Understanding the Mind from an Evolutionary Perspective: an Overview of Evolutionary Psychology

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Overview Understanding the mind from an evolutionary perspective: an overview of evolutionary psychology

Todd K. Shackelford1 and James R. Liddle2∗

The theory of evolution by natural selection provides the only scientific explanation for the existence of complex adaptations. The design features of the brain, like any organ, are the result of selection pressures operating over deep time. Evolutionary psychology posits that the human brain comprises a multitude of evolved psychological mechanisms, adaptations to specific and recurrent problems of survival and reproduction faced over human evolutionary history. Although some mistakenly view evolutionary psychology as promoting genetic determinism, evolutionary psychologists appreciate and emphasize the interactions between genes and environments. This approach to psychology has led to a richer understanding of a variety of psychological phenomena, and has provided a powerful foundation for generating novel hypotheses. Critics argue that evolutionary psychologists resort to storytelling, but as with any branch of science, empirical testing is a vital component of the field, with hypotheses standing or falling with the weight of the evidence. Evolutionary psychology is uniquely suited to provide a unifying theoretical framework for the disparate subdisciplines of psychology. An evolutionary perspective has provided insights into several subdisciplines of psychology, while simultaneously demonstrating the arbitrary nature of dividing psychological science into such subdisciplines. Evolutionary psychologists have amassed a substantial empirical and theoretical literature, but as a relatively new approach to psychology, many questions remain, with several promising directions for future research. © 2014 John Wiley & Sons, Ltd.

How to cite this article: WIREs Cogn Sci 2014, 5:247–260. doi: 10.1002/wcs.1281

INTRODUCTION as intelligently designed or irreducibly complex and revealing the natural, gradual, blind process behind arwin’s theory of evolution by natural selection1 their formation. In addition to providing a natural Drevolutionized our understanding of the com- account of complex adaptations, evolutionary theory plexity of life, taking biological features once viewed illustrates that the enormous biological diversity that we see today, from hummingbirds and humpback whales to pine trees and Streptococcus pneumoniae, ∗Correspondence to: [email protected] can all be traced to a common ancestor. Although 1Department of Psychology, Oakland University, Rochester, MI, USA Darwin largely avoided the topic in his Origin of 1 2Department of Psychology, Florida Atlantic University, Davie, FL, Species, it was clear that his theory had impor- USA tant implications for understanding the origin of Conﬂict of interest: The authors have declared no conﬂicts of humans. Indeed, it is now understood that humans interest for this article. are one branch on the tree of life, that we have

Volume 5, May/June 2014 © 2014 John Wiley & Sons, Ltd. 247 Overview wires.wiley.com/cogsci existed in our anatomically modern form for roughly of natural selection, but we now know that offspring 200,000 years,2 and that our closest evolutionary rela- inherit their parents’ genes, which provide the tive is the chimpanzee, with whom we share a common ‘recipe’ for an organism’s characteristics and are ancestor dating back 6–7 million years.3 replicated with remarkably high fidelity. Nevertheless, Despite the acknowledgment (in the scientific genetic mutations occur and play a significant community4) of human evolution, the social, behav- role in generating variability, along with genetic ioral, and psychological sciences have historically recombination through sexual reproduction and ignored this reality, resulting in theoretical assump- gene duplication.3 The discovery of genes and of tions that have not been properly informed by an particulate inheritance marked a pivotal advance evolutionary perspective. This is particularly evident in scientific understanding, with its incorporation when considering what has been dubbed the Standard into evolutionary theory described as the ‘Modern Social Science Model (SSSM),5 a predominant view Synthesis’.14,15 The gene has taken center stage in of the mind as a blank slate composed of general evolutionary theory, as it is the fundamental unit of purpose, content-independent learning and reasoning selection.16 mechanisms.5–7 Evolutionary psychology represents a If natural selection operates at the level of the relatively new, and long overdue, paradigm shift in gene rather than the level of the individual, it is which the architecture of the mind is understood as possible for traits to evolve that are beneficial to the a product of specific and recurrent selection pressures genes that built them but costly to the individuals acting over deep evolutionary time. Here we provide that house them. This possibility exists because of a broad overview of evolutionary psychology, focus- the existence of genetic relatives. Since relatives ing on the core theoretical assumptions of the field, share genes by descent, if an organism performs a highlighting some significant empirical findings, and costly behavior that increases the survival chances or describing how this approach is uniquely suited to reproduction of a genetic relative, that trait may be unify the currently disparate subdisciplines of psy- selected for because the genes related to that trait chological science within a single coherent theoretical are shared by the relative. The genes are reaping the framework. rewards of an increased possibility of transmission to the next generation, and so the trait will be selected despite the costs to the organism displaying A PRIMER ON EVOLUTION the trait. More specifically, a trait that is harmful To understand and appreciate the utility of evo- to the individual who possesses it can be selected lutionary psychology, one must be familiar with if the cost is outweighed by the benefits this trait evolutionary theory. Although there are many excel- bestows on a genetic relative, multiplied by the degree lent introductions to evolutionary theory to which of relatedness. In other words, a trait that targets readers may refer Refs 3 and 8–11, we offer a brief one’s cousin would have to confer a higher fitness review of several core concepts, beginning with natural benefit to be selected compared with a trait that targets selection. one’s sibling. In short, traits evolve as a function of Evolution is simply change over time, and their impact on inclusive fitness17 (an individual’s own natural selection is a primary driving force behind reproductive success plus the effects the individual’s this change. The process of natural selection can actions have on the reproductive success of genetic be understood with reference to three components: relatives).13 variability, inheritance, and differential survival. If Finally, for evolution by natural selection these components are present, evolution by natural to occur, there must be a differential survival of selection will occur.1 Variability refers to the fact organisms within a population. If all members of a that within any population of sexually reproducing population were equally successful at surviving and organisms, individuals differ in many ways, such as reproducing, the population would grow exponen- color, size, shape, behavior, and other traits that may tially and quickly reach unsustainability.18 Given or may not be easily perceived.12 These variations limited resources available in the environment, the provide the ‘raw materials’ for evolution,13 as there result is a struggle for existence, with some organisms would be nothing for natural selection to select if thriving while others fail to survive and reproduce. organisms did not vary. To the extent that inherited variations influence However, variability can only play a role in one’s chances of success in the struggle for existence, evolution if the traits in question are inherited natural selection will take place, selecting for traits (i.e., passed on to offspring with high fidelity). that increase one’s chances and selecting against traits Darwin1 could only speculate about this component that decrease one’s chances.

