwin (1859) predicted that the field would be based on a new foundation. There are many notable examples of psychological theories with evolutionary bases, such as Bowlby’s (1969) model of attachment, yet these are often isolated examples. In the last three decades, the evolutionary perspective has been reinvigorated with considerable theoretical advances and a continually growing array of empirical studies.
Claims for such dramatic advancements on currently held beliefs likely evoke skepticism. The massive empirical evidence accumulating for the influence of evolutionary selection pressures on psychological mechanisms will convince objective observers. It is important to note that evolutionary explanations will not necessarily replace the current models for specific psychological and behavioral phenomena, but rather integrate the “how” with the “why.” It may help to recognize that evolutionary psychology is not monolithic; there are multiple levels of theory from basic principles to specific phenomena and multiple competing explanatory theories. Disagreements occur even between those considered the founders of the modern field. For example, some believe there are psychological adaptations facilitating homicide for strategic ends (Buss, 2005) whereas others believe that homicide is the product of adaptations for sub-lethal motivations such as competition combined with lethal modern technology (Daly & Wilson, 1988).
It may also help to distinguish modern evolutionary psychology from the selective breeding programs in previous eras of human history. There is no teleology in evolution; no person or people are more highly evolved than any other persons or peoples. Everyone alive today is descended from a long, long line of successful ancestors. Yet there may be individual and group differences in psychological domains that are partially a result of differential selection pressures on ancestral populations. Humans have colonized nearly every land area on the surface of the earth, and each of these diverse ecologies could shape our psychological design. Efforts to advance human welfare may benefit from this recognition, as well as the understanding that genes are not the script for a pre-ordained destiny. Everything about us as individuals is a product of complex interactions between our genetic instructions and aspects of the environments in which they are expressed.
By providing the broader context in which research results may be interpreted, researchers across fields will facilitate the integration of a larger body of scientific knowledge. The evolution of psychology will facilitate its recognition and integration as science.
Bowlby, J. (1969). Attachment and loss, Volume. 1. New York: Basic Books.
Buss, D. M. (2005). The Murderer Next Door: Why the Mind is Designed to Kill. New York: Penguin Press.
Daly, M., & Wilson, M. (1988). Homicide. New York: Alidine de Gruyter.
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection. London: John Murray.
Wilson, E.O. (1998). Consilience: The Unity of Knowledge. New York: Alfred A. Knopf.
Robert Kurzban is Associate Professor in the Department of Psychology at the University of Pennsylvania.
There is only one known cause of the complex functional organization of matter that characterizes the biological world: evolution by natural selection. Because scientists do not typically ignore the central causal process that gives rise to the object of their study, it makes sense that 150 years after The Origin of Species, scientific research investigating the physiology or behavior of any of the roughly 1.5 million species on Earth requires graduate training in biology, in particular instruction in the theory of evolution by natural selection, a fact which stands as a tribute to Darwin’s legacy. Scholarship on each of the world’s species is motivated by hypotheses of evolved function, which in turn guides research on proximate mechanisms.
This is true for every single species on the planet with one exception: Homo sapiens.
Students who wish to study humans – psychologists, political scientists, economists, sociologists and other social scientists – are not required to take a single course in biology, and, with few exceptions, they do not. This puts much of the social sciences in the position of trying to explain human psychology without the tools of Darwinism, a circumstance akin to trying do chemistry while keeping studiously ignorant of the causal foundations of the discipline: atoms, molecules, the periodic table, and the basic forces that govern matter.
This leads to embarrassing mistakes. Biologists would never think that an explanation for complex behaviors such as dam-building in beavers would either begin or end with reference to constructs such as “protecting self-esteem,” “salience,” or “maximizing utility,” yet “explanations” of precisely this sort are pervasive in the social sciences. Biologists understand that explanations owe a Darwinian debt: evolved mechanisms have biological functions, and these must, eventually, be explicated. However, not only are explanations that begin with a theory of evolved function still rare in the social sciences, but such explanations, perversely, frequently attract scorn, ridicule, and blind incomprehension.
This situation can be remedied. I offer two suggestions. First, graduate training in all of the social sciences should require at least one class in evolutionary biology. Students entering the field should be armed with the tools that have been so productive in explaining and predicting the behavior of every other species. Psychology can lead the way in this by acting quickly. Second, editors should begin requiring that papers include an explicit hypothesis about the evolved function of the mechanisms investigated in the manuscript. Form follows function in biology, and hypotheses about the form psychological mechanisms take – how they work – should always be informed by hypotheses about function – what they’re designed to do.
