Evolutionary Psychology Controversies, Questions, Prospects, and Limitations

Evolutionary Psychology Controversies, Questions, Prospects, and Limitations Jaime C. Confer, Judith A. Easton, Diana S. Fleischman, Cari D. Goetz, David M. G. Lewis, Carin Perilloux, and David M. Buss University of Texas at Austin Evolutionary psychology has emerged over the past 15 Theoretical Background years as a major theoretical perspective, generating an increasing volume of empirical studies and assuming a The fundamental basis of evolutionary psychology dates larger presence within psychological science. At the same back to Darwin’s (1859) theory of natural selection. Dar- time, it has generated critiques and remains controversial win postulated that if variant traits could be inherited by among some psychologists. Some of the controversy stems offspring from parents, then those variants that aided an from hypotheses that go against traditional psychological organism’s survival and reproduction would be transmitted theories; some from empirical findings that may have dis- to future generations at greater frequencies than alterna- turbing implications; some from misunderstandings about tives. Variants with less beneficial effects—such as those the logic of evolutionary psychology; and some from rea- that hinder an organism’s ability to survive or reproduce— sonable scientific concerns about its underlying frame- would not replicate because the organisms possessing them work. This article identifies some of the most common would transmit them at lower rates. This causal process results in three products: (a) adaptations,1 inherited char- concerns and attempts to elucidate evolutionary psycholo- acteristics that reliably solved problems related to survival gy’s stance pertaining to them. These include issues of and reproduction better than competing alternatives during testability and falsifiability; the domain specificity versus the time period in which they evolved (example: fear of domain generality of psychological mechanisms; the role dangerous snakes); (b) by-products, artifacts without func- of novel environments as they interact with evolved psy- tional value that persist because they are inherently coupled chological circuits; the role of genes in the conceptual with adaptations (example: fear of harmless snakes); and structure of evolutionary psychology; the roles of learning, (c) noise, variations in a given characteristic that are due to socialization, and culture in evolutionary psychology; and random environmental events or genetic mutations (exam- the practical value of applied evolutionary psychology. The ple: random low base-rate fears, such as fear of sunlight, article concludes with a discussion of the limitations of that occur because of stochastic genetic or developmental current evolutionary psychology. factors) (Tooby & Cosmides, 1992). Although these principles historically have been ap- Keywords: evolutionary psychology, misconceptions, ad- plied to anatomy and physiology, there is now widespread aptation, psychological mechanisms, natural selection recognition that they also provide powerful tools for ex- plaining the origins of psychological, strategic, and behav- T he goal of evolutionary psychology is to study ioral adaptations in nonhuman animals as well as in hu- human behavior as the product of evolved psycho- mans (Alcock, 2005; Buss, 2005). Just as physiological logical mechanisms that depend on internal and adaptations solve specific problems associated with sur- environmental input for their development, activation, vival and reproduction (e.g., the immune system has and expression in manifest behavior. Although empirical evolved as a defense against disease), psychological adap- research within evolutionary psychology has grown tations too have evolved because they solved problems related to survival and reproduction (e.g., preferences for steadily over the past 15 years (Cornwell, Palmer, Guinther, & Davis, 2005), some within mainstream psy- chology remain unfamiliar with its theoretical frame- Jaime C. Confer, Judith A. Easton, Diana S. Fleischman, Cari D. Goetz, work and application, and misconceptions about it are David M. G. Lewis, Carin Perilloux, and David M. Buss, Department of common (e.g., Cornwell et al., 2005; Park, 2007). The Psychology, University of Texas at Austin. We thank Laith Al-Shawaf, Matthew Brooks, Meg Cason, Aaron T. purpose of this article is to clarify some of the tenets of Goetz, Owen Jones, William Swann, and Yla Tausczik for their insightful evolutionary psychology and to dispel several common suggestions on drafts of this article. misconceptions about it. By doing so, we hope to pro- Correspondence concerning this article should be addressed to David M. Buss, 1 University Station, Department of Psychology, University of vide some conceptual tools to facilitate a greater inte- Texas, Austin, TX 78712. E-mail: [email protected] gration of evolutionary psychology with mainstream 1 Key terms defined in the Appendix are printed in boldface the first psychological theory and research. time they appear. 110 February–March 2010 ● American Psychologist © 2010 American Psychological Association 0003-066X/10/$12.00 Vol. 65, No. 2, 110 –126 DOI: 10.1037/a0018413  not be used as solutions to the adaptive problems of mate selection (e.g., avoiding those who inflict costs) or habitat selection (e.g., choosing a site containing resources and refuge). Consequently, evolutionary psychologists suggest that the human mind is a complex integrated assembly of many functionally specialized psychological adaptations that evolved as solutions to numerous and qualitatively distinct adaptive problems—a premise about adaptations shared widely by evolutionary biologists in understanding nonhuman animals (Alcock, 2005). Psychological adapta- tions often interact with each other to produce adaptive behavior, as when a person is faced simultaneously with the adaptive problems of hunger and a threatening lion; fear of the lion will temporarily suspend hunger pangs until the threat of imminent death has passed (Buss, 2008). The list of adaptive problems extends far beyond avoiding snakes, selecting nutritious food, and favoring fertile mates. It includes problems of parental investment (e.g., socializing sons and daughters), kinship (e.g., chan- neling altruism toward close rather than distant genetic Jaime C. relatives), friendship (e.g., detecting cheaters), coalitional Confer cooperation (e.g., punishing free riders), selective aggres- sion (e.g., diffusing a violent confrontation), negotiating status hierarchies (e.g., besting close rivals; dealing with subordinate status), and many others (Buss, 2008). Psycho- statistically reliable cues of fertility in potential mates). logical adaptations are not separate “modules” in the Psychological adaptations are information-processing cir- Fodorian (Fodor, 1983) sense of informational encapsula- cuits that take in delimited units of information and trans- tion; rather, they often share components and interact with form that information into functional output designed to each other to produce adaptive behavior (see Barrett & solve a particular adaptive problem. Kurzban, 2006, for a cogent discussion of this issue). Evolved fear adaptations provide relatively uncontro- Evolutionary psychologists are not monolithic in their versial examples that are well supported empirically theoretical stances. There exist legitimate scientific differ- (Mineka & Öhman, 2002; Öhman & Mineka, 2003). ences on key issues such as the importance and meaning of Snakes and spiders, for example, signal potentially danger- individual differences (Buss, 2009; Nettle, 2006; Tooby & ous threats to survival. A programmatic series of studies Cosmides, 1990), the existence and functions of more has shown that an intense fear of snakes exists in humans domain-general psychological mechanisms such as fluid and other primates; snakes and spiders embedded in com- intelligence (Geary, 2009), and the importance of group plex visual arrays automatically capture attention far more selection (Williams, 1966; D. S. Wilson & Wilson, 2007). than do harmless objects—they “pop out” of the visual Nonetheless, evolutionary psychologists all share the view array; humans rapidly condition to fear snakes more than that understanding the evolved functions of psychological most other stimuli; and the snake fear adaptation is selec- adaptations—the problems they were “designed” by a prior tively and automatically activated, it is difficult to extin- history of selection to solve (no forward-looking intent guish, and it can be traced to specialized neural circuitry implied)—is an indispensable, not an optional, ingredient (Mineka & Öhman, 2002; see Rakison & Derringer, 2008, for a mature psychological science. Understanding the on the evolution of specialized spider detection adapta- evolved function of a psychological mechanism, or why it tions). Evolutionarily ancient dangers such as snakes, spi- exists (often referred to as an ultimate explanation) pro- ders, heights, and strangers consistently appear on lists of vides a complementary level of analysis to that of under- common fears and phobias far more often than do evolu- standing the details of how the mechanism works (often tionarily modern dangers such as cars and guns, even referred to as a proximate explanation). Both types of though cars and guns are more dangerous to survival in the explanation are required for a complete understanding, and modern environment. The functional outputs of evolved indeed they mutually inform one another. Knowledge of fear adaptations, such as