Toward a Biologically Informed

Psychology of Personality

David M. Buss

University of Michigan

ABSTRACT This article describes nine ways in which biological approachescan lnforni issues of central and long-standing concern to personality psy-chologists These include (a) developing an adequate description of humannature, (b) providing several solutions to the puzzle of within-species geneticvariability, (c) identifying the most important ways m which individuals differ,(d) giving precision to the concepts of adaptation and adjustment, (e) identi-fying the ongms of personality dispositions, (/) providing insight into person-ahty development and the life course, (g) providing conceptual and evidentialstandards for invoking personality types as opjwsed to personality dimensions,(/i) addressing the psychophysiology of personahty, and (i) focusing attentionon psychological mechanisms as evolved dispositional strategies

Biologists define biology as the study of life In that broad sense, allof psychology is subsumed by biology Within the field of psychology,however, biological approaches represent particular modes of analyz-ing psychological and behavioral phenomena Three such modes arehighlighted m this special issue evolutionary approaches, behavioralgenetic approaches, and psychophysiological approaches In the pastdecade, advances in knowledge, theoretical sophistication, and meth-odological precision have occurred in each of these subfields This issuebrings together and elucidates some of the most important of theseadvances

This report was supported m part by NIMH Grant MH-44206-01 and by a fellow-ship from the Center for Advanced Study m the Behavioral Sciences The authorthanks Steve West for helpful editorial suggestions on an earlier version of this articleCorrespondence should be addressed to David M Buss, Department of Psychology,Umversity of Michigan, Ann Arbor MI 48109-1346

Journal (^Personality 58 1, March 1990 Copynght © 1990 by Duke University PressCCC 0022-3506/90/$! 50

2 Buss

Many misunderstandings surround the biological study of person-ality One IS that there is "a" biological perspective—a smgle, unified,monolithic approach This view errs for two reasons (a) There are sev-eral distinct levels of biological analysis (e g , evolutionary, behavioralgenetic, psychophysiological), each with distinct theoretical assump-tions, content, and methods, and (b) within each level, there are oftencompeting theones about the same set of observations Biological ap-proaches to personahty are many and varied, not singular m nature

A second common misunderstanding is that biological approachesare somehow at odds with approaches that are considered "environ-mental" or "social " Many biological approaches, particularly thosethat are evolutionary, focus precisely on how organisms deal with theirenvironments As one biologist aptly put it "The whole reason forphenotypes' having evolved is that they provide flexibility in meetingenvironmental contingencies " (Alexander, 1979, p 14) Evolu-tionary approaches are not opposed to environmental approaches, theydescnbe how, through a long history of natural selection, environmentshave shaped organisms to adapt strategically to their environments

There is an important sense in which all human behavior is the prod-uct of the environment But environments vary on a temporal dimensionfrom distal to proximate The distal environment is the "environmentof evolutionary adaptedness," the ancestral conditions that forged basichuman adaptations A more proximate environment is that of ontogeny—the conditions encountered or created dunng development At themost proximate level are immediate environmental contingencies that,through organismic structures created over phylogeny and ontogeny,affect current behavior All personality phenomena are products ofthese environmental conditions Biological approaches are concemedwith identifying the mechanisms created by previous environments andthrough which current environmental contingencies operate

Most personality research is conducted without being informed bybiological perspectives Therefore, it is reasonable to ask What centralissues in personality can be illuminated by these biological perspec-tives'' This introduction highlights nine such issues The first hes at thecore of nearly all grand theones of personality—the quest for under-standing human nature

Human Natiue

Personality psychology at least since Freud has been concemed withidentifying human nature—the core human motives and psychologi-

Introduction 3

cal mechanisms shared by all (or typical of most) members of ourspecies To cite two examples, Freud posited core human instincts ofsex and aggression, universal components of the psychic apparatus (id,ego, superego), and species-typical mvariance in stage sequence (oral,anal, phallic, latency, genital) Maslow proposed the push toward self-actualization and the satisfaction of survival needs as the central drivingforces of human motivation