The continued stubborn resistance of psychologists to learn the ideas that integrate their discipline with the natural sciences will be viewed unfavorably by posterity. Change has been glacial, leaving psychology to be condemned, as Max Planck is frequently quoted as having said of science, “to advance one funeral at a time.” So, one way or another, the social sciences will, eventually, be Darwinized.
Debra Lieberman is Assistant Professor in the Department of Psychology at the University of Miami.
Martie Haselton is Associate Professor in the Departments of Communication Studies and Psychology at the University of California, Los Angeles.
200 years after Charles Darwin’s birth and 150 years after the publication of “On the Origin of Species”, the field of psychology is traveling back to its roots as a life science, integrating the same principles biologists use to understand non-human life forms to understand human behavior and cognition. Darwin’s theories of natural and sexual selection identified the primary forces that shape both physiological structures and psychological mechanisms alike. Combined with the recent theoretical advances offered by genic selection and inclusive fitness theory, Darwin’s principles have proved to be invaluable tools for mapping the structure of the modern human mind and linking it with our long evolutionary history. For instance, we now know that the threats our ancestors faced left their legacy in the particular fears and phobias humans are most likely to acquire – fears of fanged creatures like spiders and snakes, but not of modern-day threats like the guns and fast moving cars that are far more likely to kill us today. Research applying Darwinian principles has also shown that kinship is a privileged social relationship, governed by specialized psychological mechanisms that infer relatedness based on ancestrally available cues that reliably distinguished kin from non-kin and between different types of kin. We have also discovered that human females, like our mammalian cousins, have an estrus phase of the cycle in which their sexual preferences and behaviors shift in reproductively sensible ways. These, and the many other discoveries enabled by the application of evolutionary tools, would not have been made without Darwin’s grand theory.
Looking forward, the application of evolutionary principles continues to permeate different subdisciplines within psychology including clinical science, cognitive psychology and neuroscience. More and more, Darwin’s influence can be seen in research programs investigating, for instance, whether particular clinical “disorders” are in fact psychological adaptations, the domain specificity of memory and attentional processes, and the specialized circuits involved in processing particular emotions. Despite widespread application, obstacles remain. Darwin’s theory is beautiful yet deceptively simple. It is often misapplied – for example, by assuming that adaptations work for the benefit of the group or species or by side-stepping rigorous consideration of the historical selective pressures leading to the evolution of a particular capacity. This will only continue if psychologists do not receive serious training in evolutionary biology. The study of the human mind must be grounded in biology, the study of life. Of course, there are those who oppose the full integration of biological theorizing into psychology, but this is based on concerns that, at least to us, are largely outdated. It is our hope that new generations of psychologists and social scientists will be fluent in Darwinian principles and modern evolutionary biology and that just as Darwin predicted, “psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Much light will be thrown on the origin of man and his history” (Darwin, 1859, pp 428).
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection. London: John Murray.
Edouard Machery is Associate Professor in the Department of History and Philosophy of Science at the University of Pittsburgh.
Psychologists have often relied on unreliable technological metaphors to develop hypotheses about the nature of the human mind. Freud’s psychoanalytic hypotheses were inspired by the then prevalent hydraulic metaphor, which compared desires, emotions, and urges to fluids, while cognitive scientists have more recently been looking for psychological counterparts of the processes and systems making up digital computers (think of, e.g., working memory as the counterpart of a computer’s CPU and of attention as the counterpart of the allocation of computing power to different softwares). As Gigerenzer (1991) has shown, statistical tools such as, e.g., linear regression or bayesianism, have also often been turned into hypotheses about the nature of human psychological processes.
In contrast to these unreliable and often unprincipled discovery heuristics, evolutionary theory provides psychology with a well-motivated and powerful method for discovering human psychological traits. Nobody seriously denies that the mind is made of evolved traits, and, in combination with discoveries about animal behavior and psychology, archaeological findings, and anthropological data from hunter-gatherer studies, evolutionary theories can lead psychologists to develop plausible hypotheses about the nature of these evolved traits. In fact, because these are likely to be our best discovery heuristics, evolutionary theories should guide psychologists’ efforts to understand mind and behavior.
But there’s a catch: Taking evolutionary theory seriously has costs. Although psychologists rarely have the time and competence to engage with the burning controversies within evolutionary biology, they should keep up with the developments of evolutionary thinking instead of relying on somewhat outdated theories. Furthermore, showing that some psychological trait evolved and, a fortiori, that it is an adaptation is more difficult than is typically acknowledged by evolutionary-minded psychologists. These should be willing to broaden the toolbox they currently use, and to make place for the sources of evidence biologists view favorably.