freezing, fighting, and fleeing, are the human liver, for example, would be incomplete without specifically designed to solve the adaptive problems posed understanding its evolved functions (e.g., to neutralize tox- by these evolutionarily recurrent threats to survival ins) as well as the specific processes by which those func- (Bracha, 2004). tions are achieved (e.g., the lobules that produce bile and Adaptive problems are many in number. Solutions to emulsify lipids). Similarly, knowledge of psychological one problem often fail to successfully solve other prob- adaptations such as stranger anxiety in infants would be lems. Solutions to the adaptive problems of food selection incomplete without understanding their functions (to avoid (e.g., avoiding substances containing toxins) generally can- potentially dangerous humans) as well as the specific pro- February–March 2010 ● American Psychologist 111  ered or documented (the phrase design features is standard shorthand in evolutionary biology for attributes or compo- nent parts of an adaptation that have been forged or “de- signed” by a past history of natural selection; no forward- looking or intentional process is implied by this phrase; see, e.g., Hauser, 1999). A recent example comes from a research program on “adaptive memory.” Nairne and his colleagues hypothe- sized that evolved memory systems should be at least somewhat domain specific, sensitive to certain kinds of content or information (Nairne & Pandeirada, 2008; Nairne, Pandeirada, Gregory, & Van Arsdall, 2009; Nairne, Pandeirada, & Thompson, 2008). They hypothesized that human memory should be especially sensitive to content relevant to evolutionary fitness, such as survival (e.g., food, predators, and shelter) and reproduction (e.g., mating). Using a standard memory paradigm involving a scenario priming task and a surprise recall task, they found that words previously rated for survival relevance in scenarios were subsequently remembered at significantly higher rates Judith A. than words rated for relevance in a variety of control Easton scenario conditions. Furthermore, Nairne and his col- leagues conducted additional experiments that pitted sur- vival processing against well-documented powerful encod- ing techniques, such as ease of generating a visual image, cesses by which they operate—a predictable developmental ease of generating an autobiographical memory, and inten- emergence at 6 to 8 months, a special sensitivity to strange tional learning in which subjects were instructed to remem- males more than strange females, emotional systems acti- ber the words for a later test. Interestingly, rating the item’s vated internally, external behavioral signaling to caregivers relevance in the survival scenario produced better recall through crying, the activation of specific neural circuitry, performance than did any of the other well-known mem- and so on. Knowledge of ultimate functions is invaluable in ory-enhancing techniques. The researchers concluded that guiding the search for the proximate causes, just as under- “survival processing is one of the best encoding procedures standing proximate implementation informs the search for yet identified in human memory research” (Nairne & Pan- ultimate function. deirada, 2008, p. 242). Had the results failed to confirm the Evolutionary psychology uses all of the standard predictions about specialized sensitivity to survival-rele- methods of investigation available to psychologists to test vant content, Nairne’s adaptive memory hypothesis would hypotheses, including laboratory experiments, observa- have been falsified. tional techniques, questionnaires, physiological techniques, A second example of a collection of hypotheses that mechanical recording devices, genetic methods, and brain have been tested and subjected to the risk of falsification imaging techniques. In addition, evolutionary psycholo- derives from error management theory (Haselton & Buss, gists sometimes use methods not typically used by psychol- 2000, 2003, in press; Haselton & Nettle, 2006). In an ogists, such as comparative analyses across species, ethno- uncertain world in which observable cues are only proba- graphic records, archeological records, paleontological bilistically related to the true state of reality, there are two data, and life-history data (Schmitt & Pilcher, 2004). possible ways to err. One way to err is to infer that a state Common Queries About exists when it does not, as when a rustle in the leaves does Evolutionary Psychology not signal a snake but rather was caused by a gust of wind. The other way to err is to infer that a state does not exist We now turn to some common queries about evolutionary when it actually does, as in a failure to infer that a snake psychology and endeavor to provide answers to them. exists on the basis of a rustling of leaves when a snake truly does exist. According to error management theory, the 1. Can evolutionary psychological costs of making these two types of errors (false positive vs. hypotheses be empirically tested or falsified? false negative) under conditions of uncertainty were often The logic of hypothesis testing in evolutionary psychology recurrently asymmetric in the currencies of survival and is the same as hypothesis testing in all sciences (see Kete- reproduction. The cost of failing to detect a snake in the laar & Ellis, 2000, for an extended discussion of falsifi- leaves, in this example, could result in death; the cost of ability). The researcher first formulates a hypothesis about erroneously inferring a snake that does not exist results in an evolved psychological mechanism and then generates a trivial expenditure of energy in avoiding something harm- testable predictions about the attributes or design features less. If these and other asymmetric costs of errors recur of that mechanism that have not previously been discov- over generations, selection is hypothesized to favor the 112 February–March 2010 ● American Psychologist  tion is that women will prefer men as potential mates who express a willingness to invest in them and their offspring (Buss, 1995). This is derived from the hypothesis that in paternally investing species, females will use cues to a man’s willingness to invest as a criterion for mate selec- tion. In turn, this hypothesis is derived from parental investment theory, which posits that the sex that invests more in its offspring will be the choosier sex when select- ing mates (Trivers, 1972). Finally, the logic behind parental investment theory is derived from inclusive fitness theory (Hamilton, 1964a, 1964b), the modern formulation of evo- lution by natural selection. As in all realms of psycholog- ical science, the evaluation of each evolutionary psycho- logical hypothesis, as well as the broader theories within which they are embedded, rests with the cumulative weight of the empirical evidence. Although evolutionary psychology is a relatively young field, hundreds of empirical studies have been con- ducted to test a variety of evolutionary psychological hy- potheses. Some have been confirmed; others have been Diana S. falsified; and others have not yet been subjected to enough Fleischman empirical tests to render a firm scientific conclusion. Among those that have been confirmed by multiple meth- ods in multiple samples by multiple investigators are anti- free-rider adaptations in cooperative groups (e.g., Price, evolution of cognitive biases that are designed to err in the Cosmides, & Tooby, 2002); cheater-detection adaptations least costly direction. in social exchange (e.g., Cosmides & Tooby, 2005); female Error management theory logic has been used to test superiority in spatial location memory as part of a gathering hypotheses about adaptive biases in domains ranging from adaptation (e.g., Silverman & Choi, 2005); functional at- visual and acoustic perception to inferences about sexual tributes of male and female short-term mating strategies and homicidal motivation in other people. In the domain of (e.g., Greiling & Buss, 2000; Schmitt, 2005; Schmitt & visual perception, for example, the descent illusion hypoth- International Sexuality Description Project, 2003); sex- esis (Jackson & Cormack, 2007) was used to predict that differentiated deception tactics in human mating (e.g., Ha- people would make asymmetric distance estimations when selton, Buss, Oubaid, & Angleitner, 2005; Tooke & Ca- judging from the top versus the bottom of a tall structure, mire, 1991); antipredator adaptations in children (e.g., owing to the dangers associated with falling from heights. Barrett, 2005); and dozens more (see Buss, 2008, for a Tests of the descent illusion hypothesis show that people recent review). perceive distances viewed from the top of tall structures to Among those that either have been falsified or have be 32% greater than precisely the same objective distances failed thus far in empirical tests are the kin altruism theory when viewed from the bottom, confirming the prediction of male homosexuality (Bobrow & Bailey, 2001); the hy- (Jackson & Cormack, 2007). On the basis of error man- pothesis that males have an evolved preference for virginity agement theory logic, Neuhoff (2001) confirmed empiri- in selecting long-term mates (Buss, 1989); and at least one cally that humans have an auditory looming bias—they version of the “competitively disadvantaged male” hypoth- overestimate the closeness of approaching sounds com- esis about rape (Lalumière, Chalmers, Quinsey, & Seto, pared with receding sounds, because of the historically 1996). The kin altruism hypothesis of male homosexuality, greater dangers associated with approaching