evolutionary biology and personality psychologyevolutionary biology and personality psychology. the...

Evolutionary Biology and Personality PsychologyToward a Conception of Human Nature and

Individual Differences

David M. Buss Harvard University

ABSTRACT: Although personality psychology sub-sumes the study of both individual differences andspecies-typical characteristics, the field has not yetresolved several key concerns: (a) what are the mostimportant species-typical characteristics; (b) whatare the most important ways in which individualsdiffer; and (c) how can species-typical characteristicsand individual differences be reconciled within ageneral theory of personality. Evolutionary biologyprovides one set of criteria for identifying thesecharacteristics and for designating relative importanceamong them. Genotypic universality, automaticity,and adaptation are examined as potential criteriafor identifying important species-typical characteris-tics. Heritability, inclusive fitness, sexual selection,and assortative mating are evaluated as criteria fordesignating important individual differences. Sug-gestions are made for resolving some of the conceptualand operational difficulties entailed by implementingthese criteria. It is argued that, although substantialproblems remain, evolutionary biology can provideone means for identifying relations between individualdifferences and species-typical characteristics.

Evolutionary biology and personality psychology,broadly conceived, share several common concerns.Both fields seek to identify enduring organismiccharacteristics and to locate their origins and func-tional significance in environments. Both fields dealwith past and present adaptation. And both grant acentral role to individual variation, which is thefocus of most personality research and the sine quanon of evolution. These shared concerns suggestintriguing potential connections. This article attemptsto offer directions for an integrative effort, whileidentifying some of the difficulties of this endeavor.

The first section of the article identifies severalkey issues in personality psychology, with particularattention given to the separation between approachesemphasizing species-typical tendencies and thosefocusing on systematic variation around those ten-dencies. The second section identifies themes inevolutionary biology that parallel those in personality

psychology, typological and population approaches,and highlights some of the alternative aims, assump-tions, methods, and limitations of each. The thirdsection outlines three major directions for linkingevolutionary biology and personality psychology. Thefinal section attempts to identify some of the mostpromising programs for future research.

Some Key Concerns in PersonalityPsychologyAlthough disagreement exists about the definingissues in personality psychology, the following ques-tions address one set of the field's prominent con-cerns:

1. What are the major enduring commonalitiesamong people in action, motivation, and cognition?This question subsumes study of the origins of themajor commonalities, their stability and change overtime, their relations to each other, their functionalsignificance, and their consequences. One majorcurrent concern is to establish criteria for consideringsomething species typical or part of human nature.1

2. What are the important enduring ways inwhich individuals differ in action, motivation, andcognition? This question embraces study of theorigins of major individual differences, their devel-opment (stability and change) over time, the relationsamong them, and their consequences. A major taskin personality psychology is to establish criteria foridentifying the most important ways in which indi-viduals differ (Allport, 1937; Buss & Craik, in press;Cattell, 1946; Goldberg, 1972; Wiggins, 1979).

I would like to thank Leda Cosmides, Kenneth H. Craik, StephanE. Glickman, Harrison G. Gough, Richard J. Herrnstein, ArthurR. Jensen, Douglas Kenrick, Gerald A. Mendelsohn, Daniel J.Ozer, Robert Plomin, Edward O, Wilson, and especially CarolynPhinney for helpful comments on earlier versions of this article.

Requests for reprints should be sent to David M. Buss,Department of Psychology and Social Relations, Harvard Univer-sity, William James Hall, 33 Kirkland Street, Cambridge, Mas-sachusetts 02138.

1 The term human nature is used here without reference tocontent and without commitment to particular units such asmotivational states, cognitive processes, or classes of acts.

October 1984 • American PsychologistCopyright 1984 by the American Psychological Association, Inc.Vol. 39, No. 10, 1 1 3 5 - 1 1 4 7

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3. How can the relations between the majorcommonalities and the major individual differencesbe conceptualized and understood? Subsumed bythis question are the following subquestions. Howdo findings of individual variation limit or impor-tantly qualify statements about the major common-alities? How do species-typical patterns constrainthe manifested range of individual differences? Howcan theory and research on the major commonalitiesand individual differences be integrated to form acoherent understanding of our species?

In the past five decades, a separation has de-veloped in the field of personality psychology betweentheory and research. Most personality theories focuson describing human nature, on identifying, inMaddi's (1980) terms, the "core tendencies" or"core characteristics" of the species. Psychoanalytictheory, for example, posits universality of energizingforce and of psychic apparatus, as well as invarianceof stage sequence. Dimensions of individual differ-ence can be derived from such theories. For example,variation can be attributed to differences in thespeed of transition through the universal stages orto differences in the current level attained. But theseindividual differences are subsidiary to commonattributes. Species-typical characteristics have usuallytaken precedence over individual differences in per-sonality theories.

In contrast, most current personality researchfocuses on individual differences: describing, ex-plaining, and identifying covariation between theways in which individuals differ. Indeed, many cur-rent personality psychologists define the field solelyby reference to this issue: "Personality is that branchof psychology which is concerned with providing asystematic account of the ways in which individualsdiffer from one another" (Wiggins, 1979, pp. 395;Wiggins, Renner, Clore, & Rose, 1971). This focusseems to exclude from study the key issue of species-typical characteristics that forms the core of mostpersonality theories.