Gigerenzer, G. (1991). From tools to theories: A heuristic of discovery in cognitive psychology. Psychological Review, 98, 254-267
Gary Marcus is Professor of Psychology at New York University.
Few phrases in science are as powerful – or as widely misunderstood – as the words “survival of the fittest.”
The problem with the phrase (actually coined by Darwin’s contemporary Herbert Spencer) is that it is perfectly ambiguous. On the one hand, “survival of the fittest” could mean “of all the possible creatures that one might imagine, only the fittest possible creatures survive”; on the other, it could mean something considerably less lavish: not that the fittest possible creatures survive, but only that creatures that survive tend to be the fittest that happen to be around at any given moment.
This seemingly subtle difference – between “fittest among the choices that happen to be laying around” – and “fittest imaginable” – makes all the difference in the world. “Better than one’s neighbor” is a far cry from “best possible.”
Discussions of evolutionary psychology sometimes seem to be premised on the first. Human beings do such and such because such and such was the optimal (“fittest”) thing for our stone-age ancestors to do. Men like women with smooth skin because (prior to the advent of plastic surgery), smooth skin was a reliable predictor of fertility, so it was in the interest of our ancestors’ “selfish genes” to create brains with a preference for smooth skin.
While talk of function certainly has its place, examples like the injury-prone human spinal column (an unwise modification of the more sensible horizontal spine of our four-legged ancestors) suggest that the usual considerations of optimal function should be supplemented with consideration of what one might call evolutionary inertia. Just as objects in motion tend to stay in motion (Newton’s second law), evolution tends to modify what is already in place, rather than starting from scratch.
Consider human memory, which is far less reliable than computer memory. Whereas it takes the average human child weeks or even months or years to memorize something as simple as a multiplication table, any modern computer can memorize any table in an instant – and will never forget it. Why can’t we do the same?
Whereas computers organize everything they store according to physical (or logical) locations, with each bit stored in a specific place according to some sort of master map, we have no idea where anything in our brains is stored. We retrieve information not by knowing where it is, but by using cues or clues that hint at what we are looking for.
In the best case, this process works well: the particular memory we need just “pops” into our minds, automatically and effortlessly. The catch however is that our memories can easily get confused, especially when a given set of cues points to more than one memory. What we are able to remember at any given moment also depends heavily on the accidents of which bits of mental flotsam and jetsam happen to be mentally active at that instant. Our mood, our environment, even our posture can all influence our delicate memories.
Our memories may work in this fashion not because that is the optimal solution, but simply because, at the time of human evolution, cue-dependent memory was a firmly entrenched off-the-shelf part: cue-driven memory and all its idiosyncrasies has been found in just about every creature ever studied, from worms to flies, from spiders to rats, from monkeys to humans.
The structure of human memory might thus exist as it does not because it is the ideal solution (fittest possible) but simply because it was the fittest solution that was readily available (Marcus, 2008).
Marcus, G. (2008). Kluge: The haphazard evolution of the human mind. Boston, MA: Houghton-Mifflin.
Daniel Povinelli is James S. McDonnell Centennial Fellow and Professor of Biology at the University of Louisiana.
Derek Penn is Affiliate Scientist at the University of Louisiana.
Keith Holyoak is Distinguished Professor of Psychology at the University of California, Los Angeles.
Darwin believed that earthworms have a sense of consciousness and that plants can hear bassoons. He claimed that “higher” animals have an incipient capacity for empathy, logic, language, magnanimity, an appreciation of beauty and a nascent belief in God. And he believed that dogs have a “sense of humor as distinct from mere play” and “possess something very like a conscience.” (Darwin, 1871).
Darwin’s anthropomorphic view of animals was as unfounded and unnecessary as his theory of pangenesis: Nothing about Darwin’s theory of evolution requires—or even suggests—that there be a seamless psychological continuity among living species. And yet, over the last quarter century, many comparative psychologists have stubbornly championed Darwin’s quaint idea that there are no “fundamental differences” between the mental capacities of humans and animals and have made anthropomorphic claims about nonhuman cognition as unsubstantiated as Darwin’s.
Even those comparative researchers who acknowledge that there might be something qualitatively different about the human mind have largely attributed the discontinuity to particular domain-specific faculties—such as language or social-communicative intelligence—and have denied that there might be a more profound, domain-general discontinuity between human and nonhuman minds.
The evidence clearly suggests otherwise: Only humans make fire, fashion wheels, draw maps, diagnose each other’s illnesses, risk their lives for ideals, punish strangers for breaking the rules, explain the world in terms of unseen causes, plan for hypothetical scenarios, take others’ welfare into account and teach each other how to do all the above. The evolution of all these uniquely human abilities begs for explanation.