In summary, natural selection is the nonrandom results from the existence of the umbilical cord, an selection of random genetic variants that impact an adaptation for obtaining nutrients throughout fetal organism’s fitness (i.e., the likelihood of passing on development.13 Another example is the whiteness of one’s genes, determined by survival and reproductive bones, which also serves no function but results from success). There is, of course, no actual ‘selector’ in high concentrations of calcium, a likely adaptation for nature pulling invisible strings. This process is blind improved bone strength.21 Noise refers to inherited and inevitable, as genes that provide survival benefits variants that do not have any impact on survival and and improve reproductive success will necessarily reproduction and, therefore, are not selected for or become more prevalent in subsequent generations, against.20 as the organisms most likely to pass on their genes Because there are several different products of are, on average, those that possess these beneficial evolution, it is helpful to rely on specific criteria to genes. When discussing this process, it is useful to determine whether a trait qualifies as an adaptation. refer to traits being selected for or against, but it is Such criteria were developed by George Williams22 important to keep in mind that there is no conscious and continue to be relied on by researchers today.13 entity overseeing this process. Natural selection has In short, for a particular trait to be considered no goals, foresight, or morality; all that matters are an adaptation, it should exhibit reliability (reliably the effects that inherited traits have on survival and developing in all or most members of a species when reproduction. in a species-typical environment), efficiency (solving the adaptive problem well), and economy (solving the adaptive problem without imposing large costs The Products of Evolution on the organism).13,22 Although these criteria are not Much like the imperceptible geological forces that exhaustive, attention to these criteria can reduce the build mountains and sink islands, natural selection possibility of mistakenly categorizing by-products or operates over vast periods of time, selecting favorable noise as adaptations. variants within a population over thousands of generations. This gradual process eventually leads to the construction of an adaptation, which can Middle-Level Theories be defined as ‘an inherited and reliably developing The theory of evolution by natural selection is simple: characteristic that came into existence through natural organisms gradually change over many generations as selection because it helped to solve a problem of a result of inherited variants impacting survival and survival or reproduction during the period of its reproduction. However, evolutionary theory becomes evolution’ (Ref 13, p. 38). These problems of survival more complex when one considers middle-level or reproduction are selection pressures, meaning that theories, which provide key insights that are consistent any inherited trait that helps solve a particular with, but not directly derivable from, general problem is likely to become more prevalent in the evolutionary theory.13,23 Several of these theories population over time via natural selection. Given have played important roles in guiding evolutionary enough time (and the appropriate genetic variants), psychological research, and we summarize these adaptations can become quite complex, as each theories next. random and slight improvement in design is selected. A good example is the human eye, which researchers Sexual Selection have estimated can evolve in fewer than 400,000 Twelve years after Origin of Species was published, generations through a series of intermediate steps Darwin discussed human evolution and provided beginning with a patch of light-sensitive cells.19 The an important addendum to natural selection in The key is that for selection to occur, each step in the Descent of Man, and Selection in Relation to Sex.24 eye’s evolution should confer some benefit over the The addendum was sexual selection, a process that previous design, which is a justifiable assumption emphasized the importance of reproductive success given the discovery of many of these ‘steps’ in the in determining the selection of inherited traits. fossil record and in extant species.8 Although the process is technically indistinguishable Adaptations are a focal point of evolutionary from natural selection, sexual selection highlights the theory, but they are not the only products of unequal fitness benefits derived from survival and evolution. Evolution may also result in by-products reproduction. Traits that increase reproductive success or noise.20 By-products are characteristics that result at a cost to the organism’s long-term survival will be from the existence of adaptations. A good example is selected over traits that increase long-term survival the belly button, which has no adaptive function but at a cost to the organism’s reproductive success. An