rather than for example, contends that homosexuality is an adaptation receding objects and predators. In the social realm, Hasel- that involves a shift among those whose heterosexual mat- ton and Buss (2000) confirmed the existence of a commit- ing prospects are not promising from direct mating effort to ment skepticism bias in women, such that they underinfer investing in kin, such as the children of one’s brothers and levels of romantic commitment (compared with men) that sisters (E. O. Wilson, 1978). In a direct test of this hypoth- are based on cues such as declarations of love. The impor- esis, Bobrow and Bailey (2001) used samples of hetero- tant point is that had any of the empirical tests failed, they sexual and homosexual men matched for age, education, would have falsified the descent illusion hypothesis, the and ethnicity. They assayed generosity toward family auditory looming hypothesis, or the commitment skepti- members; financial and emotional investment; avuncular cism hypothesis. tendencies, such as willingness to give gifts or cash to The science of confirming and falsifying hypotheses, support nieces or nephews; and general feelings of close- of course, is typically more complex than these examples ness toward genetic relatives. The results proved conclu- indicate. Often a hypothesis is embedded within a larger sive—they found no evidence for any of the key predic- theoretical network. For example, one evolutionary predic- tions made by the kin altruism hypothesis of male February–March 2010 ● American Psychologist 113  merous domain-specific adaptations. Although these claims may have intuitive appeal, we suggest that they are of dubious validity as cogent scientific explanations. The emotion of romantic jealousy provides a concrete example. Although dozens of studies had been conducted on romantic jealousy, it was not until evolutionary psychologists hypothesized sex differences in evolved design features that such differences were discovered (Buss, Larsen, & Westen, 1992; Daly, Wilson, & Weghorst, 1982; Symons, 1979). Because paternity uncertainty has been a recurrent adaptive problem for men, evolutionary psychologists hypothesized that men’s jealousy would be especially triggered by cues to sexual infidelity, whereas women’s jealousy would center on emotional infidelity (i.e., cues that signal the long-term diver- sion of commitment and resources, such as a mate falling in love with another woman). Although both sexes experience jealousy over a part- ner’s sexual and emotional infidelity, abundant empirical evidence from studies employing more than half a dozen different experimental methods supports the existence of sex differences in the relative weighting given to cues to Cari D. Goetz infidelity (for recent reviews, see Buss, 2008; Buss & Haselton, 2005; and Sagarin, 2005). Independent replica- tion is the hallmark of the scientific method, however, and a meta-analysis of 72 studies found a strong effect size homosexuality. Subsequent empirical tests have also failed (d ⫽ 0.64) in support of the hypothesized sex differences to support the kin altruism hypothesis, finding no signifi- (Hofhansl, Voracek, & Vitouch, 2004), although an occa- cant differences between homosexuals and heterosexuals in sional study will fail to find full support for them (e.g., generosity toward kin, general feelings of familiality, or Penke & Asendorpf, 2008). The fact that the sex differ- willingness to invest in nieces or nephews (Rahman & ences have been documented using highly diverse empiri- Hull, 2005). On the basis of the current empirical evidence, cal methods, ranging from spontaneous memorial recall of we can conclude that the kin altruism theory of male cues of sexual versus emotional infidelity (Schützwohl & homosexuality has been refuted. Koch, 2004) to sex-differentiated patterns of neural activa- The key point is that precisely formulated evolution- tion using fMRI technology (Takahashi et al., 2006) sug- ary psychological hypotheses that yield specific predictions gests that the burden of proof must now shift to those who about design features that are not known to exist prior to doubt the existence of evolved sex differences in the emo- empirical testing are fully amenable to empirical confirma- tion of jealousy (Buss & Haselton, 2005). tion and falsification. Evolutionary psychological hypoth- After these previously unknown sex differences were eses, like those from other theoretical perspectives within discovered as a consequence of evolutionary psychological psychology, vary in their quality, their precision, and the hypotheses, others have offered post hoc explanations for degree to which they are anchored in well-established their existence that invoke domain-general rationality. theoretical foundations. Sloppy and imprecise evolutionary C. R. Harris and Christenfeld (1996), for example, sug- hypotheses that fail to generate precise predictions deserve gested that the sex differences are due to “rationality com- scientific criticism. The sometimes reflexive charge that bined with the general desire people have to keep what is evolutionary psychological hypotheses as a rule are mere theirs” (p. 378). They concluded by proposing that “the “just-so stories,” however, is simply erroneous, as the domain-general mechanism of rational thought” (p. 379) examples above demonstrate. can explain the findings. Although we do not deny that humans are capable of rational thought, there are several 2. Don’t people just solve problems using problems with “the domain-general mechanism of rational rationality? Wouldn’t one domain-general thought” as an alternative explanation for most hypotheses rationality mechanism be more that invoke specialized evolved psychological mecha- parsimonious than postulating many nisms: failure to predict, combinatorial explosion, poverty domain-specific mechanisms? of the stimulus, and context-specificity. We address these Some psychologists react to hypotheses about evolved psy- problems in turn. chological mechanisms by invoking concepts such as do- The failure-to-predict problem. Domain- main-general rationality as alternative explanations, sug- general rational thought, as it has been used as an expla- gesting that “people just figure it out.” Some propose that nation, is typically invoked post hoc to account for new it is more parsimonious to postulate one or a few domain- empirical findings discovered as a consequence of novel general mechanisms such as “rationality” rather than nu- evolutionary psychological hypotheses. Domain-general 114 February–March 2010 ● American Psychologist  quence of this rational deliberation? The predictability and rapidity of men’s jealousy in response to cues of threats to paternity points to a specialized psychological circuit rather than a response caused by deliberative domain-general ratio- nal thought. Dedicated psychological adaptations, because they are activated in response to cues to their corresponding adaptive problems, operate more efficiently and effectively for many adaptive problems. A domain-general mechanism “must evaluate all alternatives it can define. Permutations being what they are, alternatives increase exponentially as the problem complexity increases” (Cosmides & Tooby, 1994, p. 94). Consequently, combinatorial explosion paralyzes a truly domain-general mechanism (Frankenhuis & Ploeger, 2007). The poverty of the stimulus problem. The sexual infidelity of a man’s mate has been statistically associated with increased paternity uncertainty over deep evolutionary time. It is highly improbable, however, that men could learn this statistical regularity during develop- ment. To do so, men would have to observe a large sample David M. G. of instances of sexual infidelity (which tend to be cloaked Lewis in secrecy) and then associate them with potentially detect- able cues to lack of paternity that would be displayed nine months, or even several years, later (e.g., lack of pheno- typic resemblance to the putative father). In contrast, se- rational thought has not, to our knowledge, been used to lection can favor specialized adaptations that exploit sta- predict or discover new empirical findings. Consequently, tistical regularities that are not detectable ontogenetically. it appears to lack both heuristic and predictive power. The context-specific rationality problem. Hypotheses about motivational priorities are required to From an evolutionary perspective, there exists no domain- explain empirically discovered phenomena, yet they are not general criterion of “rationality.” It is often reproductively contained within domain-general rationality theories. A rational for a mateless man on a path toward reproductive mechanism of domain-general rationality, in the case of oblivion to take life-shortening risks to change course—to jealousy, cannot explain why it should be “rational” for engage in steep future discounting (M. Wilson & Daly, men to care about cues to paternity certainty or for women 2004). In contrast, such risk taking would be reproductively to care about emotional cues to resource diversion. Even irrational from the perspective of a man who has a wife and assuming that men “rationally” figured out that other men three young children to protect. What would have been having sex with their mates would lead to paternity uncer- rational from the perspective of an attractive high-status tainty, why should men care about cuckoldry to begin man in our ancestral past (e.g., securing multiple mates with? In order to explain sex differences in motivational through polygyny, affairs, or short-term mating) would be concerns, the “rationality” mechanism