In sum, although the prominent concerns inpersonality psychology involve individual differencesand common human characteristics, the field hasproposed few criteria for identifying the most im-portant individual differences, the important species-typical characteristics, and the relations betweenthese sets of attributes (but see Cattell, 1946; Eysenck,1967; Goldberg, 1972;Gough, 1968; Norman, 1963;Wiggins, 1979). Evolutionary biology provides oneframework for investigating these issues.

Typological and Population Approaches inEvolutionary BiologyIt is an oversimplification to speak of evolutionarybiology as though it were a single, monolithic,

consensually agreed upon set of ideas (Mayr, 1982).Major differences surround key issues such as theunit and level of selection (e.g., gene, individual, orgroup; see, e.g., Alexander, 1979; Dawkins, 1976;Williams, 1966), the amount of genetic variationwithin species (e.g., Wilson, 1978), and the explan-atory status of evolutionary theory (e.g., Searle,1978). At least two major themes may be identified,each with separate aims, assumptions, methods, andlimitations.

The first, here called "typological thinking"(adapted from Mayr, 1963), has as its aims discov-ering basic species-typical characteristics, discoveringthe adaptive significance of species traits, monitoringthe genetic basis of species behavior, and predictingspecies-typical responses from evolutionary postu-lates.2 Applied to Homo sapiens, this set of concernsreduces to attempts to uncover the "pan-humanpsyche," the "traits of human nature" (Wilson,1978, p. 34), the "human biogram" (Tiger & Fox,1971), or the "universal human ethogram" (Fuller,1983, p. 460). An assumption (sometimes implicit)of typological thinking is that within-species variationis relatively small, a deviation from the speciesarchetype, and subsidiary in importance to themajor topography of species traits.

Among recent evolutionary approaches, socio-biology has tended to emphasize species-typical traits(as has ethology). Genetic variability within speciesis recognized, but does not occupy a central role insuch approaches. According to Wilson (1978), "eitherpossibility—complete cultural determination versusshared cultural and genetic determination of vari-ability within species—is compatible with the moregeneral sociobiological views of human nature" (p.43). Indeed, even though genetic heterogeneity withinpopulations is required by evolutionary models ofsociobiology (Fuller, 1978, 1983), "sociobiology hastended to downgrade [within-species] differences andto stress the universal of human nature that havebeen shaped by similar selective forces everywhere"(Fuller, 1983, p. 470; emphasis added).

Comparative phylogenetic analysis is one of themajor methods of typological thinking (Wilson,1975),3 Species comparisons highlight the similaritiesof our species with others, generating, for example,general mammalian traits (e.g., group living) orgeneral primate traits (e.g., hierarchical social ar-rangement), as well as the unique features of eachspecies. Correlating variations in ecological nicheswith variations in the traits of species that occupy

2 It may be more accurate to speak of "evolutionary ap-proaches with typological (species-typical) emphasis," rather than"typological approaches." The latter is used here for expositionalclarity.

3 Other methods include those of behavioral ecology andlaboratory-based experiments.

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Table 1A Comparison of Typological and Population Approaches

Typological Population

Aims

Assumptionsabout variation

To discover "human nature"To discover the adaptive significance of

species traitsTo monitor the genetic basis of species

behavior

Variation is small and subsidiary inimportance to species-typicalcharacteristics

Primary methods Comparative phylogenetic analysis

Limitations Variability of traits limits utility of postulatingspecies-typical attributes

Defined so broadly that any outcome ofquantitative genetic research iscompatible

To discover variation among speciesmembers

To monitor the genetic basis of observedvariation

To discover the reasons for which variantsare selected

Variation is real and consequential •

Quantitative and population genetics

Findings limited to extant populationvariation and to environments withinwhich population is studied

Will not discover species traits

these niches provides one method for inferring theadaptive significance of species traits (see Lewontin,1978, and Williams, 1966, for detailed discussionsof adaptation).

In sum, typological thinking takes the speciesas the focal unit and through comparative phyloge-netic analysis seeks to identify the major traits ofeach species and the adaptive significance of each ofthose traits. Although within-species genetic variationis sometimes recognized, it is typically viewed assubsidiary in importance.

In contrast, "population thinking" (adaptedfrom Mayr, 1963) has as its aims discovering variationamong conspecifics, monitoring the genetic basis ofobserved variation, and discovering the forces bywhich variation itself increases or decreases, as wellas the forces causing some variants to increase ordecrease in frequency in the population. Quantitativegenetics is a primary method of population thinking,and it focuses on partitioning phenotypic variationinto genetic and environmental sources and identi-fying the interactions and correlations among thesecauses of variation.

Both typological and population approaches inevolutionary biology carry limitations due to theirprimary focus. Population approaches are limitedin that the methods of quantitative genetics will notdiscover species-typical traits. Leggedness, for ex-ample, would have a heritability near zero becausevariations from two-leggedness are due mostly toenvironmental sources (e.g., accidents), rather thanto genetic sources (Loehlin & Nichols, 1976). Find-ings are also limited to extant population variation

and to the environments within which the populationis studied. Heritability can vary as a function of therange of environments. And increases in trait dis-persion (e.g., one genetic consequence of assortativemating) may increase heritability estimates. Herita-bility estimates cannot be viewed as eternally fixed;instead, they reflect the dispersion within the popu-lation within the existing range of environments ata given period of time.