Volume 5, May/June 2014 © 2014 John Wiley & Sons, Ltd. 249 Overview wires.wiley.com/cogsci organism that lives for 100 years and fails to produce provided to offspring. This is particularly evident offspring will have the same fitness as an organism in placental mammals, in which females provide that dies immediately after birth, and both will have higher minimum obligatory investment via internal a much lower fitness than an organism that lives for a fertilization, gestation, and nursing, whereas the week and produces several offspring. minimum obligatory investment for males is the The conceptualization of sexual selection was contribution of sperm.29 motivated, in part, by the apparent contradiction Parental investment theory predicts that the sex between natural selection and the peacock’s train. which invests more heavily in offspring will also The large and vibrant train appears to be a be more discriminating when choosing mates, since characteristic that would be selected against because the greater costs associated with producing offspring it is metabolically costly to produce and negatively translate into greater costs associated with poor mate impacts survival, both by increasing visibility to choice (i.e., investing limited resources into genetically predators and decreasing ability to escape from inferior offspring that have a reduced likelihood of predators. However, Darwin realized that such a survival and reproductive success). The choosier sex characteristic could be selected if it had a positive thus represents a limiting factor for the reproductive impact on the reproductive success of peacocks, thus success of the other sex. Therefore, the sex that invests outweighing the costs to survival. Indeed, researchers less in offspring is predicted to be more competitive have demonstrated that peahens preferentially mate and to engage in riskier behaviors in the pursuit with peacocks based on the quality of their trains, of mating opportunities. This explains traits such as artificially removing or adding eyespots to trains as violent competition between stags and the sexual decreases and increases their reproductive success, dimorphism observed in gorillas and elephant seals,30 respectively.25,26 One possible explanation is that but the predictive power of parental investment peahens prefer more elaborate trains because they are theory is highlighted when observing species in which more costly for peacocks to produce, as this would the typical sex difference in investment is reversed. provide an honest signal of genetic quality—because In species such as the Mormon cricket,13 pipefish only the highest quality males would be capable of seahorse,13 and Australian cassowary,31 males invest producing and surviving with such a handicapping more in offspring than females and are the choosier characteristic.27 sex, whereas females typically compete for access to Sexual selection further expands upon general mates. evolutionary theory by describing two general contexts in which selection occurs. The preference for elaborate trains by peahens is an example of DEFINING EVOLUTIONARY intersexual selection, which refers to the selection of PSYCHOLOGY traits in one sex as a result of preferential mate choices In regard to understanding humans, the social, by the other sex. The second context in which sexual psychological, and behavioral sciences have long selection operates is intrasexual competition,which been dominated by theories that fail to take human refers to the selection of traits in one sex that provide evolutionary history into account.7 Psychologists a benefit in competing with same-sex rivals for access often focus on the immediate, salient reasons to mates. An example of intrasexual competition is for a given phenomenon; that is, they rely on two stags locking horns in combat.13 In competitions proximate explanations.12 Although proximate causes such as these, the victor gains increased mating access are important pieces of the puzzle of human nature, to members of the other sex. This type of sexual it is necessary to also examine the ultimate causes selection is observed among males in many species, to understand why certain psychological phenomena and to appreciate why this is the case, we must refer exist in the first place. Evolutionary psychology to another middle-level theory. represents an attempt to uncover such ultimate causes by relying on evolutionary theory as a foundation Parental Investment Theory for generating hypotheses about the fundamental 28 The theory of parental investment describes components of human nature.13 Next, we highlight the important role that investment in offspring some of the basic tenets of evolutionary psychology. plays in determining sex differences in intersexual selection and intrasexual competition. This theory is based on the observation that, among sexually Evolved Psychological Mechanisms reproducing species, there is often a difference between Evolutionary psychology is based on the premise the sexes in the minimum obligatory investment that the brain, like all other organs, consists of