must be coupled evolutionarily irrational from the perspective of an attrac- with auxiliary hypotheses that specify the origins of the sex tive high-status woman, who would be better off securing differences in motivational priorities. Because it requires a long-term mate of high genetic quality who is both the postulation of additional post hoc explanatory clauses willing and able to invest in her and her children (Buss & for each new empirical phenomenon, “rationality” itself Shackelford, 2008). Because what is “rational” differs acquires the “lack of parsimony” problem sometimes erro- neously leveled against specific evolutionary hypotheses. across adaptive problems, sexes, ages, and life circum- The problem of combinatorial explosion. stances, there exists no single domain-general criterion of Domain-general theories of rationality imply a deliberate cal- rationality. culation of ends and a sample space of means to achieve those In sum, domain-general mechanisms such as “ratio- ends. Performing the computations needed to sift through that nality” fail to provide plausible alternative explanations for sample space requires more time than is available for solving psychological phenomena discovered by evolutionary psy- many adaptive problems, which must be solved in real time. chologists. They are invoked post hoc, fail to generate Consider a man coming home from work early and discov- novel empirical predictions, fail to specify underlying mo- ering his wife in bed with another man. This circumstance tivational priorities, suffer from paralyzing combinatorial typically leads to immediate jealousy, rage, violence, and explosion, and imply the detection of statistical regularities sometimes murder (Buss, 2000; Daly & Wilson, 1988). Are that cannot be, or are unlikely to be, learned or deduced men pausing to rationally deliberate over whether this act ontogenetically. It is important to note that there is no jeopardizes their paternity in future offspring and ultimate single criterion for rationality that is independent of adap- reproductive fitness, and then becoming enraged as a conse- tive domain. February–March 2010 ● American Psychologist 115  Although the field of psychology lacks a complete understanding of the nature of these learning adaptations, enough evidence exists to draw a few reasonable conclu- sions. Consider three concrete examples: (a) People learn to avoid having sex with their close genetic relatives (learned incest avoidance); (b) people learn to avoid eating foods that may contain toxins (learned food aversions); (c) people learn from their local peer group which actions lead to increases in status and prestige (learned prestige crite- ria). There are compelling theoretical arguments and em- pirical evidence that each of these forms of learning is best explained by evolved learning adaptations that have at least some specialized design features, rather than by a single all-purpose general learning adaptation (Johnston, 1996). Stated differently, evolved learning adaptations must have at least some content-specialized attributes, even if they share some components. Solving the adaptive problem of incest avoidance re- quires learning to avoid sexual contact with close genetic relatives who are, from a reproductive standpoint, inappro- Carin priate sexual partners due to inbreeding depression. Em- Perilloux pirically, there is good evidence for an evolved incest avoidance learning mechanism that is highly sensitive to reliable cues of close genetic relatedness, such as coresi- dence during development (Lieberman, Tooby, & Cos- 3. Aren’t human behaviors the result of mides, 2003, 2007). Coresidence in the ancestral environ- learning and socialization, not evolution? ment was a reliable cue of genetic relatedness, although this does not always hold true in the modern environment, The framework of evolutionary psychology dissolves di- as in experiments with Israeli kibbutzim. Duration of chotomies such as “nature versus nurture,” “innate versus coresidence with a member of the opposite sex during learned,” and “biological versus cultural.” Instead it offers childhood powerfully predicts lack of sexual attraction as a truly interactionist framework: Environmental selection well as the degree of emotional repulsion at the idea of pressures shape evolved mechanisms at the phylogenetic having sex with him or her (Lieberman et al., 2003, 2007; level (Fraley, Brumbaugh, & Marks, 2005). Environmental see Tyber, Lieberman, & Griskevicius, 2009, for empirical input influences their development at the ontogenetic level evidence for three evolved functions of emotional repul- (e.g., Belsky, 2007; Bjorklund & Pelligrini, 2000; Ellis & sion—sexual disgust, pathogen disgust, and moral disgust). Bjorklund, 2005). And the environment provides cues that Food aversions, on the other hand, require a different activate psychological adaptations at the immediate proxi- set of learning mechanisms and therefore result in a differ- mal level (e.g., Buss, 1995; Cosmides & Tooby, 2000). ent set of learned behaviors. Humans, and most omnivores, Thus, it does not make sense to ask whether calluses or learn food aversions through a mechanism that efficiently mating decisions are “evolved” or “learned” or due to associates the nausea with the ingestion of toxins or patho- “nature” or “nurture.” All evolved mechanisms require gens in food recently consumed, thereby producing disgust some environmental input for their activation, be it re- and avoidance reactions to future encounters with the as- peated friction to the skin in the case of calluses or observ- sociated food (Domjan, 2004). Food aversion learning able cues to mate value in the case of mating decisions. helps organisms to avoid ingesting substances that previ- The term learning is sometimes used as an explana- ously caused nauseous reactions to substances containing tion for an observed effect and is the simple claim that toxins or infectious microorganisms. something in the organism changes as a consequence of Another set of learning mechanisms is responsible for environmental input. Invoking “learning” in this sense, incorporating information about status and prestige in the without further specification, provides no additional ex- psychology of hierarchy negotiation (Henrich & Gil-White, planatory value for the observed phenomenon but only 2001). Among hunter-gatherer societies, good hunting regresses its cause back a level. Learning requires evolved skills lead to prestige (Hill & Hurtado, 1996). In academia, psychological adaptations, housed in the brain, that enable individuals attain high prestige by publishing prominent learning to occur: “After all, 3-pound cauliflowers do not papers that are cited by other scholars (Pennebaker, 2009). learn, but 3-pound brains do” (Tooby & Cosmides, 2005, p. People learn prestige criteria, in part, by scrutinizing the 31). The key explanatory challenge is to identify the nature attention structure: Those high in prestige are typically of the underlying learning adaptations that enable humans those to whom the most people pay the most attention to change their behavior in functional ways as a conse- (Chance, 1967). By attending to and imitating the qualities, quence of particular forms of environmental input. clothing styles, and behaviors of those to whom others pay 116 February–March 2010 ● American Psychologist  Socialization is another explanation that is sometimes proposed as an alternative to evolutionary psychological explanations. Indeed, for much of 20th-century psychol- ogy, socialization was thought to be a powerful influence on human development, personality, and sex differences. This view was based on an old but extremely influential idea that “individual natures are merely indeterminate ma- terial that the social factor molds and transforms” (Durkheim, 1895/1962, p. 106). Although psychologists are increasingly recognizing that evolved tendencies play some role in psychological development, historically many theories of socialization implicitly assumed that the mind was more or less a blank slate upon which parents, teach- ers, and other agents of socialization wrote scripts (Pinker, 2002). Empirical work over the past few decades has gradually changed this assumption. Plomin and Daniels (1987), using behavioral genetic methodologies, showed that shared environmental influences within the family (those typically assumed by traditional socialization theo- ries) account for a trivial percentage of variance in person- David M. ality and other psychological variables. Furthermore, a Buss meta-analysis of 172 studies of differential parental social- ization practices of boys and girls discovered that “most effects were found to be non-significant and small” (Lytton & Romney, 1991, p. 267). the most attention, humans learn the prestige criteria of The sole exception to this conclusion, from among the their local culture (one form of social learning) (Henrich & 19 key domains examined in the meta-analysis, centered on Gil-White, 2001). sex-typed activities. Parents apparently encourage girls to These three forms of learning—incest avoidance, food play with dolls and houses and boys to play with balls, bats, aversion, and prestige criteria—require at least some con- and toy trucks. Even within this delimited domain, how- tent-specific specializations to function properly. Each op- ever, the assumption of a unidimensional parent-to-child erates on the basis of inputs from different sets of cues: direction of effects has been undermined by other studies. coresidence during development, nausea paired with food Male vervet monkeys, like boys, prefer