Typological approaches, in contrast, are limitedin that observed variability of traits within specieslimits the utility of postulating species-typical char-acteristics.4 Measures of central tendency lose indi-vidual predictability and descriptive utility as dis-persion about them increases. Because typologicalapproaches emphasize species universals, phenotypicvariation traceable to genotypic variation is oftenignored or viewed as tangential to the typologicalenterprise. Thus, the field of quantitative geneticstends to be viewed as independent when it could beintegral to more typologically oriented approaches(see, e.g., Fuller, 1983; Thiessen, 1979). Table 1summarizes the broader aims, assumptions, methods,and limitations of typological and population think-ing as they are conceived here.

These two themes of evolutionary biology canbe closely aligned with the questions identified earlieras major concerns of personality psychology (cf.

4 This limitation applies primarily to presumptively "obligate"trails (genes producing the same phenotype under all environmentalconditions commonly encountered) rather than to "facultative"traits (genes producing different phenotypes in different environ-ments). See also the section on directions for rapprochement.

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Campbell, 1975; Cunningham, 1981; Dickstein,1979;Freedman, 1971, 1979; Hoffman, 1981;Hogan,1983; Kenrick, Dantchik, & MacFarlane, 1983;Rushton, 1984; Van den Berghe & Barash, 1977).Typological approaches may aid conceptualizationof, and research on, the major commonalities of ourspecies (i.e., human nature). Population approaches,on the other hand, may be used to identify theimportant individual differences and their implica-tions. It is probably no coincidence that sociobiolo-gists, rather than behavior geneticists, have speculatedabout the implications of their (often) typologicalapproach for conceptualizing human nature (see,e.g., Barash, 1977; Symons, 1979; Wilson, 1978).Behavior geneticists, on the other hand, tend toavoid discussion of human nature, restricting studyprimarily to variation within species. It has beenargued, for example, that behavior genetics candesignate importance among individual differenceswithin our species by identifying the traits with thehighest heritabilities (see, e.g., Eysenck, 1967, 1981).

But the application of typological approachesto discussions of human nature and the use ofbehavior genetics to identify the important ways inwhich individuals differ logically follow from priorconceptual issues that must be resolved or at leastmade explicit if these applications are to be success-ful. Indeed, the absence of denning criteria for"human nature" in articles and books carrying thisphrase in their titles may reflect the implicit standingof these issues in the application of principles ofevolutionary biology to personality psychology.

Three major issues are considered here: (a)What criteria can be established for consideringsomething "part of human nature," or an importantspecies-typical commonality? (b) What criteria canbe established for considering something an impor-tant way in which individuals differ? (c) How cantypological and population approaches be reconciledwithin a general theory of personality?

Some Directions for Rapprochement

Establishing Criteria for Determining ImportantSpecies- Typical Characteristics

Explicit criteria must be established for consideringsomething part of human nature, or an importantspecies-typical characteristic. Inclusion criteria fordeciding whether or not a feature is part of humannature have not generally been made explicit. Untilsuch criteria are established, theorizing and researchon human nature remain at the level of favorite listsand constructs of convenience. It is impossible atthis early stage to provide an exhaustive list ofpotential criteria. Several candidates, some of whichhave been implicit in current formulations, can becritically examined.

Universality. Perhaps the most common im-plicit criterion for designating an attribute part ofhuman nature concerns its prevalence or universality.This criterion assumes that manifested ubiquityreflects (or correlates with) featural importance.Features found across cultures, races, and populationsare assumed to be more part of human nature thanthose features that are unique to certain subgroupsor individuals. Attributes of limited generality arenot considered to reflect human nature.

The level of analysis to which the universalitycriterion applies must first be specified. Questionssuch as "Is aggression part of human nature?" canbe posed at the phenotypic or genotypic level. Theselevels of analysis are often conflated in efforts tospecify the traits of human nature. If posed pheno-typically, the appropriate index would be whetheror not each species member displays aggression.If posed genotypically, however, universalities ofaggression displayed by each species member wouldnot be required, although other conditions wouldhave to be met (see below). In contrast to thephenotypic and genotypic levels, questions such as"Do all human groups develop dominance hierar-chies?" are posed at the level of social structure.Therefore, universalities across groups, rather thanacross individuals, would be required. However for-mulated, the universality criterion calls for someform of pan-species invariance at a particular levelof analysis.

Within-species variability, therefore, poses amajor problem for the universality criterion. Phe-notypically, the greater the manifested dispersionwithin species, the more statements of universalitymust be qualified. Similarly, at the level of socialstructure, variability among groups empirically un-dermines statements of universality. Problems at thegenotypic level are even more pronounced becausethey involve limitations imposed by current meth-odologies as well as by empirical findings.

The universality criterion at the genetic levelincurs problems when the methods for establishingthe genetic basis for a trait are examined. The fieldof behavioral genetics has developed methods forapportioning phenotypic variation into genetic andenvironmental sources (Falconer, 1960; Fuller &Thompson, 1978; McClearn & DeFries, 1973;Plomin, DeFries, & McClearn, 1980). That is, ob-served differences between people are traced toheritable and nonheritable differences. These geneticmethods cannot be applied to discerning the majorcommonalities because, by definition, commonalitiesare attributes for which little individual variationexists.