250 © 2014 John Wiley & Sons, Ltd. Volume 5, May/June 2014 WIREs Cognitive Science Understanding the mind from an evolutionary perspective design features that are the result of natural selection adaptive problem that mechanism evolved to solve, operating over human evolutionary history (as well which in turn activates a particular decision rule: fear as design features that were inherited from ancestral and increased attention to the stimuli. The outcome species that existed millions of years before humans). of detecting a snake meets the final criteria, as the Evolutionary psychologists argue that the brain is evolved mechanism for fear of snakes induces us to take action to remove ourselves from the danger, not a domain-general problem-solving device. When clearly a physiological response evolved to preventing one examines any other organ, such as the heart bodily harm. (p. 180) or kidneys, it is clear that they represent evolved solutions to specific adaptive problems. Therefore, The concept of an EPM provides a powerful given the variety of functions accomplished by the framework for generating hypotheses about how the brain, and the fact that adaptations result from mind works by guiding researchers to identify the recurrent and specific problems of survival and design features that are necessary for the mind to reproduction, evolutionary psychology posits that solve the various adaptive problems faced by our our brains consist of a multitude of domain-specific ancestors. adaptations, each of which solves a unique adaptive problem. Evolutionary psychologists refer to these adaptations as evolved psychological mechanisms The Environment of Evolutionary (EPMs),13 and propose that they possess the following Adaptedness properties: Another important concept within evolutionary psychology is the environment of evolutionary adaptedness (EEA), which ‘refers to the statistical (1) An EPM exists in the form that it does composite of selection pressures that occurred during because it solved a specific problem of survival an adaptation’s period of evolution responsible or reproduction recurrently over evolutionary for producing the adaptation’ (Ref 13, p. 39). For history. every adaptation, there is a unique EEA, a unique (2) An EPM is designed to take in only a narrow set of selection pressures that played a role in the slice of information. process of selecting for inherited variants that led (3) The input of an EPM tells an organism the to the construction of the adaptation in question. particular adaptive problem it is facing. Importantly, the EEA should not be misconstrued (4) The input of an EPM is transformed through as referring to a single environment or time period. decision rules into output. In fact, the concept of an EEA serves as a reminder that one should not think of a single ‘ancestral (5) The output of an EPM can be physiological environment’ as providing an accurate summary activity, information to other psychological of the selection pressures influencing the evolution mechanisms, or manifest behaviors. of the mind. Throughout our evolutionary history, (6) The output of an EPM is directed toward the humans have experienced and adapted to a variety of solution to a specific adaptive problem (Ref 13, environments and selection pressures. By focusing on pp. 48–50). the specific selection pressures faced by our ancestors, based on defensible assumptions and informed by Liddle et al.12 illustrate how these properties can sources of data such as archeological records and 13 be applied to a hypothesized EPM for detecting and observations of hunter-gatherer societies, evolution- inducing a fear of snakes: ary psychologists develop testable hypotheses about the existence and function of specific EPMs. First, such an adaptation would clearly solve a specific problem of survival: avoiding dangerous Misconceptions of Evolutionary Psychology environmental stimuli. Second, a module for detecting One useful way to illustrate the basic tenets of evo- dangerous organisms may indeed take in only a limited lutionary psychology is to explain what evolutionary type of information—it may induce us to pay special psychology isn’t. Evolutionary psychology has been attention to serpentine forms and to motivate a fear met with a variety of criticisms, most of which response only towards a specific type of phenomena: perception of snakes. The third and fourth criteria stem from misconceptions or misunderstandings. are also met, in that the input—sensory processing Although a number of excellent rebuttals to such 12,32–38 of a snake or something snake-like—provides the criticisms already exist, we highlight a few organism with the information to produce a response misconceptions of the field to illustrate additional appropriate to that particular input based on the core ideas underlying evolutionary psychology.

Genetic Determinism storytelling to explain psychological phenomena (i.e., One common criticism of evolutionary psychology ‘just-so stories’)8 and that evolutionary psychological is that it implies genetic determinism,39–41 which hypotheses are untestable and unfalsifiable.46 First, suggests that behavior is exclusively controlled by it is simply not the case that evolutionary genes without environmental influence.13 In reality, psychological hypotheses are unfalsifiable.22 As in evolutionary psychology highlights the importance of any other scientific field, hypothesis testing is a both genes and the environment, arguing that the vital component of evolutionary psychology, with two interact in important ways. Duntley and Buss hypotheses standing or falling with the weight of argue that ‘environmental interactions are essential at the evidence. Not only do evolutionary psychologists every step of the causal chain—from the origins of empirically test hypotheses, but they do so using a adaptations, to their development during ontogeny, wide variety of methods,13 including comparisons to their expression in manifest behavior’ (Ref 42, p. between different species,47 cross-cultural studies,48,49 31). EPMs operate by processing specific types of physiological measurements,50 brain imaging,51 and environmental input. Without the necessary input, behavioral 52 and molecular genetic53 methods, along an EPM will not be activated and will not generate with experimental methods traditionally used in output. psychological science. Furthermore, evolutionary psychologists posit As for the charge of generating just-so stories, it that novel environmental inputs can have significant is the case that one strategy evolutionary psychologists effects on psychological mechanisms that evolved use to generate hypotheses is observation-driven. under different conditions. For example, humans However, researchers do not consider their job have an evolved preference for sweet and fatty complete after generating such ad hoc explanations, 43,44 foods, which would have been adaptive for our but rather empirically test their hypotheses using ancestors who often faced food scarcity. Those who the methods described earlier. Furthermore, a large had a preference for calorically dense foods would proportion of research in evolutionary psychology is have been more likely to survive and reproduce theory-driven rather than observation-driven,13 with compared to those who lacked the EPM that researchers generating hypotheses to discover new generated such a preference. However, in societies facts rather than explain previously acknowledged in which sweet and fatty foods are cheap and facts.12 For example, on the basis of inclusive plentiful, such a mechanism can result in obesity, fitness theory, Daly and Wilson54 hypothesized and coronary disease, and other medical problems. This subsequently confirmed that children living with one mismatch between adaptations and the environment genetic parent and one stepparent face a significantly exists because the novel environmental stimulus has higher risk of being abused. In fact, such children not existed for enough time to select against the are 40 times more likely to be physically abused than evolved mechanism. Another example of the mismatch children living with both genetic parents,54 making between an EEA and the current environment is the step-parenthood ‘the single most powerful risk factor fear of snakes, an adaptive response to stimuli that for child abuse that has yet been identified’ (Ref 55, posed a threat recurrently over human evolutionary pp. 87–88). Such findings powerfully illustrate the history. People reliably develop such fears, even in importance of generating and testing evolutionarily contemporary environments in which the associated informed hypotheses. adaptive problems (i.e., likelihood of receiving a poisonous bite) no longer exist.13,45 Conversely, people do not reliably develop fears of stimuli that Critiques of Massive Modularity are threatening in contemporary environments (e.g., Evolutionary psychologists argue that the human automobiles, electrical outlets) because these threats brain is massively modular, consisting of a large have not persisted long enough for the selection of number of domain-specific EPMs, or modules. The specialized adaptive solutions (e.g., fear, avoidance). nature and degree of modularity in the brain is In summary, evolutionary psychologists not only a topic of considerable debate (for an in-depth acknowledge the importance of environmental stimuli discussion of the modern evolutionary psychological in determining behavior, but illustrate specific ways view of modularity, see Ref 56), although a significant in which novel stimuli are predicted to interact with proportion of opposition to the view of massive evolved psychological mechanisms. modularity stems from the outdated and unnecessarily narrow Fodorian conception of modularity.57 Fodor’s Just-So Stories concept of modularity includes a list of likely features Another criticism of evolutionary psychology is the of modules, such as automaticity and encapsulation. assertion that researchers in the field resort to ad hoc Although Fodor himself argued against such features

