The Princeton Guide to Evolution

VII.12

Evolutionary Psychology

Robert C. Richardson

OUTLINE

1. The Darwinian background for evolutionary psychology

2. The modern-day program of evolutionary psychology

3. Psychological evidence

4. The application of evolutionary models in evolutionary psychology

5. Evolutionary alternatives

Evolutionary psychology is an approach to cognitive psychology that aims to inform work in psychology with evolutionary ideas and to reform cognitive science by placing it in an evolutionary context, that is, by focusing on how psychological traits such as aggression, mate selection, and social reasoning were adaptive in ancestral environments. This methodology involves a variety of psychological and behavioral evidence that is relatively independent but may be interpretable in evolutionary terms; in other cases, it involves psychological models that depend on evolutionary models. One such example is incest aversion, which can be interpreted in terms of kin selection or inclusive fitness. There are problems in integrating the two domains. More specifically, the evolutionary interpretations often lack empirical evidence. In general, it seems evolutionary psychology could benefit from a more inclusive and contemporary infusion of evolutionary theory.

GLOSSARY

Computational Mechanisms. Algorithms that compute determinate input-output functions, dependent only on the structure of representations involved; sometimes called Turing computability.

Ecological Rationality. A hypothesis characteristic of evolutionary psychology that what counts as a rational procedure is relative to the ecological context in which it is applied, and cannot be determined without knowing the context in which it is applied.

Environment Of Evolutionary Adaptedness (EEA). The environment characteristic of human evolution, both physical and social; sometimes the EEA is thought of as the environment of Pleistocene ancestors in the African savanna; sometimes it is treated as a statistical composite of ancestral environments.

Incest Taboos. General social prohibitions against sexual relations among more closely related individuals; among humans, the paradigm is the prohibition of sexual activity between individuals closer than second cousins. This is often held to be a human universal.

Modules. Cognitive mechanisms that operate in relative independence from other mechanisms that govern other domains (e.g., face recognition is a capacity in humans that is relatively independent of other capacities).

Social Exchange. Any exchange of value among individuals, with costs and benefits attached; in more interesting cases, these involve iterated exchanges in which reciprocal altruism can be effective.

Evolutionary psychology (EP) is a field of research that seeks to rely on evolutionary biology in the development and elaboration of specifically human psychological hypotheses or psychological mechanisms; more generally, EP looks to the integration of cognitive psychology with evolutionary biology in explaining and interpreting human behavior. EP developed from sociobiology, with the perspective that because humans, like all other organisms, have evolved, and the principles of evolutionary biology are universal, evolutionary theories will help us understand our own origins and features (see chapters II.18 and VII.11). Yet the application of principles of evolution to humans still engenders debate and skepticism (see chapters VIII.11–VIII.15), as it did when the idea of evolution was first introduced.

The primary range of the cognitive models in EP includes such issues as attraction to mates, patterns of jealousy, reasoning applied to social exchange, probabilistic reasoning, and incest taboos; it also includes more controversial topics such as the evolution of rape, differences between male and female aggression, and the patterns of child abuse. Psychological mechanisms are assumed to be subject to shaping by natural and sexual selection, and as a result, current behavioral patterns can be understood in terms of human evolutionary history. EP assumes that current psychological mechanisms are adaptations to ancestral environments and not to contemporary environments. Dietary preferences, for example, that may have been adaptive in ancestral environments, such as a preference for sweet food, are not adaptive in our current environment. The same disparity should apply to other psychological patterns and mechanisms.

Interpreting psychological hypotheses and mechanisms in terms of evolutionary principles is not a simple matter. In one view, evolutionary theory may be used primarily as a heuristic for defining and elaborating psychological hypotheses. Some cases seem to fit in this category, such as gender differences in the sorts of traits that are attractive in potential mates. This use seems relatively unproblematic but makes no substantive use of evolutionary theory.

In contrast, evolutionary models may be integrated into the evidence for the psychological hypotheses, supposedly contributing to their evidential credentials. For example, incest taboos are argued to have evolved by natural selection and function to avoid inbreeding. Examples of this sort are the most controversial. Here, the specific evolutionary models are often not supported by adequate evidence. When this is so, the psychological hypotheses are correspondingly uncertain, or at least no more certain than otherwise warranted by the psychological evidence. For example, human language is plausibly an evolutionary adaptation: given the complexity of the underlying structure and function of the mechanisms, the incorporation of recursive grammars, and the complex patterns in the acquisition of children’s languages, this is the sort of complex mechanism we should expect to be an adaptation. In this portrayal, human languages are adaptations for human communication, which is no doubt true given the importance of language; but we are left in the dark about the specific features of human languages, such as their recursive structure, that likely make them adaptations. Communication, for example, is adaptive, but the connection of a general appeal to communication to recursive structures is not clear. In such cases, there is a disconnection between the supposed adaptation and the adaptive model.