playing with “mas- ingestion, and group attention structure. Each has different culine” toys such as trucks; female vervet monkeys, like functional output: avoidance of relatives as sexual partners, most girls, prefer “feminine” toys such as Barbie dolls disgust at the sight and smell of specific foods, and emu- (Hines, 2004). Levels of prenatal exposure to androgens lation of those high in prestige. It is important to note that influence within-sex differences, with females exposed to each form of learning solves a different adaptive problem. higher levels of androgens displaying more masculine ap- There are four critical conclusions to draw from this pearance and behavior (Hines & Green, 1991). The relative admittedly brief and incomplete analysis. First, labeling dearth of evidence for the potency of parental socialization something as “learned” does not, by itself, provide a sat- on subsequent psychological development has caused some isfactory scientific explanation any more than labeling theorists to shift their emphasis away from parental social- something as “evolved” does; it is simply the claim that ization practices shared by children within families and environmental input is one component of the causal process instead to focus on peers (J. R. Harris, 2007) and within- by which change occurs in the organism in some way. family birth order (Sulloway, in press) to explain the non- Second, “learned” and “evolved” are not competing expla- shared environmental influences that behavioral genetic nations; rather, learning requires evolved psychological methods have shown must exist. mechanisms, without which learning could not occur. Given the large amount of effort that parents in all Third, evolved learning mechanisms are likely to be more cultures expend on socializing their children, it would defy numerous than traditional conceptions have held in psy- evolutionary logic if there were no adaptations in parents chology, which typically have been limited to a few highly associated with socialization practices. Consequently, evo- general learning mechanisms such as classical and operant lutionary psychologists fully accept the potential impor- conditioning. Operant and classical conditioning are impor- tance of environmental influences, whether coming from tant, of course, but they contain many specialized adaptive parents, siblings within families, or peers, but they suggest design features rather than being domain general (Öhman that socialization theories will become more powerful if & Mineka, 2003). And fourth, evolved learning mecha- informed by evolutionary psychological analyses (Buss, nisms are at least somewhat specific in nature, containing 2008; Ellis, Jackson, & Boyce, 2006). particular design features that correspond to evolved solu- The daughter guarding hypothesis—the idea that par- tions to qualitatively distinct adaptive problems. ents have evolved adaptations designed to socialize their February–March 2010 ● American Psychologist 117 daughters and sons differently in the sexual realm— exem- place greater value on a potential mate’s physical attrac- plifies one approach to evolution-based theorizing about tiveness (Gangestad & Buss, 1993; Gangestad, Haselton, & socialization (Perilloux, Fleischman, & Buss, 2008). Ac- Buss, 2006). In fact, ecological variation in parasite prev- cording to the daughter guarding hypothesis, greater paren- alence accounts for 52% of the cultural variation in the tal constraint on the sexuality of daughters would have importance placed on physical appearance in a mate. The provided three functional benefits: (a) protecting their key point is that evolutionary hypotheses about evoked daughter’s sexual reputation, (b) preserving their daugh- culture can explain some forms of cultural variation, al- ter’s mate value, and (c) preventing their daughter from though obviously the extent to which it can do so remains being sexually exploited (Perilloux et al., 2008). Using two to be determined. separate data sources, young adults and their parents, Per- Transmitted (or adopted) culture describes a second illoux and her colleagues found that parents were more form of cultural phenomena (e.g., Norenzayan, 2006). Ex- likely to control their daughter’s than their son’s mating amples of transmitted culture include beliefs about the decisions (e.g., by imposing an earlier curfew); reported afterlife, local moral repugnance at the thought of ingesting more emotional upset over their daughter’s than their son’s certain foods such as beef or pork, the content of certain sexual activity; and exerted more control over their daugh- stereotypes about outgroup members, and information ter’s than their son’s mate choice decisions. In a massive transmitted through gossip. In a series of studies testing cross-cultural study, Low (1989) found support for the hypotheses about adaptations for information transmission, evolution-based daughter guarding hypothesis in a study of McAndrew, Bell, and Garcia (2007) found predictable pat- 93 cultures— girls across cultures are taught to be more terns of content and persons to whom gossip is spread. sexually restrained than boys. Individuals spread damaging gossip rapidly and widely These studies illustrate just one evolutionary approach about rivals but tend to conceal it about friends or lovers. to socialization practices (see Geary & Flinn, 2001, for an Sexual rivals tend to target potential mates as recipients of evolutionary analysis of parenting, and Ellis et al., 2006, derogatory rumors about their competitors. And men and for an evolutionary theory of adaptive phenotypic plastic- women differ in the content of gossip they spread about ity). Much theoretical and empirical work remains to be their rivals, with men focusing more on deficiencies in done, but we can already see that evolutionary and social- athletic and professional prowess and women focusing ization explanations are not necessarily mutually exclusive; more on appearance and sexual conduct (Buss & Dedden, in some cases, they can be usefully integrated to provide 1990; Campbell, 2004). No one claims to have articulated novel predictions. a comprehensive explanation of any particular instance of transmitted culture. Nonetheless, satisfying explanations of 4. How does evolutionary psychology take transmitted culture will involve at least the following ele- culture into account? ments: (a) psychological adaptations in individuals de- Culture is sometimes advanced as competing with expla- signed to selectively transmit ideas, beliefs, or representa- nations that invoke evolved psychology, most frequently tions to others; (b) psychological adaptations in receivers when cross-cultural variability is observed. Cultural expla- designed to selectively accept some ideas from among the nations sometimes invoke the notion that differences be- available welter of ideas (Flinn, 1997); and (c) psycholog- tween groups are prima facie evidence that “culture” is an ical adaptations designed to discount or reject other ideas autonomous causal agent that creates the content of the from the available cultural pool on the basis of qualities human mind (see Buss, 2001, for an extended discussion of such as source credibility and conflicts of fitness interests the myth of culture as a causal explanation). Although the (Cosmides & Tooby, 2000). As Allport and Postman ambiguous referent “culture” is generally presumed to have (1947, p. 314) astutely observed long ago, “Rumor is set a single, coherent meaning, it can actually refer to at least into motion and continues to travel based on its appeal to two distinct concepts: (a) evoked culture— differential the strong personal interests of the individuals involved in output elicited by variable between-group circumstances the transmission.” operating as input to a universal human cognitive architec- Transmitted culture, if it is passed down over many ture; and (b) transmitted culture—the subset of ideas, generations, can in turn create new selection pressures for values, and representations that initially exist in at least one novel human adaptations (Cochran & Harpending, 2009). mind that come into existence in other minds through An adaptation for the digestion of dairy products, a product observation or interaction (Tooby & Cosmides, 1992). Nei- of the cultural invention of agriculture, provides one well- ther of these distinct senses of culture, however, can be documented physiological example. A possible psycholog- divorced from the content-structuring, evolved organiza- ical example is the modern rise of sociopathic traits as a tion of the human mind. consequence of living in large cities, which lowers the Evoked culture arises from human cognitive architec- reputational costs of pursuing a strategy of deception, ture and expresses itself differentially according to local cheating, and defection (Buss & Duntley, 2008; Mealey, conditions. Mate preferences for physical appearance, for 1995). Just as transmitted culture rests on a foundation of example, have been hypothesized to vary across cultures evolved psychological adaptations, novel adaptations can according to local levels of pathogen prevalence. Parasites evolve as a consequence of transmitted culture. Whether are known to degrade physical appearance. In geographic humans have evolved psychological adaptations over the regions with higher parasite loads, both men and women past 10,000 years for other culturally transmitted inven- 118 February–March 2010 ● American Psychologist tions, such as cash economies, remains an open question. (Buss, 2000). In the modern world, however, if a man’s Although ground has barely been broken