252 © 2014 John Wiley & Sons, Ltd. Volume 5, May/June 2014 WIREs Cognitive Science Understanding the mind from an evolutionary perspective being necessary or defining, the modern view of organs like the heart and lungs are clear examples modularity promoted by evolutionary psychologists of adaptations related to survival, evolutionary argues that it is premature to speculate about the psychologists argue that a host of psychological design features of a module without first considering mechanisms have also evolved in response to selection the function of that module. Barrett and Kurzban pressures related to the adaptive problems of survival. emphasize that: For example, food preferences represent an evolved solution to a particular problem of survival: the ...the concept of modularity should be grounded problem of acquiring nutrients.13 In addition to in the notion of functional specialization rather than preferences for foods that are calorically dense, any specific Fodorian criteria. Biologists have long humans have aversions to certain foods. Foods that held that structure reflects function, but that function contain toxins taste bitter or sour.44 These preferences comes first. That is, determining what structure one and aversions can be found as early as infancy13 and expects to see without first considering its function have been observed in the Hadza hunter-gatherers is an approach inconsistent with modern biological 59 theory. (Ref 56, p. 629) of Tanzania. Qualities such as sweetness and bitterness are not intrinsic to the food in question, Opposition to modularity also stems from the but rather represent output produced by psychological Fodorian concept of spatial localization. Although the mechanisms designed to influence eating behavior in idea of a module may seem to imply localization in adaptive ways. There is nothing intrinsically ‘sweet’ a particular part of the brain, the modern view of about sugar molecules, but our ancestors are those modularity avoids this assumption. Modules consist with mechanisms that rewarded the consumption of a collection of neurons, and there is no reason of such calorie-rich molecules by generating the to suspect that these neurons must be restricted to pleasurable experience of ‘sweetness’.60 a particular section of brain tissue. As with other Pregnancy sickness, or morning sickness, pro- design features of modules, the degree of spatial vides a compelling example of the degree of specificity localization likely varies with respect to the function of that can be found in regard to adaptations. Pregnancy the module in question. Until technological advances sickness is hypothesized to serve the adaptive function in neuroscience, particularly brain imaging, provide of reducing exposure to toxins that are particularly us with a more detailed view of the brain in action, harmful to a developing fetus.61 In support of this the physical instantiation of hypothesized modules hypothesis, the foods that trigger pregnancy sickness will remain nebulous, and as such the modern are correlated with toxicity, pregnancy sickness view of modularity remains largely agnostic on such occurs between the second and eighth weeks of issues. pregnancy when the fetus is most vulnerable to toxins, and women who do not experience pregnancy sickness during the first trimester are three times more EMPIRICAL FINDINGS likely to spontaneously abort.13,61 Although evolutionary psychology represents a Another important adaptation related to relatively new approach to psychology, researchers problems of survival is fear. Humans appear to have in the field have amassed an impressive theoretical evolved predispositions for developing particular 13,58 fears, namely fears of stimuli that posed a threat and empirical literature. Although it is beyond 13 the scope of this article to provide an in-depth to our ancestors. The ontogenetic development summary of this literature, we next highlight of particular fears also points to functional special- representative empirical findings in evolutionary ization. Specific fears develop in relation to when psychology, organized in terms of the broad adaptive the relevant stimuli would have been most likely problems of survival and reproduction.13 encountered by our ancestors. For example, the fear of heights and strangers emerges at 6 months of age, which coincides with the age that infants begin to Survival crawl away from their mothers, thus increasing the One of the fundamental suites of adaptive problems likelihood of encountering such threats to survival.62 faced by humans throughout their evolutionary Similarly, animal fears emerge at 2 years of age, as history has been the problems of survival. Survival children begin more expansive explorations of the is a necessary precondition of reproductive success, environment.13 These results indicate that fears are not and so to the extent that increased survival benefits arbitrary,13,63 but rather correspond to stimuli that one’s chances of reproducing, adaptations for solving were recurrent threats throughout our evolutionary the problems of survival have been selected. Although history.