1. THE DARWINIAN BACKGROUND FOR EVOLUTIONARY PSYCHOLOGY

Attempts to unify evolution and psychology date to Darwin, but contrary to the modern pursuit of identifying the mechanisms of evolution such as kin selection or natural selection that shape human psychology, Darwin’s focus was more on arguing that human psychology has evolved than on exploring or suggesting how psychology evolves. There are few mentions of human evolution in On the Origin of Species, but Charles Darwin did write in the final chapter that The Origin would “open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history” (1859). Out of context, this appears to suggest that interpreting psychology in the light of natural selection would put psychology on a “new foundation.” However, this passage appears in a section that lists topics that Darwin felt would be transformed by acknowledging evolution. He did not discuss the mechanism of evolution in this section, just its existence. Thus, while he tackled head-on the most contentious area of all—what would seem to differentiate us from all other animals—human psychology, he did so without reference to the mechanism of natural selection.

Darwin developed his ideas on human evolution more fully in The Descent of Man and Selection in Relation to Sex (1871). In the opening passage of The Descent he writes:

He who wishes to decide whether man is the modified descendant of some pre-existing form, would probably first enquire whether man varies, however slightly, in bodily structure and in mental faculties; and if so, whether the variations are transmitted to his offspring in accordance with the laws which prevail with the lower animals; such as that of the transmission of characters to the same age or sex. Again, are the variations the result, as far as our ignorance permits us to judge, of the same general causes, and are they governed by the same general laws, as in the case of other organisms? (Darwin 1871, 9; italics added.)

In both The Origin and in this passage from The Descent, Darwin is discussing evolution, or common descent, and not specifically natural or sexual selection. There is appeal to variations and to inheritance, and to the “laws” governing each of them, but there is not a hint of competition or the “struggle for existence,” much less of natural selection. Here in The Descent, he initially recapitulates the argument for common descent from The Origin, extending it to what he calls the “mental faculties” of man, saying at the outset that his object “is solely to shew that there is no fundamental difference between man and the higher mammals in their mental faculties” (1871). Darwin is clear that this commitment to evolution is meant to include what he calls the “moral sense.” This was crucial for Darwin. It meant, among other things, that our capacities for social interaction and our psychological propensities were meant to be within the purview of his evolutionary theory.

Darwin was neither the first nor the last to bring evolutionary insights to the discussion of our social sentiments and reasoning. In the nineteenth century, Herbert Spencer had placed his discussion of psychology in an explicitly evolutionary setting before the publication of The Origin; William James’s psychology was inspired by Darwinian insights, as were other important psychologists at the turn of the century. In the twentieth century, there were other ventures into the evolution of human psychology, some of which are in retrospect less well regarded, such as Desmond Morris’s The Naked Ape. With the elaboration of models designed to capture social behavior in the middle of the last century, Sociobiology by E. O. Wilson dealt with the task of capturing animal behavior in evolutionary terms, and almost as an appendix extended that project to the domain of human social behavior. From Wilson’s book, the field of sociobiology was born and thrived in the 1980s, with various attempts to extend sociobiology to encompass the human case. This brings us to the most recent approach, evolutionary psychology, which takes up the Darwinian idea that evolution should shed light on human psychology, and which has usurped sociobiology.

2. THE MODERN-DAY PROGRAM OF EVOLUTIONARY PSYCHOLOGY

Contemporary evolutionary psychology is not a homogeneous collection of views, even with respect to its evolutionary commitments, though it is possible to articulate a loose set of claims that are broadly endorsed, and typical of contemporary adherents. In large part, these are commitments consistent with evolutionary theory as it was articulated during the “evolutionary synthesis” years in the first half of the twentieth century (see chapter I.2), updated by evolutionary models from the 1960s. Not every advocate of EP is committed to precisely the same set of claims, but it is possible to provide a kind of portrait. The following are some characteristic commitments:

• Psychological mechanisms are the result of natural selection and sexual selection. While it is generally acknowledged that evolution has some outcomes that are owing to chance or are by-products of selection for other traits, the focus of EP is on traits presumed to be adaptations and therefore that reflect evolution from selection—traits centered on problems such as finding a mate, cooperative activities like hunting, or the raising of offspring. The assumption is that natural selection will tend to “solve” problems like this with considerable efficiency. Possible alternatives to selection are rarely considered, nor are alternative selectionist regimes.