on adaptation– wife is taking birth control, male sexual jealousy adapta- culture coevolutionary processes (sometimes called gene– tions could be considered superfluous in the prevention of culture coevolution), theoretical work suggests that this is a investing in unrelated offspring. promising avenue of research (Cochran & Harpending, If the relevant adaptation were a general motivation to 2009; Richardson & Boyd, 2005). maximize reproductive success (which cannot evolve in The stance of evolutionary psychology toward culture principle—see Tooby & Cosmides, 1990), then the wife’s can be summarized by several key points: (a) Cultural contraception should short-circuit or dispel her partner’s phenomena are real and require explanation; (b) labeling sexual jealousy. If the relevant adaptation is a powerful something as “culture” is simply a description, not a causal emotion triggered by modern instances of ancestral cues, explanation; (c) it is useful to distinguish between different such as witnessing one’s mate kissing a rival, then knowl- forms of cultural phenomena, such as evoked culture and edge of the wife’s contraception would not prevent the transmitted culture; (d) explaining evoked cultural phe- activation of her partner’s sexual jealousy. Psychological nomena requires an understanding of the evolved psycho- adaptations will be activated by the cues, or close approx- logical mechanisms and the relevant environmental input imations of those cues, that those adaptations were de- involved in their elicitation; (e) explaining transmitted cul- signed to detect, regardless of whether the adaptations ture requires the invocation of evolved psychological currently serve the functions for which they originally mechanisms in both transmitters and receivers; and (f) evolved. transmitted culture, if recurrent over generations, can in- In order for contraception to affect male sexual jeal- fluence the evolution of novel adaptations, which in turn can affect transmitted culture, theoretically producing ad- ousy, the selection pressures introduced by its invention aptation– culture coevolutionary processes. would have to fashion new, or modify existing, psycholog- ical mechanisms. For such complex adaptations to evolve, 5. How do recent novel environmental the selection pressures responsible for their design must be phenomena affect human evolutionary reliably and recurrently faced for multiple generations. psychology? Effective contraception is a very recent invention on the timescale of human history. Consequently, there has not Extremely novel recent environments, of course, have not been enough time for natural selection to forge or modify had enough time to influence the evolution of psychologi- complex psychological adaptations to effectively utilize the cal adaptations. Humans live in modern environments that evolutionarily novel inputs associated with birth control. In differ in some important ways from the ancestral environ- some ways, there exist mismatches between the social ments to which our hominid ancestors adapted. Rather than world in which humans evolved and the modern worlds collecting food via hunting or gathering, people eat at they now inhabit. restaurants and buy prepackaged food at the grocery store. The other way in which novel environmental influ- Modern computers have partially replaced face-to-face ences can affect evolved psychology is when they hijack communication, whereas increased geographic mobility our evolved psychological mechanisms by closely mimick- has brought together previously isolated peoples. Through ing ancestral cues that the psychological adaptation was contraceptive technology, modern humans can sever the designed to detect. Pornography, for example, can activate link between sexual intercourse and reproduction. How do a man’s sexual arousal adaptations, despite his inability to these recent environmental novelties affect human evolved copulate and impregnate a two-dimensional image of a psychology? woman on a page or computer screen (Symons, 1979). This There are at least two approaches to answering this question. First, mismatches between modern and ancestral activated adaptation was designed to elicit arousal at the environments may negate the adaptive utility of some sight of an attractive, unclothed, sexually receptive woman. evolved psychological mechanisms. Mechanisms that once In ancestral environments, such a sight would only have influenced reproductive success may no longer have the occurred in a woman’s actual presence, reliably indicating same effect. Our evolved taste for foods containing fat and a sexual opportunity. Consequently, there would have been sugar, adaptive in the past, now leads to obesity and Type no selection pressure to design a mechanism capable of 2 diabetes in modern environment replete with highly pro- distinguishing between a situation in which a nude woman cessed foods available cheaply and in great abundance. was seen and a sexual opportunity existed, and a similar Second, novel environmental stimuli, such as media images situation lacking such sexual opportunity (e.g., a photo- or pornography, may trigger, hijack, or exploit our evolved graph of a nude woman). In short, modern cues that mimic psychological mechanisms. ancestral cues can artificially hijack evolved psychological Male sexual jealousy provides a specific example of a adaptations in ways that may not be currently adaptive. In mismatch between ancestral and modern environments. some ways, of course, modern environments do resemble Male sexual jealousy almost certainly evolved, in part, to ancestral environments, and adaptations functional in the serve a paternity certainty function—motivating men to past may continue to be functional in the present, so the ward off mate poachers, deterring a mate from sexually continuity of functionality must be determined on a case- straying, and avoiding investing in a rival’s offspring by-case basis. February–March 2010 ● American Psychologist 119 6. What role do genes play in the expression of that trait. Most characteristics are polygenic, framework of evolutionary psychology? that is, they are regulated by complex interactions of many genes in conjunction with specific forms of environmental This is a complicated question that can be analyzed at a input. Single genes rarely code for any complex mecha- minimum of three levels— genetic determinism versus in- nism, be it physiological or psychological. Although the teractionism, genes underlying adaptations, and inferences discovery of the sickle cell gene led some scientists to be about adaptations in the absence of knowledge of molecu- optimistic about finding other single-gene adaptations, de- lar-genetic substrate. cades of subsequent research have revealed few others. All Genetic determinism versus interaction- adaptations, by definition, must have a genetic basis. If they ism. Genetic determinism is the view that genes deter- did not, they could not have evolved by the process of mine phenotypes, such as morphology, psychology, or be- natural selection. This applies as much to brain-based psy- havior, with little or no environmental influence. chological adaptations such as snake fear and cheater de- Evolutionary psychology forcefully rejects a genetic deter- tection as it does to body-based physiological adaptations minism stance and instead is organized around a crisply such as the kidneys and lungs. formulated interactionist framework that invokes the role Can you provide evidence for a psycho- of the environment at every step of the causal process. logical adaptation without documenting its Environmental input includes (a) the selection pressures genetic basis? Evolutionary psychologists often for- that give rise to psychological adaptations; (b) the environ- mulate hypotheses about adaptations, although some test ments needed for the proper development of these mecha- hypotheses about by-products of adaptations (e.g., Kurz- nisms in individual humans; and (c) immediate proximate ban, Tooby, & Cosmides, 2001) or noise generated by inputs necessary for their activation. Indeed, adaptations mutations (e.g., Keller & Miller, 2006). Adaptations are exist in their current form precisely because they histori- typically defined by the complexity, economy, and effi- cally solved adaptive problems posed by various environ- ciency of their design and their precision in effecting spe- mental contingencies. cific functional outcomes, not by the ability of scientists to To illustrate the inaccuracy of the notion that evolu- identify their complex genetic bases (Williams, 1966). For tionary psychology holds to a genetic determinist stance, example, the human eye is indisputably an adaptation de- consider one example of an environmental variable acti- signed for vision, based on the design features for solving vating a plausible evolved psychological mechanism—the the particular adaptive problems such as detecting motion, decrease in testosterone that occurs when men enter a edges, colors, and contrasts. The reliably developing, uni- long-term relationship such as marriage (Burnham et al., versal, and complex design features of the eyes provide 2003) and the further decrease in testosterone after the birth abundant evidence that they are adaptations for specific of their first offspring (Gray, Yang, & Pope, 2006). This functions, even though scientists currently lack knowledge candidate adaptation is hypothesized to function to redirect of the specific genes and gene interactions involved in the the effort that a man allocates away from mating and visual system. toward the adaptive problem of parenting. The evolved