In addition to these methodological difficulties,which can be overcome in principle, empirical find-ings from the field of behavior genetics pose a second


problem for postulating universality at the geneticlevel. Empirically, most personality traits such asdominance, aggression, and extraversion appear toshow moderate heritability (e.g., Fuller & Thompson,1978; Loehlin & Nichols, 1976). That is, phenotypicdifferences are traced partly to genetic differenceswithin our species, rather than to varying environ-mental conditions that differentially potentiate auniversal genetic substrate. Findings of genetic vari-ability compromise the invocation of a universal"human biogram" and throw into question whetheraggression (or any other trait) is a "trait of humannature" rather than just a feature in the behavioralrepertoires of a subset of individuals.

Spontaneity, automaticity, and intractability. Asecond potential criterion by which attributes canbe evaluated for potential inclusion as part of humannature is whether they are produced spontaneously,with little or no environmental impetus or incentive.The tendency of pigs to revert spontaneously torooting behavior in spite of training and reinforce-ment schedules to the contrary, for example, wouldconstitute evidence that rooting is part of pig nature(Breland & Breland, 1961, 1966). Similarly, if dom-inance hierarchies spontaneously emerge in groupsof adults and children, where no particular instruc-tions or external requirements were presented, thiswould fulfill the spontaneity criterion, and suchstructures would be included as potential candidatesfor being part of human nature. In Skinner's (1938)terms, these behaviors are operants, emitted spon-taneously when no strong environmental forces reign.

A related criterion, one that builds in an envi-ronmental contingency in its specification, is auto-maticity—the extent to which a behavior or attributeis reflexively displayed in response to a given envi-ronmental elicitor. The knee-jerk and moro reflexeswould be examples of respondent attributes. Attach-ment (Bowlby, 1969), emotional expression (Darwin,1872/1965; Plutchik, 1980), and fixed action patterns(e.g., Tinbergen, 1951) might be additional examples.The spontaneity and automaticity criteria both in-voke innate, unconditioned, and relatively difficultto modify features of behavior, with the two differingonly in the role played by the specific, efficient,antecedent cause. Features of behavior that arespontaneously emitted and automatic are presumedto be more a part of the "human biogram" thanthose features requiring extensive training or condi-tioning,

The automaticity criterion encounters difficul-ties in that well-learned responses can become quiteautomatic in the ease or rapidity with which theyare elicited. Yet their origins lie with previous learn-ing history rather than being intrinsic to the species.Similarly, what appears to be spontaneously emittedalso may depend upon prior learning schedules,

which may vary from individual to individual, cultureto culture, and even generation to generation. Thereis no guarantee that spontaneity derives from anintrinsic species nature and not from particularreinforcement schedules, although there may beconditions (e.g., smiling behavior in congenitallyblind persons) that may permit reasonable infer-ences.5

A criterion related to spontaneity and auto-maticity is intractability—the idea that species attri-butes that are difficult to alter are more part ofhuman nature than more ephemeral features, orthose easily altered by environmental forces. Thecriterion of intractability has the advantage that itsoperationalization is straightforward, entailing as-sessment of the effectiveness of imposed contingen-cies. Its drawback is that limitations or gaps inknowledge could preclude change, easily observedonce effective environmental contingencies are dis-covered.

Adaptation. A third criterion embedded inevolutionary biology involves adaptation. Speciesfeatures that solve ecological problems and enableorganisms to function well (survive, reproduce) intheir niches can be viewed as more important orintrinsic to our "human biogram" than featureslacking adaptive functions. Implicit in this criterionis a defensible means for partitioning the behavioralstream into functionally significant units (e.g., actsor classes of acts). In addition, some reasonablecategorization of niches is required for calibratingorganismic behavioral units with corresponding en-vironmental units so that functional significance canbe identified.

Assuming these requirements can be met, theconcept of adaptation carries two additional prob-lems (Lewontin, 1978; Williams, 1966). First, theconcept implies a preexisting environment that posesa problem to which adaptation is the solution. Theconcept appears to bypass the role of the organismin creating the niche. Niches may be created specif-ically to correspond to preexisting organismic attri-butes (Buss, 1981, 1984a, 1984b, in press; Plomin,DeFries, & Loehlin, 1977; Scarr & McCartney,1983). It makes little sense to postulate adaptivefunctions for ecological problems when the ecologyitself is forged by the organism.

In addition, species traits are often involved ina variety of functions. It is particularly difficult todesignate the specific problem solved by a giventrait, or to infer the selective pressures that mayhave been operating previously to create the trait. Aplausible function can be invented for any charac-teristic. But such post hoc arguments, however per-

5 Even this smiling behavior, however, could arise throughoperant conditioning.


suasive, do not provide clear criteria for orderingspecies traits by importance (but see Alexander,1979, for a set of hypotheses and predictions; seeGlickman, in press, for a more general historicaltreatment in the context of comparative psychology).

In summary, attributes under consideration forinclusion as part of human nature can be evaluatedagainst certain biological criteria. Each of thesecriteria possesses problems. Some are conceptual(e.g., at what level of analysis should the universalitycriterion be invoked?). Others are empirical or op-erational (e.g., findings of genetic variation; unde-veloped methods for discerning genetic commonali-ties within species). And still others remain prob-lematic, even after conceptual clarification andoperationalization, due to untested premises (e.g.,assumptions about environmental effects). Additionaland related criteria could have been discussed (e.g.,speed of learning, developmental invariance), andshould be examined in the future. The more limitedaims of this section, however, are to highlight severalcriteria that are currently implicit in existing con-ceptions of human nature, to point to problems andlimitations in their application, and to underscorethe requirement that explicit criteria must be estab-lished for considering something part of our "humanbiogram" if evolutionary biology is to be effectivelylinked with personality psychology.

