How Can EvolutionaryPsychology Successfully ExplainPersonality and IndividualDifferences?David M. BussUniversity of Texas, AustinABSTRACT—Although evolutionary psychol ogy has beensuccessful in explaining some species-typical and sex-differentiated adaptations, a large question that haslargely eluded the field is this: How can the field success-fully explain personality and individual differences? Thisarticle highlights some promising theoretical directions fortackling this question. These include life-history theory,costly signaling theory, environmental variability in fitnessoptima, frequency-depend ent selection, mutation load,and flexibly contingent shifts in strategy according to envi-ronmental conditions. Tackling the explanatory questionalso requires progress on three fronts: (a) reframing somepersonality traits as forms of strategic individual differ-ences; (b) providing a nonarbitrary, evolutionary-basedformulation of environments as distributions and salienceprofiles of adaptive problems; and (c) identifying whichstrategies thrive and which falter in these differing prob-lem-defined environments.With the growing acceptance of evolution as a metatheory forpsychology, more and more personality psychologists are trying toconceptualize personality within an evolutionary framework.—Penke, Denissen, & Miller, 2007, p. 553.Evolutionary psychology is a relatively new metatheoreticalparadigm that synthesizes the modern principles of psychologywith the core principles of evolutionary biology. It is anchored onan idea captured eloquently by George Williams, one of theleading biologists of the 20th century: ‘‘Is it not reasonable toanticipate that our understanding of the human mind would beaided greatly by knowing the purpose for which it was de-signed?’’ (Williams, 1966, p. 16). Although some of the roots ofevolutionary psychology can be traced to Charles Darwin (Buss,2009b), it is only recently that new theoretical syntheses haveallowed the field to blossom empirically.The scientific successes of evolutionary psychology have beenmost prominent in domains such as survival (e.g., evolvedhabitat preferences), sexuality (e.g., multiple functional moti-vations for intercourse), mating strategies (e.g., universal sexdifferences in mate preferences), sexual conflict (e.g., predict-able patterns of sexual deception), parenting (e.g., adaptationsin males to scale back on investment when faced with cues topaternity uncertainty), kinship (e.g., altruism preferentiallychanneled to kin as a function of degree of genetic relatedness),cooperation (e.g., discovery of cheater-detection and anti-free-rider adaptations), and aggression (e.g., predictable cir-cumstances in which men adopt risky social strategies; Buss,2008). These theoretical and empirical advances, however, havebeen achieved primarily at the levels of species-typical and sex-differentiated adaptations. Personality tra its and other individ-ual differences—profound and integral to human psychology—have been relatively neglected, with some notable exceptions(e.g., Buss, 1991; Buss & Greiling, 1999; Figueredo et al., 2005;Hawley, 1999, 2006; MacDonald, 1995; Nettle, 2006; Wilson,1994; Wilson, Near, & Miller, 1996). Indeed, it is only recentlythat cogent evolutionary analyses have been devoted toexplaining individual differences within nonhuman species(e.g., Gosling, 2001; Wilson, 1998; Wolf, van Doorn, Leimer, &Weissing, 2007).The key question is this: How can evolutionary psych ologysuccessfully explain personality and individual differences?To begin answering this question, one might find it usefulto first examine why individual differences have beenAddress correspondence to David M. Buss, Department of Psy-chology, University of Texas, Austin, TX 78731; e-mail: [email protected] ON PSYCHOLOGICAL SCIENCEVo l u m e 4 — N u m b e r 4 359Copyright r 2009 Association for Psycholo gical Scienceneglected and why they are so impor tant that they cannot beneglected.WHY HAVE PERSONALITY AND INDIVIDUALDIFFERENCES BEEN NEGLECTED?There are important reasons for this neglect, starting with apaucity of powerful theories. Cogent evolutionary theories existfor predicting and explaining sex differences—the most notablebeing the theory of sexual selection (Buss, 1995; Geary, 1998).Evolutionary theories of species-typical adaptations, such askin-selection theory and the theory of reciprocal altruism, alsoprovide powerful tools. Evolutionary psychologists have syn-thesized these theories with principles of modern psychology tocreate unique theories such as social-contract theory (Cosmides& Tooby, 2005), sexual-strategies theory (Buss & Schmitt,1993), and error-management theory (Haselton & Buss, 2000),which, in turn, have led to important empirical discoveries suchas adaptive cognitive biases (Haselton, Nettle, & Andrews,2005). In sharp contrast, comparably powerful theories that canpredict and explain personality and individual differences havelargely eluded both evolutionary psychologists as well as psy-chologists who fail to take advantage of its conceptual toolkit.Another reason for the neglect of individual differences canbe traced to the foundational assumption in evolutionary biologythat natural selection has reduced or eliminated heritable in-dividual differenc es because traits that are advantageous tend tospread over time to fixation and become species -typical. Con-sequently, some of the key founders of evolutionary psychologyhave argued that heritable individual differences are bestviewed as ‘‘noise’’ and are thus irrelevant to the basic func-tioning of the psychological machinery, much like differences inthe colors of the wires of a car engine do not affect its basicfunctioning (Tooby & Cosmides, 1990). This assumption seemedreasonable 20 years ago, but it has been seriously challenged bykey theoretical developments in evolutionary biology and ad-vances in evolutionary foundations of psychology (Keller, 2007;Nettle, 2006).WHY INDIVIDUAL DIFFERENCES AREEVOLUTIONARILY IMPORTANTIndividual differences acquire importance for several reasons—some empirical and some theoretical. First, profound andconsequential individual differences have been welldocumented—individual differences in personality character-istics (e.g., dominance vs. submissiveness; agreeableness vs.aggressiveness), general intelligence and more specific abilities(e.g., spatial location vs. spatial rotation abilities),