Mating pursue a long-term mating strategy, this preference Finding and securing a mate are crucial adaptive prob- reverses. Long-term mating for men typically consists lems, as one’s fitness is determined by reproductive of investing significant resources in offspring, and success (inclusive fitness notwithstanding). As such, so men are predicted to be particularly sensitive we can assume that there have been strong selec- to the adaptive problem of paternity uncertainty: tion pressures throughout human evolutionary history the possibility that the offspring they are raising for the construction of EPMs that increase one’s are not genetically related to them, an adaptive chances of successfully mating. Evolutionary psychol- problem absent among women. As such, men looking ogists have generated a large body of theoretical and for a long-term mate value chastity and sexual empirical work on human mating psychology.13,58,64 inexperience, as these traits signal a reduced likelihood 13 The theories of sexual selection and parental invest- of partner infidelity. In summary, by focusing on ment have proved valuable in generating hypotheses the adaptive problems unique to our male and female regarding sex differences in mating psychology. More ancestors, evolutionary psychologists have advanced specifically, the concept of intersexual selection, along our understanding of mating psychology. with assumptions of the different adaptive problems faced by men and women as a result of different levels of minimum obligatory parental investment, UNIFYING THE PSYCHOLOGICAL has guided the generation and testing of hypotheses SCIENCES regarding mate preferences. Evolutionary psychology is best viewed as an Because of a greater minimum obligatory of approach to psychology rather than a subdiscipline investment in offspring and a lower capacity for the of psychology. Evolutionary psychologists are not number of offspring that can be produced (due to limited to studying particular topics such as social periods of gestation and nursing), women are the behavior, cognitive development, or personality; an choosier sex, preferring to engage in a long-term evolutionary perspective can be applied to any area mating strategy, and exhibiting preferences for a man’s of psychological research. Evolutionary psychology ability and willingness to invest resources in a woman 13,64 is uniquely suited to provide a unifying theoretical and any offspring produced with her. Women framework for the currently disparate subdisciplines across cultures view traits such as good financial 65 of psychology. Indeed, the current division of areas prospects, high social status (which is correlated of psychological study is shown to be unnatural with access to resources),13 slightly older age (which 13 66 and arbitrary when one adopts an evolutionary is correlated with increased status), and ambition perspective. Buss illustrates this well by discussing as more desirable in a long-term mate than do men.64 13 stranger anxiety, a proposed behavioral adaptation Signals of good health are also important to women, that emerges reliably in infants around 6 months of particularly masculine features that appear to signal age and functions to reduce the likelihood of infants high levels of testosterone. Because a high level of being harmed by dangerous humans, particularly male testosterone is known to compromise the immune strangers: system in humans, the preference for such traits may be analogous to the peahen preference for elaborate In which subdiscipline of psychology does stranger trains: masculine features may provide honest signals anxiety belong? It obviously involves information 67 of a man’s genetic quality. processing and this could be claimed by cognitive Men, on the other hand, do not have their psychology. It shows a predictable ontogenetic reproductive potential restricted in the same way as unfolding, so it could be claimed by developmental do women, and therefore are more likely to pursue psychology. It is activated by interactions with others, short-term mating strategies.13 Also, men across so it belongs to social psychology. Individual infants cultures exhibit a higher sex drive than women,13 differ in the intensity of stranger anxiety, so it even in societies with high levels of gender equality.68 falls within the province of personality psychology. Furthermore, as the reproductive value (the number The mechanism can malfunction in a minority of infants, so it’s relevant to clinical psychology. And of children one is likely to have in the future) of its biological substrate must include the brain, so women declines much more sharply than that of neuroscience can also lay claim. Obviously, stranger men, men across cultures place a greater emphasis anxiety belongs simultaneously to all or to none. (Ref on youth and physical features that serve as signals 69, pp. xxiv–xxv) of youth and fertility.13,64 Finally, men who pursue a short-term mating strategy exhibit a preference for An evolutionary perspective provides a nonar- sexual experience in a mate.13 However, when men bitrary method for dividing up psychological science