Establishing Criteria for Identifying ImportantIndividual Differences

A second major task for integrating evolutionarybiology with personality psychology concerns iden-tifying the important individual differences fromamong the hundreds or thousands that are availableor imaginable.6 Criteria can be instated to orderindividual differences by importance and priority.Differences in ear-wiggling ability must be distin-guished from variations in dominance or in accessto sexual partners—individual differences with dem-onstrated evolutionary consequences. From thestandpoint of evolutionary biology, four criteria maybe employed to designate importance: heritability,inclusive fitness, sexual selection, and assortativemating.

Heritability. Heritability may be offered as onecriterion for ordering individual difference variablesfrom most to least important. Differences amongindividuals that are traceable to genetic differencescan be considered important because they providethe variation necessary for evolution. "These indi-vidual differences are of the highest importance forus, for they are often inherited . . . and they thusafford materials for natural selection to act on and

6 This problem is logically related to discovering importantequivalence classes in behavior (see, e.g., Herrnstein, 1977).

accumulate" (Darwin, 1859, pp. 59-60). Estimatesof heritability can be derived from family studies,twin studies, and adoption studies, and convergenceamong these methods can be examined empirically.Heritability is particularly attractive as a criterionbecause it provides a direct link between evolutionarybiology and personality psychology.

Several considerations, however, may seriouslyqualify the standing of heritability as a criterion forimportance. The first is the argument marshaled byThiessen (1972), in which individual differencesshowing high heritability are viewed as "geneticjunk." Attributes may show great variability preciselybecause of their unimportance in evolutionary his-tory. Variability may be low for traits that have beensubjected to severe selection pressure and are there-fore ubiquitous among species members (see alsoFalconer, 1960). Nonetheless, whatever the priorevolutionary standing of heritable differences, suchdifferences do provide "materials for natural selectionto act on and accumulate" and may acquire impor-tance for that reason.

A second problem is more pragmatic. Theresolution of behavior genetic studies has not beensufficiently articulated to establish differential herit-ability, at least within the personality realm (see,e.g., Loehlin, 1978; Loehlin & Nichols, 1976). Ifmoderate heritability can be shown for all personalitytraits, then the heritability criterion cannot be em-ployed to order dispositions by importance. Opti-mism has been expressed, however, that differentialheritability must exist (e.g., Loehlin, 1978), andthere are preliminary indications that can be discov-ered empirically (Carey, Goldsmith, Tellegan, &Gottesman, 1978; Loehlin, 1982;Zonderman, 1982).

A third qualification of the heritability criterionis that estimates are inextricably linked to the existingpopulation distributions and to the particular rangeof environments during the time period withinwhich estimates are made. Thus, two major factorscould alter the ordering of dispositions by heritabilitymagnitudes: (a) changes in the range and type ofenvironments and (b) changes in population distri-butions on dispositions.

Changes in environment could be of the formof increasing uniformity (e.g., due to increases incommon influences such as television and othermedia), which might amplify heritability estimatesof some traits. Alternatively, compensatory environ-ments and "coercion toward the biosocial norm"(Cattell, 1973) could decrease heritability on thoseattributes toward which provisions and coercion aredirected. Magnitude orderings of dispositions byheritability can and probably do change as a functionof altered environments.

Heritability estimates can also change as afunction of changes in population distributions. One


such change may be brought about by assortativemating (Eckland, 1968; Jensen, 1978; Vandenberg,1972), which can increase phenotypic and genotypicvariance and hence heritability. Differential assor-tative mating for some traits and not for otherscould produce shifts in the ordering of traits usingthe heritability criterion. Thus, changes in populationdistributions—both genotypic and phenotypic—canalter heritability estimates.

In linking evolutionary biology with personalitypsychology, heritability provides one criterion bywhich dimensions of individual difference may beordered by importance. Care must be taken, however,not to infer previous evolutionary adaptation solelyfrom findings of heritability.7 In addition, presentheritability ordering should not be viewed as fixed.Heritability estimates can change with alterations inenvironments and population distributions. Finally,application of the heritability criterion awaits suffi-ciently precise demonstrations of differential herita-bility.

Inclusive fitness. Individual difference variablesmay be ordered by their correlation with geneticperpetuation or inclusive fitness (Hamilton, 1964).The most direct method for operationalizing inclusivefitness would be to derive indices of genetic perpet-uation that include offspring of biological relativesas well as an individual's own offspring. The degreeto which different traits covary with subsequent generepresentation would rank these traits by importance.

Traits showing significant heritabilities that arecorrelated with measures of inclusive fitness take onintrinsic importance within evolutionary biologybecause they are the traits for which frequencies canbe expected to increase (positive correlation) ordecrease (negative correlation) in succeeding gener-ations. Such traits become part of the complexselection process that forms the core of evolutionarybiology. They acquire additional importance forpersonality psychology because such changes couldproduce alterations in the mean frequencies of man-ifested acts that are encompassed by these traits.