Several articles m this special issue propose ways in which evolu-tionary biology can inform the study of human nature—conceptionsconsonant with what is known about the forces of natural selection thatgovern organic hfe Tooby and Cosmides provide a compelhng argu-ment that the core of human nature is to be found m universal functionaldesign They argue that the human mind cannot be understood prop-erly without articulating the adaptive problems that humans have facedm our evolutionary history—problems ultimately linked with survivaland reproduction Psychological mechanisms, they argue, have evolvedto solve these adaptive problems These species-typical psychologicalmechanisms, together with the adaptive problems toward which theyare directed, comprise the core of human nature

Tooby and Cosmides illustrate this evolutionary approach with an im-pressive body of research They argue that in order for us to engage ef-fectively m elaborate social exchange without being exploited, humansmust have evolved psychological mechanisms to detect cheaters—thosewho benefit m a social exchange without reciprocating by paying a costA psychological mechanism for detecting such cheaters, they argue,must be species-typical of humans, a part of human nature that solvesone important adaptive problem

Tooby and Cosmides argue further that because humans have had tosolve many, distinct adaptive problems, the psychological mechanismsthat have evolved to solve these problems must similarly be numerousand specific (cf Symons, 1987) General, all-purpose psychologicalmechanisms such as "learn from your environment" or "maximize in-clusive fitness" could not have evolved Such mechanisms would beanalogous to programming a computer chess strategy with a singlegeneral instruction such as "win" or "make good moves " Such a pro-gram, like postulated general all-purpose psychological mechanisms,are too underdetermined to generate the complex, flexible, and spe-cific contingent patterns of behavior that we observe Thus, m additionto illustrating one empincal method for studying the core of humannature, they present a compelhng view that our species-typical psycho-logical mechanisms are likely to be specific, numerous, and complex

4 Buss

This view implies that personality theories that posit only one or a fewmotives, and only a handful of psychological mechanisms, are almostsure to be wrong from the outset

Daly and Wilson use evolutionary reasoning to argue that Freud'spostulation of a universal Oedipal complex is inconsistent with whatIS now known about evolutionary biology Usmg Tnvers's (1974) nowclassic theory of parent-offspnng conflict, they argue that Freud cor-rectly noted the pervasiveness of confiict and ambivalence in parent-child relations But Freud failed to distinguish between two sorts ofconfiict—an early nonsexual father-son confiict over how a mother'sreproductive efforts are to be expended, and a later potential rivalrythat IS sexual, but over possible mates and not the mother

Daly and Wilson review previous data and present new data showingthat the central Freudian claim of a same-sex contingency in parent-offspnng antagonism during the Oedipal phase is not supported Theyargue, like Tooby and Cosmides, that theones of human nature shouldbe consistent with what is known about the pnnciples that govem or-ganic life generally, and the evolution of social interaction specificallyEvolutionary theory, while not a theory of human nature, provides apowerful guide or metatheory for articulating a possible basis for humannature

The Enigma of Human Genetic Variability

Although once considered to be highly controversial, the major dimen-sions of personality are now known to show moderate heritabihtyRoughly half of the observed variance m personality traits, tradition-ally conceived, has been shown to be due to genetic differences amongindividuals This general conclusion has emerged repeatedly over thepast several decades from research using different methods (e g , twin,adoption, family) and different investigators, many of whom contrib-uted to this special issue (Bouchard, Eysenck, Hom, Loehlin, McGue,Nesselroade, Plomm, Rushton, Willerman) Now that genetic van-abihty within our species has been documented, scientists are tumingto the next logical question Why is it there"'

The question is not a tnvial one, for it is known that natural selec-tion tends to eliminate genetic vanability Virtually all humans havetwo legs, two eyes, and five fingers on each hand—these are species-typical charactenstics, and deviations from them are due mostly toenvironmental vanance (e g , accidents) rather than to genetic van-

Intioduction 5

ance (Loehlin & Nichols, 1976) Although there are forces that createand maintain genetic variability (e g , mutation, frequency-dependentselection, genetic drift), natural selection tends to move traits in apopulation toward fixation or species uniformity

Several contributors to this special issue attempt to solve the enigmaof human genetic diversity, but differ sharply with one another in theirproposed solutions Tooby and Cosmides offer the most general con-ceptual treatment by outlining the major possibilities Within-speciesgenetic variance can be due to (a) frequency-dependent selection foraltemative morphs or strategies (e g , white- and dark-colored mothsexist within the same species due to frequency-dependent selection thatoperates against those that become too common), (b) "genetic noise"(e g , due to mutation) that is selectively neutral, and hence was noteliminated by natural selection, or (c) concomitants of parasite-dnvenfrequency-dependent selection for biological individuality per se