It is possible that each of our uniquely human kinds of cognition results from a separate, domain-specific innovation. And it is possible that they all somehow arise from language. But it seems much more likely to us that some central cognitive capability co-evolved with and continues to subserve all our uniquely human abilities. According to our hypothesis (Penn, Holyoak, and Povinelli, 2008), this central cognitive capability was the ability to reason about higher-order relations and the core innovation that gave rise to the human mind was our brain’s ability to approximate the relational capabilities of a physical symbol system.
We are not sure that our hypothesis is correct; but we are sure of this: It is time for comparative psychologists to move beyond a faith-based belief in the “mental continuity” between all species and to invest as much effort in identifying the differences between human and nonhuman minds as they have invested in identifying the similarities. Only then will comparative psychology be able to take its rightful place at the roundtable of cognitive science.
Darwin, C. (1871). The descent of man, and selection in relation to sex. London, John Murray.
Penn, D. C., Holyoak, K. J., & Povinelli, D. J. (2008). Darwin's mistake: Explaining the discontinuity between human and nonhuman minds. Behavioral and Brain Sciences, 31(2): 109-178.
Wendy Wood is Professor of Psychology and Neuroscience and Professor of Marketing at Duke University.
Alice Eagly is Professor of Psychology at Northwestern University.
Charles Darwin, while offering a brilliant analysis of species development and change, struggled to understand human distinctions of race, class, and gender. In Darwin’s analysis, these distinctions arose from sexual selection processes. For example, he explained the presumably superior beauty of the aristocracy as due to upper class men successfully competing for and choosing the most attractive women from all social classes. Like Darwin, contemporary evolutionary psychologists explain men’s universally greater size and strength along with their tendencies toward psychological aggressiveness and competitiveness as due to sexual selection mechanisms of male competition and female choice.
Modern evolutionary thinking has progressed beyond such a simple analysis, in part because evidence from comparative studies of primates questions whether human sex differences originated in sexual selection. The human male-female size difference is low in magnitude compared with other primate species, and species with low dimorphism have a large variety of behavioral patterns and social structures (Plavcan & van Schaik, 1997). In addition, both female and male size are products of multiple selection pressures. Such findings call for more complex evolutionary accounts of humans’ physical and psychological sex differences.
Our evolutionary analysis of sex differences takes into account humans’ considerable behavioral flexibility in response to local circumstances. This characteristic feature of humans reflects their evolution in diverse environments with changeable conditions that impinged in differing ways on survival and reproductive outcomes (Wood & Eagly, 2002, in press). For example, in the late Pleistocene era, climate appears to have been highly variable. Also, humans and their ancestors engaged in extensive niche construction, meaning that their activities altered the environments in which they lived. Accommodating to such changes required behavioral flexibility, enabled by an evolved capacity for innovating and sharing information through social learning, yielding a cumulation of culture. Humans’ flexibility is evident in their various novel solutions to the problems of reproduction and survival, including tolerance for a wide range of foods, ecologies, and living arrangements.
Given these selection pressures on human ancestors, sex differences in behavior arise flexibly from a biosocial interaction in which sociocultural and ecological forces interact with humans’ biology as defined by female and male physical attributes and reproductive activities (Wood & Eagly, 2002). Specifically, women bear and nurse children, and men possess greater size, speed, and upper-body strength. Given these attributes, a division of labor arises such that certain activities are more efficiently accomplished in certain societies by one sex than the other. For example, women are limited in their ability to perform certain tasks incompatible with childcare (e.g., requiring speed, uninterrupted activity). Therefore, women in foraging, horticultural, and agricultural societies generally eschew tasks such as hunting large animals, plowing, and conducting warfare. Nonetheless, under certain social conditions that lessen these constraints, women have taken on roles of warriors and hunters. Recently, the division of labor and gender hierarchy have become less pronounced, reflecting the declining importance of physical sex differences due to lowered birthrates and decreased importance of size and strength for high status roles. The resulting political and social changes give women access to a greater range of social roles and have altered female psychology.
Plavcan, J. M., & van Schaik, C. P. (1997). Interpreting hominid behavior on the basis of sexual dimorphism. Journal of Human Evolution, 32, 345–374.
Wood, W., & Eagly, A. H. (2002). A cross-cultural analysis of the behavior of women and men: Implications for the origin of sex differences. Psychological Bulletin, 128, 699-727.
Wood, W., & Eagly, A. H. (in press). Gender. In S. T. Fiske, D. T. Gilbert, & G. Lindzey (Eds.), Handbook of social psychology (5th ed.). New York: McGraw Hill.