Assessing trait-fitness linkages is a never-endingprocess, however, because changes in culture, envi-ronment, and even trait distributions are likely toalter such correlations. One speculative example isthat the current trend toward increased longevitymay have an impact on trait-fitness correlations.Although increases in longevity may have little effecton direct production of offspring, longer life spansmay increase the opportunities to facilitate the geneticperpetuation of distant kin, such as great grandchil-dren and grandnieces. This implies a shift in theratio of inclusive to individual fitness opportunitiesthat could tilt selection in favor of traits that arecorrelated with altruism, such as generosity or em-pathy (Hoffman, 1981).

In addition to continuous evaluation of changesin trait-fitness associations, care must be taken notto erroneously infer trait-fitness covariation solelyfrom evidence of heritability (see footnote 7). Finally,the concept of inclusive fitness must be adequatelyoperationalized before it can be effectively employedfor the conceptual functions it could serve.

Sexual selection. Darwin initially proposedthe concept of sexual selection to account for attri-butes such as the plumage of peacocks that hebelieved could not be explained by the process ofnatural selection (Darwin, 1871). Sexual selectionsubsumed two related processes: intrasexual selection,or competition between members of one sex foraccess to members of the opposite sex, and intersex-ual (epigamic) selection, the differential choice ofmating partners possessing preferred characteristics.Although sexual selection operates through differ-ential reproduction of individuals and is now rec-ognized as being subsumed by natural selection orinclusive fitness (cf. Campbell, 1972; Trivers, 1972),the processes defined by sexual selection can beimportant proximate mechanisms through whichdifferential gene representation is achieved.

By identifying individual differences central tomate choice and competition, the study of sexualselection provides a third basis for designating im-portance among individual differences. Potentialpartners with preferred attributes will be more fre-quently chosen for mating; those relatively deficientin these qualities will tend to be excluded andtherefore will be less represented in subsequentgenerations. Species-typical preferences exist to theextent that consensus occurs among species membersabout the valued attributes in potential mates (e.g.,intelligence, cooperativeness). Traits can thereforebe ordered on the degree to which they are consen-sually valued, and are thus intrinsic to mate choice.As with the heritability and inclusive fitness criteria,however, preferred mate characteristics may be time-and culture-bound. The extent to which character-istics central to sexual selection transcend culturesand eras remains unanswered empirically (cf. Bate-son, 1983; Campbell, 1972), but poses an intriguingresearch agenda (Buss & Barnes, 1984).

Assortative mating? A fourth criterion thatmay be employed for identifying the most importantindividual differences involves patterns of nonrandom

' It is possible, however, that certain patterns of heritabilitymay permit inferences about directional selection. Findings oflow heritability in the narrow sense (additive genetic variance)combined with high heritability in the broad sense (due to geneticdominance) are theoretically consistent with directional selectionfor a trait in evolution (Fisher, 1930; Jensen, 1983).

8 Although assortative mating is discussed here in the contextof individual variation, it has also been examined as a species-typical trait (Fox, 1979; Thiessen & Gregg, 1980).


mating. Assortative mating, the coupling of individ-uals based on resemblance on one or more traits, iscurrently the most striking deviation from randommating, or panmixia.9 In the past several decades,moderate levels of assortment have been documentedfor cognitive and intellectual abilities (e.g., Johnsonet al., 1980; Price & Vandenberg, 1980; Watkins &Meredith, 1981; Zonderman, Vandenberg, Spuhler,& Fain, 1977), and low but consistently positivelevels of assortment have been observed for person-ality and interest variables (e.g., Buss, 1984a, 1984b;Jensen, 1978; Vandenberg, 1972; Price & Vanden-berg, 1980).

Within the context of evolutionary biology, theimportance of assortative mating for identifyingimportant traits lies with the profound genetic con-sequences that follow from assortment. For example,assortative mating can increase the variability of atrait in the next generation, can create correlationsamong traits that were initially uncorrelated (e.g.,between physical beauty and IQ), and can increasecorrelations among certain biological relatives (e.g.,between parents and offspring) on those traits forwhich assortment occurs. Because of these wide-ranging genetic and social consequences, assortativemating can be used as a fourth criterion for orderingindividual difference traits by importance.

The primary problem with assortative matingas a criterion is that differential assortative mating(i.e., greater assortment for some traits than forothers) has not yet been demonstrated, at leastwithin the personality domain. Failure to find dif-ferential assortment may be due to inadequacies ofthe measuring instruments. Built-in covariationamong traits due to item overlap can preclude thediscovery of differential assortment that might bepresent. Alternatively, low but consistently positivelevels of assortment may be the rule in the personalitydomain, with little or no differential assortmentoccurring.

Although differential assortative mating providesa potential criterion for identifying the most impor-tant within-species differences, application of thiscriterion must await reliable demonstrations of dif-ferential assortment. Even when more precise as-sessment becomes available, this criterion will belimited by the fact that the traits on which assortmentoccurs may change from generation to generation.Continuous population monitoring will be necessaryto prevent outmoded generalizations.

In sum, four criteria have been proposed bywhich important individual differences may be iden-tified and ordered: heritability, inclusive fitness, sex-ual selection, and assortative mating. All providedirect links between evolutionary biology and per-sonality psychology. Other criteria could be invokedfor identifying and ordering dispositions by impor-

tance, such as covariation with access to resources(cf. Buss & Craik, in press). Although difficultiesqualify these criteria, each provides a starting point.Once important human commonalities and individ-ual differences have been identified, the further stepof specifying the relations between them must betaken.

Specifying Relations Between HumanCommonalities and Individual Differences

The relations between important species-typicalcharacteristics and important individual differencesshould be identified and their implications under-stood.