Tooby and Cosmides outline the stringent standards of evidence re-quired if genetic variability is to be considered to be of the first variety—altemative adaptive strategies They argue that these standards areunlikely to be met, given the constraints that sexual recombination andthe complexity of adaptations impose on functional design Thus, thenotion that different "personality types" represent genetically altema-tive adaptive strategies is seen as improbable by Tooby and Cosmides,although they acknowledge that future empirical evidence might even-tually document this possibility

Gangestad and Simpson argue that at least some genetic differ-ences represent frequency-dependent adaptive strategies They developa model of female sociosexuality, with "restricted" as one type (thosewomen who require more time, attachment, and commitment pnorto entenng into a sexual relationship) and "unrestricted" as the other(those women who require less time, attachment, and commitment)The model posits that these two types of females evolved and are main-tained by frequency-dependent selection, with different reproductivebenefits accruing to the two types Restncted females could have bene-fited by eliciting greater male parental investment in their offspringUnrestricted females, in contrast, could have benefited by increasingthe quality of the genes passed on to their sons

Gangestad and Simpson test predictions from their model with sev-eral empirical studies They show, for example, that sociosexualitytends to be bimodally distributed (supporting the notion of two types)and that females genetically predisposed to be unrestricted tend to pro-

6 Buss

duce more sons than those predisposed to be restricted Although fur-ther tests of the theory are needed, their results provide encouragingsupport for the possibility that at least some genetic personality van-ability withm our species represents alternative adaptive strategies Thenext decade of research will undoubtedly lead to continued progressin unraveling the mystery of the moderate heritabihty of personalitycharactenstics

Identifying the Most ImportantIndividual Differences

Personality psychologists have long been concemed with identifyingthe most important ways in which individuals differ From among thethousands of dimensions of difference, which ones should galvanize theattention of personality theonsts and researchers'' As Goldberg (1972)succinctly phrased the question Why measure that trait'' Rationales fordesignating some individual differences as particularly important havecome from folk psychology (Gough, 1968), factor analysis (Cattell,1957), lexical analysis (Norman, 1963), and the act frequency approach(Buss & Craik, 1985), to name a few An important altemative rationaleresides in biological cntena for importance (cf Buss, 1984)

Individual differences closely linked with the components of natu-ral selection, sexual selection, and important hfe-history strategies arecmcial when using an evolutionary rationale for designating impor-tance Kenrick, Sadalla, Groth, and Trost demonstrate that personahtycharactenstics such as dominance, friendliness, and emotional stabilityare intimately tied in with sexual selection in that they are centralto mate choice (cf Buss, 1989) The possibihty that these individualdifferences have previously been, and may currently be, linked withevolution by sexual selection grants them special importance from anevolutionary perspective, when contrasted with those individual differ-ences not linked to evolution Gangestad and Simpson, arguing froma different perspective, propose that individual differences surroundingsociosexuality are especially important because they represent alter-native adaptive strategies for achieving reproductive success Becausethey are so closely linked with reproductive success, these differencesacquire special importance withm an evolutionary framework

These two examples highhght the need to move to extemal cntenafor evaluating the importance of dimensions of individual differencesThey suggest that individual differences take on a larger significance

Introduction 7

to the extent that they play an integral role m important processes thatgovem organic life—those of evolution by natural selection

Adaptation and Adjustment

Concepts such as adaptation and adjustment have been central to nearlyall frameworks of personahty As typically used, these concepts signifyan ability to deal effectively with the varied demands of everyday livingThe related dimensions of Neuroticism (Eysenck, 1953), Emotional In-stability (Norman, 1963), and Trait Anxiety (Spielberger, 1972), whichimply ineffective negotiation of life's tasks, are central to nearly everytaxonomic system of personality These concepts represent a particularmanner of coping, typically one with high levels of subjective distress,intrapsychic discomfort, large fiuctuations of mood or affect, and arelative inability to terminate negative subjective states