The same logic applies to psychological adaptations. mechanisms involved in the regulation of androgens in Just as empirical research has documented snake fears, the males obviously have a genetic basis, but in this context auditory looming bias, and the descent illusion as adapta- they are designed to use specific social cues in adjusting tions without identifying their genetic basis, so too can their levels (Gray, Parkin, & Samms-Vaughan, 2007). The research confirm the existence of adaptations for kin altru- key point of this example, whether or not further research ism, differential parental investment, and cheater detection confirms its nature and hypothesized function, is that evo- in social exchange without identifying their genetic basis. lutionary psychology furnishes an interactionist perspec- The demonstration of specialized functional design pro- tive. Psychological adaptations are designed to respond to vides some of the most compelling forms of evidence for social conditions such as being mated or unmated, being a adaptation. Identifying the genetic basis of a trait is neither parent or being childless, being high or low in the status necessary nor sufficient for demonstrating that the trait is hierarchy, and more generally, confronting one suite of an adaptation. adaptive problems rather than another. Although evolution- Nonetheless, we expect that developments in molec- ary psychology clearly rejects a blank slate view of the ular genetics will be increasingly useful in both testing and human mind, it just as clearly rejects genetic determinism informing evolutionary psychological hypotheses. Individ- and instead provides a detailed interactionist framework ual variations in the alleles of the DRD4 gene provide one (Pinker, 2002). example. The 7R allele has been linked with novelty seek- Is there a gene for each behavioral adap- ing and extraversion (Ebstein, 2006), and it occurs at dra- tation? Evolutionary psychologists sometimes receive matically different rates in different geographical regions questions such as “Is there a jealousy gene?” or “Is there a (e.g., higher in North America than in Asia). This 7R allele gene for male aggression?” Genes do not code directly for of the DRD4 gene has been hypothesized to be advanta- particular physiological or behavioral characteristics; geous in exploiting resources in novel environments (Chen, rather, they code for specific proteins. When geneticists say Burton, Greenberger, & Dmitrieva, 1999; Penke, Denissen, there is a gene for a particular trait, this is a shorthand way & Miller, 2007). The finding that the 7R allele is substan- of indicating that the gene is involved in some way in the tially more common in nomadic than in sedentary popula- 120 February–March 2010 ● American Psychologist tions supports this evolutionary psychological hypothesis Another practical application is in the area of law. (Eisenberg, Campbell, Gray, & Sorenson, 2008). In short, Legal scholars are increasingly using knowledge of although molecular genetic analysis is not necessary for evolved psychological adaptations to understand how to carrying out the research program of evolutionary psychol- better regulate human behavior and guide policy decisions. ogy, we anticipate that it will become increasingly useful to Evolutionary psychology helped to inspire the formation of test and to inform some evolutionary psychological hypoth- the Society for Evolutionary Analysis in Law (SEAL: see eses. www.sealsite.org), which now has over 400 members span- ning more than 30 countries. The practical legal applica- 7. What is the practical value of evolutionary tions include uncovering conflicts among simultaneously psychology? pursued legal policies; sharpening the cost– benefit analy- ses that often influence legal policymaking; increasing our Evolutionary psychology is a basic science, and as such it understanding about human decision making; disentan- seeks a fundamental understanding of human nature— our gling the multiple causes of various law-relevant behaviors; evolved mechanisms of mind. Its conceptual framework exposing a variety of unwarranted assumptions underlying was developed as a metatheory for aiding psychological legal approaches for inspiring behavioral changes; assess- science, not specifically as an aid for practical applications. ing the comparative effectiveness of legal strategies used to Nonetheless, insights provided by evolutionary psychology change specific behaviors; and identifying undernoticed have increasingly been applied to practical societal prob- and unintended selection pressures that legal systems can lems. themselves create (Jones & Goldsmith, 2005). One practical application is in the field of clinical Evolutionary analysis sheds light on the complex fac- psychology. The evolutionary clinical psychologist Steve tors behind sexual harassment, sexual assault, and infanti- Ilardi has developed effective treatment for depression cide, helping guide legal initiatives to aid in deterring based on what he has hypothesized to be plausible mis- future assaults (Jones, 1997, 1999; Jones & Goldsmith, matches between ancestral and modern environments. In 2005). To take one concrete example, sexual harassment ancestral times, people spent much of their day outdoors in and stalking laws typically invoke what is called a “rea- sunshine, engaging in high-activity tasks such as hunting, sonable person standard,” which means that a person can gathering, building shelters, making tools, and tending to be liable for behavior if a reasonable person would have children. They lived in small tightly knit groups rather than found the behavior to be sexually harassing or fear induc- isolated nuclear families. And their diets differed from ing. Evolution-guided research, however, has shown that most modern diets. His six-step treatment for depression women consistently judge a variety of acts to be more includes increasing omega-3 fatty acid consumption, get- sexually harassing than do men, and women experience ting at least 30 minutes of daily exposure to sunlight, greater levels of fear than do men in response to specific ramping up exercise, socializing daily with friends and acts of being stalked (Buss, 2003; Duntley & Buss, in family, engaging in antiruminative activity, and adopting press). But if reasonable women and reasonable men have patterns of good sleep hygiene—all procedures designed to different reactions, how exactly can a jury apply an ungen- mimic ancestral living conditions. This evolution-based dered “reasonable person standard”? Although no firm therapy has produced a 14-week success rate of 75.3%, solutions have been attained, legal scholars are beginning defined as a greater than 50% reduction in depressive to evaluate evolution-based findings such as these for con- symptoms, compared with a 22% success rate for a wait- structing laws that are more effective for their intended list control group (Ilardi et al., 2007). purpose, such as the reduction of illegal sexual harassment These results do not imply that depression was absent and stalking (Jones & Goldsmith, 2005). in ancestral times. Indeed, there is evidence that some Although evolutionary psychology is a scientific dis- forms of depression (or low mood) may serve different cipline with no social agenda, greater knowledge of adaptive functions, such as disengaging from strategies that evolved psychological mechanisms and the environmental are not working, aiding people to be more objective in cues that activate them can, in principle, aid the effective- reassessing their goals, signaling to loved ones that they are ness of preexisting widely desired social policies. Reducing in need of help, or conserving energy (e.g., Andrews & the rate of child abuse, for example, could be facilitated by Thompson, 2009; Keller & Nesse, 2006; Nesse & Wil- knowledge of the evolutionary forces that affect its occur- liams, 1994; Stevens & Price, 1996; Watson & Andrews, rence. The fact that an evolutionary hypothesis led to the 2002). Nonetheless, rates of depression appear to have discovery of the single largest predictor of child abuse to increased dramatically in the modern environment (Kessler date—the presence of a step-parent in the home—provides & Üstün, 2008). A deeper evolutionary understanding of a “lever” for social policies designed to reduce the inci- the common modern problem of depression, including its dence of child abuse (Daly & Wilson, 2005; Herring, 2009; possible functions as well as the ways in which mismatches Jones & Goldsmith, 2005). between ancestral and modern environments exacerbate it, can lead to better prevention and treatment. Although clin- 8. What are the limitations of evolutionary ical applications are just beginning, evolutionary psychol- psychology? ogy is starting to yield practical benefits from clinical Although we have articulated what we see as some of the treatment and lifestyle recommendations. valuable properties of evolutionary psychology and clari- February–March 2010 ● American Psychologist 121 fied some common misconceptions, it is also important to ancestors engaged in hunting for at least the past one highlight its limitations— both empirical phenomena that million years (Leakey & Lewin, 1992; Milton, 1999); that are genuinely perplexing and current conceptual con- our ancestors lived in small groups, ranging in size from a straints. One class of limitations pertains to phenomena that few dozen to 150 (Dunbar, 1993); and that our ancestors are truly puzzling from an evolutionary perspective, such made and used tools for hunting, gathering, cooking, and as those that appear to reduce an