• Psychological mechanisms can be thought of as computational mechanisms. Among such mechanisms are included cognitive processes (e.g., probabilistic reasoning or problem solving) as well as emotional responses (e.g., jealousy or fear). The idea that psychological mechanisms are computational is a common assumption among a range of cognitive scientists, though its prevalence has faded considerably in the last decade or so. Alternatively, these computational mechanisms can be thought of as exhibited in and causing behavioral strategies for responding to environmental challenges, where the strategies are genetically specified.

• Psychological mechanisms evolved in response to relatively stable features of ancestral environments, often collectively referred to as the environment of evolutionary adaptedness (EEA). EP asserts that because most of human evolution took part during the Pleistocene (roughly 2.6 million to 12,000 years ago), and presumably in the later Pleistocene, what is seen today in terms of psychology evolved to be adaptive in this hypothetical EEA. Often, the EEA is identified with the savanna of the African Pleistocene, with a hunter-gatherer lifestyle. The EEA can also be identified with a kind of statistical aggregate of the total range of ancestral environments.

• Because psychological mechanisms are adaptations to ancestral environments, there is no assumption that they are adaptive in contemporary circumstances. Social environments are a significant part of the environment and are obviously crucial to human evolution. If we assume with EP that our ancestral social environment consisted of small, nomadic bands of relatives, then the difference between that and our contemporary culture suggests that whatever strategies were adaptive for our ancestors, may not be so for us. Likewise, if we assume that our distant ancestors lived in a sugar-deprived environment, then our fondness for sweets might be “natural” though no longer adaptive. In general, EP assumes that evolutionary responses are too slow to have had any significant effect in the last 12,000 years or so, the earlier advent of agriculture and sedentary life.

• The human mind is a kind of mosaic of mechanisms, each with some specific adaptive function, rather than merely a general-purpose learning machine. Different adaptive problems will require different solutions and different strategies for dealing with them. So, for example, a mechanism for mate selection is unlikely to be of much use in foraging. At least some of this machinery must be domain specific, specialized for particular tasks, and some of these mechanisms may count intuitively as instincts. Several advocates of EP treat these mechanisms as modules, though others insist that all that is required is distinct domain-specific mechanisms.

3. PSYCHOLOGICAL EVIDENCE

Evolutionary psychologists make use of an array of techniques to evaluate their psychological models, most of which do not specifically depend on the evolutionary assumptions. These methods include, among others, the use of questionnaires, controlled experiments, observational methods, and brain imaging (functional magnetic resonance imaging [fMRI] and positron emission tomography [PET]). Some also make use of a variety of less standard techniques, including ethnographic records, paleontological information, and life history data. Evolutionary assumptions do come into play in advancing and formulating hypotheses, as suggestive of psychological hypotheses to test. Whether they are more than merely heuristic is sometimes not clear.

Evolutionary psychologists have articulated and tested a wide array of psychological hypotheses inspired by evolutionary thinking. These include human propensities for such matters as cooperation and cheater detection, differences in spatial memory, and short-term mating preferences. Some simple examples may be sufficient to illustrate the method. Assume that human memory is sensitive to items that affected fitness among our ancestors, such as food items, shelter, or possible mates. Using standard experimental memory probes within psychology that are concerned with recall and recognition for lists of words, researchers found that recall for survival-oriented terms was significantly better than recall for more neutral words. Similarly, theories of parental investment suggest that given monogamous coupling, females will tend to prefer mates that are more likely to invest in offspring. From an EP perspective, this also suggests that males and females will differ in the patterns of jealousy, with females on average more sensitive to emotional infidelity (as a risk of abandonment) and males more sensitive to actual sexual infidelity (as a risk to paternity). These predictions have been supported by straightforward evaluations of preferences using questionnaires, spontaneous recall, and fMRI.

4. THE APPLICATION OF EVOLUTIONARY MODELS IN EVOLUTIONARY PSYCHOLOGY

Relying on work in paleoanthropology and ethnography relating especially to contemporary hunter-gatherers, evolutionary psychologists have elaborated a portrait of ancestral social life. While their description is plausible, it is also controversial among anthropologists. EP assumes ancestral hominids lived in relatively compact kin-based groups of no more than 100 members. It is presumed that a sexual division of labor existed, with males more engaged in hunting and females more engaged with gathering, and that stable male-female bonds existed, as well as long periods of biparental care. Within each kin group, there was cooperative foraging. Much more is known about the biotic and abiotic environment that existed. For example, it is known that during this time, humans were subject to a variety of predators and pathogens and considerable variance in the availability of resources.