Perhaps the largest division in modern psy-chology concerns what Cronbach (1957) has called"the two disciplines of scientific psychology." Cron-bach framed the division as a contrast betweencorrelational and experimental psychology, but theseterms may be too closely tied to particular methods.Another way to conceptualize this division is to askwhether general laws are sought (common to mostor all species members) or whether individual vari-ation is the primary focus of study. In spite ofCronbach's (1957) incisive statement of the problemand its reiteration nearly two decades later (Cron-bach, 1975; see also Underwood, 1975; Vale & Vale,1969), little progress has been made in bringingthese two disciplines into closer alignment. Linkingevolutionary biology and personality psychology mayfoster this integration.

In examining the relations between speciestraits and within-species differences, it is necessary(a) to separate the different conceptual levels ofanalysis across which relations can be identified and(b) to separate the descriptive properties that areapplicable to each level. Three levels of analysis areparticularly relevant for considering the relationsbetween species traits and individual differences:genotypic structure, phenotypic structure, and social(group) structure.

When species-typical characteristics and within-species dispersion are examined at different levels ofanalysis, questions about their causal connectionsbecome salient.10 Within this hierarchy, causal rela-tions are commonly viewed as extending primarily

9 Other deviations from panmixia include selective mating,polygamy, inbreeding, and outbreeding.

10 It is possible, of course, to examine the relations betweendifferent attributes occurring at the same level. Interactions ofalleles at a single locus on the homologous chromosome (domi-nance) and of alleles at different loci (epistasis) are types ofrelations at the genetic level. Similarly, individual differences inheight can causally affect individual differences in dominance,both at the phenotypic level. This section, in contrast, is concernedwith causal relations between dispersion and species typicality,and it is at different levels of analysis that these relations are mostapparent.


from lower to higher levels. Species-typical genotypicstructures, for example, can affect phenotypic dis-persion through interaction with features of theenvironment. Phenotypic dispersion, in turn, cancreate species-typical emergents at a higher level.But species-typical emergents such as social hierar-chies can also feed back causally to the lower levels,both genotypic and phenotypic. Examples of theserelations between levels are given below:

1. Genotypic species-typical traits can producephenotypic individual differences. One type of con-nection occurs when species-typical traits cause ob-served dispersion through their interaction with fea-tures of the environment. For example, nepotism(altruism toward kin) may be a genotypic species-typical characteristic, yet enduring phenotypic dif-ferences in altruistic behavior are predictable fromthe number and genetic closeness of conspecifks inproximity. Sherman (1980) found that individualdifferences in alarm calling when faced with apotential predator (an altruistic act because it callsattention to and endangers the alarm-caller) arepredictable among female ground squirrels from thenumber of genetic kin in the immediate surround-ings. Thus, nepotism may be a genotypic species-typical characteristic, with phenotypic differencescaused by the interaction of this proclivity withenvironmental features. Evolutionary biology is use-ful in identifying these relations because it highlightsthe important environmental variables, such asnumber of kin in proximity, that are likely tointeract with species proclivities, such as nepotism,to produce individual variations.

2. Phenotypic individual differences can causeemergent social characteristics that are species-typi-cal. A second type of connection occurs when indi-vidual differences create a species-typical emergent.Because resources are often finite and individualsdiffer in their ability to attain them, the formationof status hierarchies based on access to resourcesmay be a species characteristic that inevitablyemerges in human groups (Hogan, 1983; Lopreato,1984). Thus, the existence of individual differencesin attributes such as intelligence, dominance, andcooperativeness may invariably lead to the formationof hierarchies. Evolutionary biology is useful inidentifying these relations because it calls attentionto those individual differences that may be linked tosurvival (e.g., intelligence) and to gene representation(e.g., dominance)—individual differences likely toform the basis for the hierarchy. Thus, individualdifferences can causally produce emergent species-typical characteristics,

3. Common species structures can produce anincrease (or decrease) in genotypic and phenotypicvariation. More subtle causal connections occurwhen emergent group structures affect genotypic

and phenotypic variation. One function that hier-archies serve, for example, is to govern who mateswith whom. Empirically, strong marital assortmentoccurs for socioeconomic status. As a consequence,assortative mating will also occur for those attributes(e.g., intelligence, dominance) that tend to covarywith status. Assortative mating increases genotypicvariance on those attributes for which assortmentoccurs, producing greater frequencies of individualsat the tails of the distribution (Crow & Kimura,1970; Jensen, 1978; Vandenberg, 1972). Thus, asocial hierarchy, because it is linked with assortativemating, can causally affect the amount of genotypic(and phenotypic) variation occurring in subsequentgenerations—an increase in the dispersion that wasin part responsible for the construction of the hier-archy to begin with. At the same time, geneticdispersion may be partly responsible for the creationof the hierarchy. In sum, the causal and reciprocalconnections between species-typical and individualdifference characteristics can extend across all threelevels of analysis.

4. Individual differences can be unrelated tospecies-typical traits. In many cases, species traitsand within-species dispersion may be unrelated toeach other. Separate levels of analysis are oftenuseful precisely because they provide insights notavailable through, or connected with, alternativelevels. There need not be ubiquitous connectionsbetween levels of analysis. And where there areconnections, they need not occur between species-typical and individual difference characteristics. Thedescriptive task entailed by analysis of both types ofattributes requires identifying the absence, as wellas the presence, of these connections.