individual’s reproductive warfare (Ambrose, 2001; Bingham, 2000; Klein, 2000). success, and cannot be explained by mismatches with, or We also know that bipedal locomotion (Tattersall, 1995), hijacking of, our psychological mechanisms by modern- extended childhood (Harvey & Clutton-Brock, 1985), day novel environmental inputs. The most obvious exam- long-term pair bonds, biparental investment (Hawkes, ple is homosexual orientation, which has been called “the 2004), and relatively concealed ovulation (Sillén-Tullberg Darwinian paradox.” Exclusive homosexual orientation & Møller, 1993) distinguish our ancestors from their clos- seems to defy evolutionary logic since it presumably fails est primate relative, the chimpanzee. to increase an individual’s reproductive success. Although Evolutionary psychologists also use evidence from evolutionary hypotheses have been proposed for homosex- anthropology, archaeology, primatology, comparative biol- uality, as discussed earlier, none have received empirical ogy, and ethology to elucidate some aspects of an other- support thus far (e.g., Bobrow & Bailey, 2001). wise scientifically uncertain ancestral past. For example, Another puzzling phenomenon is suicide. In the the paleontological evidence is rife with ancient caches of United States, more than 30,000 individuals intentionally skulls and skeletons showing patterned lethal injuries, cor- take their own lives each year (Gibbons, Hur, Bhaumik, & responding in size and shape to ancient weapons discov- Mann, 2005). It is more common among males than fe- ered in the vicinity. When combined with cave art depic- males and shows age spikes in adolescence and old age. De tions of fighting and many other sources of evidence, the Catanzaro (1995) and others (Brown, Dahlen, Mills, Rick, cumulative findings yield reasonable inferences that human & Biblarz, 1999) argued that suicide is most likely to occur warfare was a potent hostile force of nature for human in those who have a dramatically reduced ability to con- ancestors, that males were far more often perpetrators and tribute to their own reproductive fitness. In several studies, victims of homicide than females, and that the majority of they found that ill health, burdensomeness to kin, and ancestral attackers were right handed (Duntley & Buss, failure in heterosexual mating were strong predictors of 2008). suicidal ideation. Although burdensomeness to kin pro- Evolutionary psychologists also use a task analysis of vides a plausible explanation for some suicides among the existing mechanisms to provide a window into the past. elderly, it strains credulity to argue that it would be bene- Just as astronomers use phenomena such as three degree ficial to a healthy adolescent’s reproductive success to end black body radiation and an expanding universe to develop his or her life permanently, regardless of the current mating cogent and testable theories about events in the universe’s prospects. Such suicides are likely to be nonadaptive by- past, evolutionary psychologists can use the functional products of evolved mechanisms that malfunction (Wake- design of the human psychological architecture to make field, 2005). In brief, there are puzzling phenomena such as cogent inferences about the past. The universal and reliably homosexuality and suicide that remain at least somewhat developing fear of snakes, even in modern environments inexplicable on the basis of current evolutionary psycho- containing few snakes, reveals that snakes were a hostile logical accounts.2 force of nature in our evolutionary past (Nesse, 1990). Another limitation is that we lack detailed knowledge Similar scientific inferences can be drawn from universal of many selection pressures that humans faced over the human taste preferences (e.g., the substances humans find millions of years of their evolution. We do not possess a bitter often contain toxins), landscape preferences (e.g., videotape of deep time that would reveal in precise detail desire for habitats containing prospect and refuge), and all of the selective events over millions of years that have mate preferences (e.g., attraction to observable cues statis- led to the current design of the human body and mind. tically correlated with fertility). Humans are living fossils. Nonetheless, this limitation is not total. There is a surpris- They are walking archives that provide a wealth of knowl- ingly abundant amount of information about the human ancestral environment that we do know to a reasonable edge of the past. In sum, although convergent evidence degree of certainty. For example, ancestral humans “had from independent data sources yields especially reasonable two sexes; chose mates; had color vision calibrated to the inferences about some past selection pressures, evolution- spectral properties of sunlight; lived in a biotic environ- ary psychology, and indeed the entire field of psychology, ment with predators; were predated on; bled when wound- will always be limited by incomplete knowledge of past ed; were incapacitated from injuries; were vulnerable to a selection pressures. large variety of parasites and pathogens; and had deleteri- A final limitation of evolutionary psychology centers ous recessives rendering them subject to inbreeding depres- on a current relative deficiency in explaining cultural and sion if they mated with siblings” (Tooby & Cosmides, individual differences. Evolutionary psychology has been 2005, pp. 23–24). Scientists also know that fertilization occurred inter- 2 Modern-day adoption of genetically unrelated children may repre- nally within females, not within males; that females, not sent another empirical puzzle; for detailed discussion of adoption in males, bore the metabolic costs of breastfeeding; that our evolutionary perspective, see Silk (1990). 122 February–March 2010 ● American Psychologist far more successful in predicting and explaining species- the debate. Psychological Review, 113, 628 – 647. doi:10.1037/0033- typical and sex-differentiated psychological adaptations 295X.113.3.628 Belsky, J. (2007). Childhood experiences and reproductive strategies. In than explaining variation within species or within the sexes R. Dunbar & L. Barrett (Eds.), Oxford handbook of evolutionary (Buss, 2009). Although evolutionary psychologists have psychology (pp. 237–254). Oxford, England: Oxford University Press. discovered a female superiority in spatial location memory Bingham, P. M. 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Changes in the nucleotide sequence ties crafted by natural selection to solve the specific problems of a gene as a result of a copying error during cell posed by the regularities of the physical, chemical, develop- division, ultraviolet radiation, viruses, or other muta- mental, ecological, demographic, social, and informational gens. environments encountered by ancestral populations during the Naturalistic fallacy. The misconception that course of a species’ or population’s evolution” (Tooby & ethical principles can be derived from what happens in the Cosmides, 1992, p. 62). natural world. Also called the “is– ought fallacy,” that By-products. “Characteristics that do not solve because something does exist it should exist. adaptive problems and do not have functional design; they Noise. Random phenomena produced by forces such are “carried along” with characteristics that do have func- as chance mutations, sudden and unprecedented changes in tional design because they happen to be coupled with those the environment, or chance effects during development. adaptations” (Buss, 2008, p. 39). Polygenic. An effect of two or more gene loci on Cuckoldry. When a “mate poacher” has sex with an a single phenotypic character. already married woman. Genetic cuckoldry occurs when a Paternity uncertainty. See also cuckoldry; As child results from this act. Evolutionarily, a cuckolded man a consequence of internal female fertilization, females can risks investing unknowingly in another man’s offspring. be 100% sure they are the mother of their offspring. For Design features. Attributes or component parts this same reason, males can never be 100% certain they are of an adaptation that have been forged or “designed” by a past history of natural selectiion (no forward-looking or their offspring’s father. intentional process is implied by this phrase). Proximate explanation. Explaining how a Evoked culture. “Cultural differences [that] are mechanism works and is implemented, which can include explained simply by invoking a universal shared evolved identifying how a mechanism develops, the stimuli that mechanism combined with local between-group differ- activate it once it has developed, the procedures by which ences in input into that mechanism” (Buss, 2008, p. 417). it operates, and the behavioral output it produces. Group selection. “Selection operating between Transmitted culture. The subset of ideas, val- groups of individuals rather than between individuals. It ues, and representations that initially exist in at least one would produce attributes beneficial to a group in competi- mind that come into existence in other minds through tion with other groups, rather than attributes beneficial to observation or interaction (Tooby & Cosmides, 1992). individuals” (Ridley, 2004, p. 685). Ultimate explanation. Explaining the at- Inbreeding depression. Occurs when genetic tributes of a mechanism by identifying its evolved func- relatives together reproduce offspring that have decreased tion, or the adaptive problem it was “designed” by fitness due to the compounding of recessive alleles. selection to solve. 126 February–March 2010 ● American Psychologist