Assuming this broad portrait of early human social life and abiotic influences allows evolutionary psychologists to construct a variety of evolutionary scenarios. Depending on the case, they use a variety of resources from evolutionary biology, including theoretical models concerning reciprocal altruism (see chapter VII.9), parental investment (see chapter VII.8), kin selection (see chapters VII.10 and VII.12), and evolutionary game theory (see chapter VII.3). Beginning with the relevant dimensions assumed to be typical in the EEA, evolutionary psychologists construct an account of the adaptive functions that must be satisfied, rather like a design specification. The problem for EP is then to reverse engineer a solution to the adaptive problem and test it in modern populations.

Reverse engineering is a powerful theoretical tool, but it can lead to difficulties and criticism, especially if the “adaptive problem” is not clear. If it is poorly articulated, then it is not clear whether the evolutionary solution is the right one. To use a nonhuman example, if the ecological “problem” is how insects can walk on water, knowing the adaptive “solution” depends on the surface tension of water, and that in turn depends on knowing the saline content; absent the determinate content of the problem, there is no general solution to crucial issues such as foot structure, though the specific solutions are solved readily. In the human case, language is certainly involved in communication, but this fact offers no explanation for the peculiarities of human language—its recursive structure, for example—which are plausibly adaptations.

EP also raises issues about connecting the psychological hypotheses and the evolutionary interpretations. Consider a Darwinian theory of the evolution of incest avoidance. Incest is a very interesting case of a social prohibition, since psychological studies show that disapproval of it survives even the recognition that it will produce no actual harm. It has been a very significant issue, first for sociobiologists and now for evolutionary psychologists. There is a straightforward case against incest from an evolutionary perspective based on inbreeding. Inbreeding can result in reduced fitness, termed inbreeding depression (see chapter IV.6). Where inbreeding depression exists, there should be an evolutionary pressure against inbreeding.

The importance of inbreeding depression leads EP advocates to suggest that there is a natural tendency—sometimes a psychological “module”—for incest avoidance. Debra Lieberman, together with Cosmides and Tooby, has suggested that humans have a specialized kin recognition system (there are such mechanisms in other animals). They observe that incest avoidance could facilitate an avoidance of any deleterious consequences associated with inbreeding depression and suggest that this leads to selection for incest avoidance.

The Westermark hypothesis posits a mechanism of the sort Lieberman, Cosmides, and Tooby predicted, suggesting that children raised together develop a sexual disinterest, or even a sexual aversion, to one another. The proposed function is to avoid incest, since those who are raised together are most often closely related. Lieberman assumes, reasonably, that coresidence during periods of high parental investment should be a reliable indicator of kinship or would have been a reliable indicator in the EEA with small kinship bands. Together with Cosmides and Tooby, Lieberman shows considerable support for the conclusion that duration of coresidence is psychologically predictive of sexual aversion.

The evolutionary interpretation that incest avoidance evolved because of selection against inbreeding is plausible on the surface but nonetheless problematic. The association cannot be directly tested in ancestral populations, but it does fit the patterns of some contemporary “hunter-gatherer” populations (which may be considered a proxy for ancestral populations), though not all. It is, of course, true that siblings would typically be associated with one another during childhood, but the proper question is whether in ancestral groups the set of people that an individual may have selected from when choosing a mate included the siblings with which one interacted as a child.

A more straightforward and general problem with EP is assuming a single EEA. We know that our Pleistocene ancestors did not have simply one lifestyle in one region but lived on the African savanna, in deserts, next to rivers, by oceans, in forests, and even in the Arctic, employing very different foraging methods and living off diverse diets, with technologies ranging from the simple chopping tools of Homo habilis to the rich and sophisticated stone, bone, and antler toolbox of late Pleistocene Homo sapiens. There is little reason to think that there was a single form of social structure associated with the full range of human physical environments, much less that contemporary “hunter-gatherer” populations are typical of ancestral groups. For the hypothesis of incest avoidance as a mechanism to avoid inbreeding depression, it is hard to know whether association will be limited to siblings, or more likely to be with siblings, absent a fairly specific account of social organization, including the relative viscosity of the groups and issues such as group size.