The concept of adaptation is also central to evolutionary biology,although It IS defined somewhat differently Adaptations in evolution-ary biology refer to evolved solutions to problems posed by the com-plex tasks of survival and reproduction Not all features of behavioror morphology are considered to be adaptations, and the evidentialstandards for considenng somethmg an adaptation are complex andoften difficult to meet (WiUiams, 1966) Nonetheless, the effectivenesswith which reproductive problems are solved provides a biologicallyanchored meaning of "adjustment "

Perhaps the most important contribution of the articles in this issueto an understanding of adaptation and adjustment comes from the workof Draper and Belsky, Gangestad and Simpson, Rushton, and Zucker-man Draper and Belsky articulate an intriguing evolutionary theoryof altemative reproductive strategies based on the environments thathumans encounter in early childhood They propose that m environ-ments and cultures where fathers are present during early childhood, thereproductive strategy tends to involve delayed puberty, delayed onsetof sexual activity, stability of adult pair-bonds, and a set of concomi-tant personality charactenstics that facilitate this strategy such as lowself-momtonng and high cooperativeness In environments and cultureswhere fathers tend to be relatively absent, an alternative personahty con-stellation and reproductive strategy is followed—one involving earlyonset of puberty and sexual activity, mstabihty of adult pair-bonds, lowparental investment, high self-monitonng, and high aggressiveness

What IS lntnguing about this theory is that it posits that both strate-

8 Buss

gies are part of our species-typical repertoire—we all have the capacityto follow either strategy But which one we do follow depends on theenvironment that we encounter while growing up Draper and Belskyemphasize that it is possible to rear children successfully under bothregimes, and that neither strategy is inherently superior or inferior tothe other—they are both "adaptive" in the environmental contexts inwhich they occur This implies that conceptual clarity might be achievedfrom anchoring definitions of "adjustment" and "adaptation" m theeffectiveness with which reproductive problems are solved m particularenvironments The definition of adjustment would shift from the contentof the strategy (l e , it is not necessanly maladjusted to be impulsive oraggressive) to the success of the strategy m a specific environment

An analogous shift in the meaning of adjustment and maladjustmentIS implied m the articles by Gangestad and Simpson, Rushton, andZuckerman But rather than positing a universal species-typical reper-toire of strategies that are differentially evoked by environments, theyposit genetically altemative strategies that were forged during our an-cestral environments It would be mappropnate to consider one strategy(e g , the "unrestricted sociosexuality" proposed by Gangestad andSimpson the high sensation seeker proposed by Zuckerman, or ther-strategy proposed by Rushton) as either inferior or supenor to thealternative strategies

In sum, evolutionary thinking has imphcations for how personalitypsychologists might conceptualize adaptation and adjustment It sug-gests that equating maladjustment with strategies that might appeardistasteful or repugnant (e g , those that are aggressive, impulsive, orwanton) is inappropriate These strategies may be functional m the par-ticular environments m which they occur (e g , where resources areunstable or unpredictable) The effectiveness with which survival andreproductive problems are solved is one biological criterion by whichthe concepts of "adapted" and "adjusted" can be anchored

The Ontogenetic Ongins ofPersonality Dispositions

Although the field of behavioral genetics is sometimes seen as pre-occupied with estimating hentability, the articles in this special issuediicument the equally important use of behavioral genetic methodsfor illuminating the magnitude and nature of environmental infiuenceson personality dispositions Two anicles m this special issue, one by

Introduction 9

Eysenck and the other by Bouchard and McGue. contribute to the under-standing of environmental and genetic sources of vanance underlyingmajor dimensions of personality

Both articles come to similar conclusions, based on different sets ofdata Reviewing twin and family studies on Extraversion, Neuroticism,and Psychoticism, Eysenck concludes that roughly half of the varianceon these dimensions is due to additive genetic vanance Furthermore,there is little evidence for the importance of shared environmental in-fluences (environmental features that are the same for siblings), whichimplicates unique, individual nonshared environmental influences ascritical—a conclusion m line with most of the behavioral genetic evi-dence over the past decade (Plomm & Daniels, 1987)