254 © 2014 John Wiley & Sons, Ltd. Volume 5, May/June 2014 WIREs Cognitive Science Understanding the mind from an evolutionary perspective by emphasizing the different categories of adaptive emphasis on are more distressed by cues to a partner’s problems faced by humans.64 In addition to the sexual infidelity (an evolved solution to the problem particular problems of survival and reproduction of paternity uncertainty), whereas the same effects in highlighted earlier, evolutionary psychologists have women are triggered more by a partner’s emotional generated and tested hypotheses related to a wide infidelity (an evolved solution to the problem of variety of specific adaptive problems, such as prob- long-term diversion of investment by a mate).50 Based lems associated with parenting (e.g., the problems on these findings, Schutzwohl¨ and Koch81 predicted of paternity uncertainty, determining how much and found that men and women exhibit greater to invest in offspring, mother–offspring conflict in memory recall for cues to sexual and emotional utero,70 and parent–offspring conflict over mating71), infidelity, respectively. Findings such as these suggest kinship (e.g., the problems of kin recognition,72,73 that mechanisms often viewed as domain-general incest avoidance,72,73 and grandparental investment problem-solvers include design features built to aid in in offspring74), and group living (e.g., the problems of solving domain-specific adaptive problems. promoting cooperation and avoiding free riding75,76). Nevertheless, the existing subdisciplines of psychology are unlikely to be dismantled any time soon, even Evolutionary Social Psychology as evolutionary psychology becomes increasingly Humans are an intensely social species, a fact that incorporated into mainstream psychology,77 and so it some argue was the primary selection driving the is useful to illustrate how an evolutionary perspective rapid increase in brain size throughout hominid can be applied to some of these areas of research. evolution.3 Given the important influence of social behavior on survival and reproduction throughout human evolutionary history, one can reasonably posit Evolutionary Cognitive Psychology that the mind includes many EPMs devoted to solving Cognitive psychology has traditionally focused on adaptive problems related to social life.13 Traditional the study of the information-processing mechanisms social psychology has generated a large number of of the mind. However, cognitive psychologists have descriptions of psychological phenomena, such as the also traditionally viewed cognitive architecture as fundamental attribution error,82 self-handicapping,83 general purpose and content free.13 Evolutionary and the confirmation bias,84 but the field lacks an over- psychology presents a stark contrast to this view, arching, ultimate explanation for such phenomena.85 resulting in hypotheses that can uncover previously Several theories that inform evolutionary psy- unnoticed features of mechanisms that have been chological thinking are directly related to social behav- extensively studied by cognitive psychologists.78 For ior. For example, cooperative and altruistic behavior example, applying an evolutionary perspective to can be partially explained by referring to inclusive mechanisms underlying attention and memory can fitness theory and the benefits that such behavior highlight the evolved functions of these systems. One can confer to genetic relatives. However, humans general prediction regarding these systems is that they often cooperate with individuals to whom they are should be sensitive to the types of information that not genetically related. Evolutionary theory has have historically been related to adaptive problems.79 provided insights about this type of behavior as well, In terms of attention, an analysis of 736 front- particularly with the theories of reciprocal altruism75 page newspaper stories spanning eight countries and indirect reciprocity.86 Reciprocal altruism posits and 300 years revealed a large degree of content that altruistic behavior between genetically unrelated uniformity, with a focus on the following themes: individuals can be selected if altruistic acts are death, murder or physical assault, robbery, reputation, reciprocated in the future by the beneficiaries of such heroism or altruism, suicide, marital problems, harm acts, whereas indirect reciprocity posits that altruistic or injury to offspring, abandoned or destitute family, behavior can be selected without direct reciprocity if taking a stand or fighting back, and rape or sexual such acts positively impact the altruist’s reputation assault.80 As Buss explains, these results ‘provide and status in the community. Empirical studies suggest naturalistic evidence that human attention is specially that all three theories provide a partial explanation for targeted toward information content of maximal the variety of cooperative and altruistic acts exhibited relevance for solving adaptive problems that have by humans87 and other animals.13 These and other recurred for humans over deep time’ (Ref 13, p. 394). insights from evolutionary theory offer a framework An evolutionary perspective can also lead to within which currently disjointed social psychological unique predictions about memory performance. explanations of phenomena can be understood and For example, regarding jealousy, men place greater unified.