In many animal species, there is a tendency for animals to disperse prior to mating; they move away from their familial unit. This clearly has the effect of reducing inbreeding, though there doesn’t seem to be any consensus on whether incest avoidance is particularly significant in supporting dispersal. Among chimpanzees, when males come of reproductive age, they tend to emigrate from the ancestral clan. There is no need for incest aversion, since they move away from their siblings. To know how to apply inbreeding avoidance models to ancestral human groups, it would be necessary to know whether both males and females remained with the ancestral groups or emigrated. There is some evidence that among early Homo, the males tended to move out of their ancestral groups once they were reproductive, as with chimpanzees, but it doesn’t matter whether this is correct. The important point is that absent such information, the relevance of the evolutionary models of selection to incest aversion is not clear. If reproductives tend to emigrate, then there is little need for incest aversion, and none for incest taboos.

5. EVOLUTIONARY ALTERNATIVES

The preceding general assumptions that form the backdrop for EP are characteristic of only a selected subsample of work in evolutionary biology. More generally, evolutionary biology incorporates a wide variety of disciplinary perspectives that do not feature in the work within EP. Many of the assumptions characteristic of EP may be problematic because they do not incorporate more recent advances in our understanding of how evolution works. There are many recent developments in genetics, in evolutionary biology, and in developmental biology that might improve EP considerably, bringing it more in line with more recent evolutionary thinking. Several are briefly noted here.

• Natural selection and sexual selection are doubtless potent evolutionary forces. There are alternative evolutionary factors that can, and do, affect evolutionary trajectories. Evolutionary psychologists acknowledge such factors as genetic drift (though it plays no role in their scenarios, and they do not address it as an alternative) but do not incorporate phylogeny or comparative biology. As a result, our primate kin do not typically feature in EP explanations. One salutary change would be to take account systematically of our relatedness to our primate kin and the possibility that features exhibited by humans are inherited from a common ancestor. At the very least, this would provide expectations as to social patterns that feature so prominently in EP, and give testable alternative hypotheses. In particular, it would downplay the commitment to natural selection acting on specifically human social capacities.

• EP typically assumes that the relevant selection forces are relatively ancient and that recent changes would be insignificant. From the perspective of EP, modern humans are Pleistocene relics. Yet we know that there have been substantial changes in the human genome over even the last 10,000 years and that these changes are ongoing (see chapter VIII.12). Many of the evolutionary changes reflect the adoption of agriculture and the domestication of animals, environmental changes that surely impose selection.

• The environment of the Pleistocene is known to have been highly spatiotemporally variable. The environment of the early Pleistocene was very different from, say, the late Pliocene. Moreover, humans came to be widely dispersed, occupying a variety of distinctive environments. Given what we know, it would be reasonable as well to think that social structures would be different in different physical environments—for example, some would be more conducive to sedentary lifestyles, and others to more mobile ones. Though humans are not as genetically diverse as many other animals, there is sufficient genetic variation to support genetic changes in relatively short amounts of time.

• Human behavior is both adaptive and malleable. EP tends to assume, by contrast, that evolved computational programs are species specific and species universal. When there is within-species variation, the assumption is that the strategies are conditional, evoked in different conditions. The validity of this is questionable. Evolutionary biologists have found that the rate of evolution can be much faster than EP tends to assume (see chapters III.7 and III.8). Advances in our understanding of epigenetics and developmental plasticity provide alternatives to conditionality that are typically not incorporated in EP. It is not that EP assumes some form of genetic determinism; rather, the point is that the kind of interplay seen among genetic factors, epigenetic influences, and learning makes universals less likely.

• There are significant alternatives to the typical emphasis of EP on individual- and gene-centered models of evolution. This is an issue beyond the problems of applying EP’s preferred modes of analysis. Gene-culture coevolution may be an important source of evolutionary changes (see chapter VIII.10). This is becoming a well-developed alternative, emphasizing the role of cultural practices in modifying the human brain. In general, gene-culture dynamics can enhance and accelerate rates of evolution, even if we do not yet know how important these are. Multilevel selection models are also being developed. With distinctive, genetically isolated groups that compete as groups, it is possible to develop models for the evolution of social behavior that do not assume the typically individual- and gene-oriented perspective of EP.

There are alternatives that could enrich the work within EP but that typically remain beyond its purview. Darwin was right to think that evolution should reshape our understanding of human psychology. There are many avenues yet to explore in seeing how an evolutionary perspective can contribute to our understanding of human psychology. Most of these avenues are ahead of us.