Bouchard and McGue present findings on the California Psychologi-cal Inventory (CPI), using data from the Minnesota study of monozy-gotic and dizygotic twins reared apart From these unique data, they findthat approximately half of the vanance on CPI scale scores is heritable,that few scales show any evidence of common family environmentalinfluences, and that reanng environment, at least as gauged by retro-spective reports, explains little variance in adult personality, but doesaccount for variance on a Consensuahty factor that includes the CPIscales of Socialization and Conununality Taken together, these arti-cles represent the contribution that behavioral genetics has made towardidentifying, m a broad sense, the parameters of the major classes ofcausal influence on basic dimensions of personality

Personality Development andLile-History Analysis

One of the most important new developments m behavioral genetic ap-proaches to personality is a focus on personality change over time Twoarticles m this special issue treat this topic Plomin and Nesselroadeprovide a theoretical analysis and Loehlin, Horn, and Willerman em-pincally illustrate the behavioral genetic approach using data from theTexas Adoption Project

Plomin and Nesselroade consider several types of personahty change—changes m heritability over ages, the relative contnbution of ge-netic differences and environmental differences to personality change mchildhood and adulthood, and genetic influences on short-term changesin personality (e g , days, hours, minutes) Based on a review of theempincal evidence, they come to some provocative conclusions Hent-

10 Buss

abihty tends to increase, rather than decrease, when developmentalchanges m hentability are found Genetic involvement is substantialm personality change m childhood, but shght in personality change madulthood Finally, behavioral genetic methods can be applied to ex-amine lntra-mdividual differences in general and short-term personahtychanges from situation to situation

In an important ongoing behavioral genetic study, Loehhn, Horn, andWillerman examine Extraversion, Socialization, and Emotional Sta-bility over a 10-year interval They reach two important conclusions(a) The main source of individual change is based on individual ex-perience and/or measurement error, a finding that is consistent with anow large body of evidence suggesting the importance of nonsharedenvironmental influences (Plomin & Daniels, 1987), and (b) changesin group means of adopted and natural children on these personahtyvariables indicate an average shift toward mcreased similanty with theirgenetic parents Both articles herald major new uses of behavioral ge-netic methods in the analysis of personality

Personality Types VersusPersonality Dimensions

A long-standing debate in personality psychology is whether individualdifferences are best charactenzed by continuous dimensions or by dis-crete types Biological perspectives can inform this debate by articulat-ing theoretical arguments for the plausibility or lmplausibility of types,offenng new standards of evidence that could demonstrate the existenceof distinct types if these standards are met, and providing empincalfindings that bear on the theoretical arguments and standards of evi-dence

The contributors to this special issue diverge m their views of whethertype concepts will be supported by the biological evidence Tooby andCosmides argue that three cntena must be met for considering per-sonality differences to be typological (a) Are personahty differencesbest represented by single quantitative vanables or by an entire rangeof vanables that covary m an organized, coordinated fashion*^ (b) Dothe vanables covary m ways that fulfill cntena for adaptation'' (c) Dothe alternative types show evidence of frequency-dependent selection(l e , that the adaptive payoff of one type decreases when it increasesin frequency in the population)''

Biological sex certainly meets these standards and is perhaps the

Introduction 11

clearest case of a typology A host of traits covary with malenessand femaleness in an organized, coordinated fashion (Symons, 1979,Trivers, 1972, Williams, 1975), they do so m ways that fulfill critenafor adaptation (Buss, 1989, Symons, 1979), and the adaptive payoffof a given trait for each sex decreases as the members of that sex be-come more numerous (Fisher, 1930) Tooby and Cosmides argue thatno existing personality typologies fulfill these cntena

Gangestad and Simpson, in contrast, are more optimistic about typo-logical concepts in personality psychology They present evidence thatrestricted and unrestricted pattems of sociosexuality fulfill some ofthese cntena In addition to evidence of bimodahty, they make a plau-sible case for the adaptive benefits of each suite of covarying character-istics The third criterion—that the types fit an analysis consistent withfrequency-dependent selection—will probably be the most difficult todocument empirically