Evolutionary Developmental Psychology mental disorders as ‘harmful dysfunctions’ of par- Humans face different sets of adaptive problems ticular mechanisms. More specifically, a dysfunction throughout their development or ontogeny.13 This results from ‘the failure of an internal mechanism to implies the existence of ontogenetic adaptations, perform a natural function for which it was designed’ or adaptations limited to a particular period of (Ref 92, p. 374). EPMs can dysfunction in several development.88 This is the focus of evolutionary ways, such as failing to become activated when con- developmental psychology, a perspective that guides fronted by the relevant stimuli, becoming activated researchers to consider, for example, particular by the wrong stimuli, or failing to properly coordi- characteristics of childhood not as incomplete versions nate with other mechanisms.13 If the dysfunction in of adult characteristics, but as adaptations that question is shown to ‘cause significant harm to the per- solve adaptive problems related to that period of son under present environmental circumstances and development.88,89 For example, boys more than according to present cultural standards’ (Ref 92, p. girls engage in rough-and-tumble play. Although the 383), then it qualifies as a mental disorder. Focus- common view of such behavior is that it represents ing on the function of EPMs, these criteria, along an incomplete version of adult fighting, evolutionary with other insights from evolutionary psychology, can developmental psychologists have emphasized the alter our understanding of what constitutes a mental apparent functions of this behavior, such as disorder. constituting a way for boys to establish hierarchies Evolutionary clinical psychology can provide a and leadership within the peer group by assessing each fresh perspective for understanding phenomena that other’s strengths.88,90 Another example is children’s are thought to result from mental disorders. One overestimation of their own abilities. This could be example is suicide, which may seem, even from viewed as an example of immature cognition, but an evolutionary perspective, to serve no adaptive an evolutionary developmental perspective led to the function. However, some cases of suicide may proposal that there may be a functional explanation be best understood in terms of inclusive fitness. for this phenomenon. Indeed, overestimation appears Suicide may serve an adaptive function under specific to be beneficial in certain contexts and at specific circumstances, namely: when one’s reproductive ages. As Bjorklund and Pellegrini explain, ‘As a result potential is very low, and one is a burden to genetic [of overestimation], bright young children continue to relatives. From a gene’s-eye view, the possibility of experiment with new behaviors and practice old ones, being passed on to future generations may increase improving their skills at a time when trial-and-error if someone meeting these criteria were to take learning is so important’ (Ref 88, p. 203). their own life. The fitness impact of no longer These examples only scratch the surface, because being able to reproduce would be negligible if like evolutionary psychology, evolutionary develop- one’s reproductive potential is already low, and mental psychology is not a particular subdiscipline, reducing the burdensomeness to genetic relatives may but instead represents an approach that can be applied increase their fitness, resulting in a higher inclusive to any psychological phenomena.13 Indeed, as Buss fitness when compared to not committing suicide. describes, ‘Because few psychological mechanisms Consistent with this hypothesis, variables related to emerge at birth fully developed, a developmental per- reproductive success (e.g., sex in the last month, sex spective will necessarily be an essential part of the in the last year, success in heterosexual relationships, proper description and understanding of nearly every number of children) and perceived burdensomeness to psychological mechanism’ (Ref 13, p. 410). family members are correlated with the likelihood and strength of suicidal ideation.93 This analysis Evolutionary Clinical Psychology is not meant to suggest that suicide under these or any conditions is desirable, but an evolutionary Traditional clinical psychology consists of defining, perspective can highlight contexts that may trigger categorizing, and providing proximate explanations a historically adaptive suicidal response, thereby for mental ‘disorders’, along with developing inter- ventions for treatment. However, the determination facilitating the prevention of such behavior. of what constitutes a mental disorder has historically lacked a theoretical foundation, evidenced by the CONCLUSION extensive revisions that have led to the fifth and current version of the Diagnostic and Statistical Manual Evolutionary psychology emphasizes the fact that the of Mental Disorders.91 An evolutionary perspective design features of the brain, like all other organs, can provide such a foundation. For example, by view- have been built as a result of evolution by natural ing the mind as a collection of EPMs, one can interpret selection. General evolutionary theory, along with

256 © 2014 John Wiley & Sons, Ltd. Volume 5, May/June 2014 WIREs Cognitive Science Understanding the mind from an evolutionary perspective several middle-level theories, provides a powerful remain many exciting areas for future research. For framework within which specific hypotheses regarding example, evolutionary psychologists have historically the evolved psychological mechanisms of the mind can avoided theorizing about individual differences, focus- be produced. Although evolutionary psychology has ing instead on species- or sex-typical adaptations. been met with many criticisms, these largely stem This trend has begun to change over recent years, from misconceptions of the field, and the gradual particularly with the advancement of evolutionary but steady integration of evolutionary psychology personality psychology,94 but researchers are still in into mainstream psychology suggests that these the early stages of testing hypotheses that have been misconceptions are fading. This integration includes generated within this subdiscipline. Related topics the incorporation of an evolutionary perspective into that will benefit from increased empirical attention many of the traditional subdisciplines of psychology, are culture and cultural differences.13 For example, although there is still much work to be done before although the literature on the evolution of religious evolutionary psychology is accepted as the unifying beliefs and behaviors has expanded rapidly in recent framework of psychological science. A necessary years,95–98 there is still considerable debate as to component of this transition is the acknowledgment whether religion is best viewed as an adaptation, a of the currently arbitrary division of subdisciplines by-product, or a combination of the two. In addition within psychology. Evolutionary psychology provides to these topics, areas that have received considerably a nonarbitrary method of creating subdisciplines by more attention, such as mating psychology, still focusing on the categories of adaptive problems harbor many unanswered questions and debated over evolutionary history and the psychological issues. Some examples include whether female orgasm mechanisms relevant to these problems, but it remains is an adaptation or by-product,99 the evolution of to be seen whether mainstream psychology will adopt concealed ovulation13 and whether men have adapta- this system of classification. tions for detecting ovulation,100 and the evolution of Despite the large theoretical and empirical homosexuality.13 In short, the rich behavioral reper- literature that has been generated by evolutionary toire and complex cognitive architecture of humans psychologists in recent years, evolutionary psychology ensures that an evolutionary psychological perspective is still a relatively young field, and as such there still has much to tell us about human nature.

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FURTHER READING Evolutionary Psychology: an open-access peer reviewed journal. http://www.epjournal.net/ Kurzban R. Why Everyone (Else) Is a Hypocrite: Evolution and the Modular Mind. Princeton, NJ: Princeton University Press; 2010.