The Psychophysiology of Personality

If studies m behavioral genetics have yielded evidence that personahtytraits are moderately heritable, they leave unanswered the critical ques-tion of what the genetic differences code for at the level of physiologyTwo important articles in this issue address this question Stelmackreviews the evidence on the psychophysiology of Extraversion conclud-ing, contrary to Eysenck's (1967) original hypothesis, that there is httlecompelling evidence that introverts and extraverts differ in basal levelsof physiological activity or arousal At the same time, there is excel-lent evidence that introverts show greater physiological reactivity orsensitivity to sensory stimulation than extraverts, as indicated by bothelectrodermal and electrocortical activity This finding helps explainwhy introverts tend to avoid certain classes of stimulation m socialbehavior, whereas extraverts seek those classes of stimulation

Zuckerman summarizes a large literature on the psychophysiologyof sensation seeking He concludes that high sensation seekers showstronger onenting responses to novel stimuh of moderate intensity thando low sensation seekers The lows show heart-rate acceleration, sug-gesting defensive responses to novel stimulation At the cortical level,high sensation seekers show an augmented reaction to intense visual andauditory stimuh, whereas low sensation seekers tend to be unresponsiveto vanations in stimulus intensity

An mtnguing feature of Zuckerman's analysis is that it provides

12 Buss

direct, although speculative, links with evolutionary biological ap-proaches He proposes that sensation seeking and sensation avoidancerepresent two different strategies for adapting to a dangerous environ-ment in which novelty can be rewarding or threatening to survival Atrait of sensation seeking might provide an organism increased accessto food and mates, but also entails an increased risk The sensationavoider, in contrast, avoids the risks, but only at the expense of losingthe foraging and mating advantages that would be associated with ap-proach The psychophysiological mechanisms summarized by Zucker-man are consistent with the pattems of information processing com-patible with these respective strategies

In conclusion, although studies of the psychophysiology of person-ality historically have proceeded in relative isolation of other biologicalapproaches to personality, they may be hnked in two possible waysFirst, they illuminate the nature of the mechanisms associated with ge-netic differences found by behavioral geneticists And second, they canprovide information about the reproductive strategies adopted by thosewho differ in personahty dispositions

Psychological Mechanisms as EvolvedProblem-Solving Strategies

Psychological mechanisms have long been central phenomena m thefield of personahty These range from the defense mechanisms andmtrapsychic structures proposed by Freud to more recent concepts suchas ego control (Block, 1971), species-typical emotions (e g , Plutchik,1980), and defensive pessimism (e g , Cantor & Kihlstrom, 1987) Sev-eral articles in this issue view psychological mechanisms as the centralphenomena of personahty These mechanisms have a distinct biologicalstatus—they represent strategic solutions that have evolved because mour evolutionary past they successfully solved reproductive and survivalproblems

Kennck and his colleagues, for example, present data showing thatsex-differentiated mate preferences are evolved solutions to the some-what different mating problems faced by men and women Zuckermanargues that the psychological and physiological mechanisms hnked withsensation seeking and sensation avoidance may represent two distinctstrategies for adapting to a dangerous environment in which new stimuhcan be reproductively advantageous or reproductively damaging Rush-ton argues that epigenetic rules (routes from genes to behavior that m-

Introduction 13

elude psychological mechanisms) have evolved to solve complex prob-lems posed by social interaction

Tooby and Cosmides contend that evolved psychological mechanismsare likely to be highly specific rather than global or domain-generalMale sexual jealousy, for example, may solve the specific reproductiveproblem of increasing patemity certainty (e g , by producing behav-ior that fends off mtrasexual competitors or hmits a mate's access tothose competitors) But the psychological mechanism of jealousy con-tnbutes nothing toward solving a host of other reproductive problemssuch as finding a reproductively valuable mate or reanng offspring toreproductive age

A focus on psychological mechanisms as evolved strategies yields aunique perspective on the debate over personality consistency It sug-gests that consistency may not be found at the level of overt behaviorbecause different adaptive problems are confronted over time and oversituations, and each psychological mechanism can generate a host ofphenotypically diverse acts (e g , jealousy could produce the acts ofconcealing one's mate or derogating one's competitors) This suggeststhat the appropriate level of analysis for searching for personality con-sistency resides with endunng psychological mechanisms rather thanwith overt behavior

The concept of psychological mechanism as evolved strategy, how-ever, requires further theoretical and empirical elaboration At thisinchoate stage, it is not clear (a) precisely how psychological mecha-msms are to be described and demarcated from one another, (b) whatstandards of evidence should be required for invoking the concept ofevolved psychological mechanism, and (c) whether a comprehensivetaxonomy can be developed of the adaptive problems that humans haveevolved to solve The next decade of work in evolutionary psychologywill hkely center on this unit of analysis m personality psychology

CONCLUSION

Biological approaches to personality psychology are not new They dateat least back to the humoral theory of Galen that posited four bodilyfluids as biological determinants of personality What is new is sub-stantial progress in the accumulation of knowledge, m the precision ofmethodology, and in the sophistication of theoretical models With eachadvance, the biological approaches have confronted new frontiers

In the psychophysiology of Extraversion, for example, early theones

14 Buss

postulated individual differences in basal or tonic levels of physiologi-cal arousal It is now known that there is little evidence m support ofthe concept of "general arousal" across physiological subsystems Thisphysiological theory has been replaced by one that is replicably docu-mented—that introverts do show greater reactivity to sensory stimula-tion than do extraverts Although Eysenck's onginal hypothesis of dif-ferences in general arousal tumed out to be wrong, it catalyzed researchthat eventually led to a better understanding of the psychophysiology ofExtraversion

In previous decades, the field of behavioral genetics was centrallyconcerned with estimating the hentability of major personality dimen-sions such as Extraversion and Neuroticism It is now known that thesemajor dimensions show moderate hentability—conclusions reached byindependent investigators using different methods Attention has nowtumed to new issues such as the nature of environmental influence(shared versus nonshared), the ways in which genotypes and environ-ments correlate and interact, and the use of behavioral genetic methodsfor studying personality change over the life course

Perhaps the most exciting developments in the biological analysisof personahty have come from evolutionary theory Earlier evolution-ary approaches, although valuable m their time, were mired m nowoutdated notions of biological determinism and intractability, in super-ficial and misleading analogies with other primate species, in incorrectanalyses of adaptation and units of selection (e g , now discredited no-tions such as adaptations that exist for the "survival of the species"),in a lack of testable hypotheses, and m a virtual absence of empincalmethodology

Evolutionary approaches have moved beyond these earlier limitationsand are offenng exciting contributions to the analysis of personality Ata theoretical level, evolutionary biology informs central issues such asthe definition of human nature, criteria for identifying important indi-vidual differences, the enigma of genetic vanabihty, and conceptionsof adaptation and adjustment Perhaps the most salutary sign is the de-velopment of specific evolutionary models of particular domams (e g ,social exchange, mate selection, parent-child mteractions) that generatetestable empincal predictions

Beyond the exciting advances within all three biological approachesIS the begmnmg of bndges between the levels Evidence on the hent-ability of personality dispositions, for example, suggests to psycho-physiologists the personality dispositions that might be the most prom-

Introduction IS

lsing for an identifiable physiological substrate At a different level,the findings by Zuckerman on the psychophysiological proclivities ofsensation seekers and sensation avoiders point to the possible evolution-based strategies that each may be following The withm-species geneticvanabihty documented by behavioral geneticists was long ignored byevolutionary biologists who studied human behavior Now the enigmaof genetic variability is a major topic that evolutionists are confrontingAlthough each of these three levels of biological analysis proceeds withIts own models, assumptions, and methods, findings at one level arenow informing the models developed at the other levels

This optimistic assessment of the advances made m the biologi-cal analysis of personality no doubt underrepresents the problems thatface each approach Psychophysiological approaches are constrained bymeasurement techniques, the measurement samples required by manybehavioral genetic designs are often prohibitively large, and the amountof hard empirical data needed to test specific evolutionary hypotheses,although increasing rapidly, is still far from optimal

The advances represented by the contributions to this special issue,however, presage a central role for biological approaches to the studyofpersonality But biology will not cannibalize personality psychologyNor will It supplant the many approaches that have characterized thefield since its inception Indeed, a strong case can be made for fer-tilization both ways Traditional personality research paradigms havegenerated basic findings that biological perspectives must confront andaccount for It no longer makes sense, however, for the field to ignorethe biological foundations of its central phenomena These foundationsinclude evolutionary biology, behavioral genetics, and psychophysi-ology All three levels of analysis contribute to a biologically informedpsychology of personality

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Manuscript received July 30 1989 revised September 20, 1989