DiscussionThe field of personality psychology is centrally con-cerned with the traits that characterize our speciesas well as with the major ways in which individualscharacteristically differ. Although most current per-sonality research focuses upon individual differences,personality theories almost invariably postulate corehuman characteristics. Within such theories, indi-vidual differences typically take on derivative andsubsidiary roles. These themes are also seen inevolutionary biology, where attempts to discover the"human biogram," the "pan-species psyche," or the"traits of human nature" form disciplines separatefrom biological approaches, such as quantitativebehavior genetics, that focus more centrally on vari-ation within species.

It has been argued that certain conceptual andoperational inadequacies must be addressed beforeintegration is possible. Methodologically, researchinstruments must be coordinated with the central


questions posed by evolutionary biology. Explicitconceptual criteria should be instated for consideringa given feature, attribute, or pattern as a speciestrait. Criteria for determining the most importantways in which individuals differ also must be estab-lished. Perhaps most important, conceptual frame-works should be developed within which importantindividual differences can be integrated with species-typical characteristics. Evolutionary biology providesone framework for addressing these issues.

This article has attempted to highlight thefeatures of evolutionary biology most relevant to theconcerns of personality psychology. To examine thespecies-typical questions posed by evolutionary bi-ology, methods could be developed that do notderive meaning solely from comparisons with otherindividuals (or with other attributes within individ-uals), as do nearly all current methods within per-sonality psychology. Inclusion criteria for consideringa given feature a species trait (e.g., genotypic uni-versality, adaptation) should be critically examined.Criteria implicit within existing conceptions can bemade explicit and operationalized so that potentialcandidates for inclusion are examined empirically.In the individual difference domain, four preliminarycriteria—heritability, inclusive fitness, assortativemating, and sexual selection—can be operationalizedcurrently and warrant further attention. Becauseproblems exist with each of these criteria, it may beasked what this analysis has achieved. Given thenumerous difficulties, what are the most promisingfuture directions?

The field of personality psychology currentlyseems fragmented. The study of isolated traits with-out explicit rationale remains the norm, not theexception (Goldberg, 1981; Wiggins, 1979). Linkagewith evolutionary biology provides one set of criteria,albeit incomplete, for identifying traits and for es-tablishing relative importance among them. Thisdoes not undermine the importance of nonbiologicalcriteria (see Buss & Craik, in press, for an extendeddiscussion of alternative criteria). But it is likely thattraits linked with biological processes such as assor-tative mating, sexual selection, and inclusive fitnessmay be precisely those that fulfill other importantcriteria such as those of psychological, social, andsociological importance.

This potential link between biological and socialscience phenomena suggests that importance itselfmay be identified by the range of impact within andacross disciplines. Mate selection, for example, ac-quires importance because it is a social and culturalprocess based partly on biological principles thathave genetic consequences and sociological impli-cations (Buss, 1984a, 1984b). Traits linked withcriteria anchored in evolutionary biology acquireimportance partly through their range of impact

across different disciplines and across different levelsof analysis.

In addition to identifying a set of criteria forimportance, this analysis points to some of theresearch efforts needed to utilize them. In the domainof individual differences, for example, research couldexamine preferences in mate selection, differentialassortative mating, and differential heritability.Loehlin (1982) has provided a promising lead fordiscovering differential heritability by finding thatthe factors of extraversion and neuroticism havehigher heritabilities than do factors orthogonal tothese traits, such as stereotyped masculine interests,intolerance of ambiguity, and persistence.

By illustrating some ways in which typologicaland population approaches can be linked, this anal-ysis also aligns the study of individual differencesmore closely with the search for general laws thatoccupies most of psychology. One important direc-tion for this rapprochement involves identifyingstable classes of environmental factors that consis-tently moderate phenotypic expression. Hypothesesderived from evolutionary theory, such as the envi-ronmental conditions under which nepotism willand will not occur, provide one means for identifyingsuch factors.

In this context, assessment methods are neededthat produce interpretable central tendencies forcomparative analysis. One such method can bederived from the act frequency approach (Buss &Craik, 1983a, 1984), in which frequencies of actswithin certain categories (e.g., dominance or aggres-sion) are recorded by observers in the natural audi-ence over a specified period of observation. Acrosspersons and environments, these frequency countsyield "modal human tendencies" (Buss & Craik,1983b, p. 396). The concept of modal tendency isperhaps more appropriate than "species-typicalcommonality" because the presence of a centraltendency rarely precludes individual variation forbehavioral categories. Indeed, by identifying thedegree of variation, these methods can specify thedegree to which different traits approximate speciestypicality.

From this perspective, emphasis on central ten-dencies as well as on variability highlights the relativelyneglected descriptive task that faces personality psy-chology. The finding that altruistic acts may occurtwice as frequently as do aggressive acts, for example,or that the ratio of the two modal frequencies maybe predictable from environmental factors such aspopulation density and longevity, shows importantdescriptive characteristics of our species even priorto causal analysis. Different modal tendencies withindifferent classes of environments yield general lawsin the form of hypothetical propositions, or "if. . .then" statements, and suggest that individual variation


deriving from stable occupancy of different environ-ments may be systematic and predictable rather thanrandom or capricious. Even more transient interin-dividual and intraindividual differences may be trace-able to such "if, . . then" algorithms. This schemegrants a central role to individual variation in thecontext of these contingent relations.

Detailed analysis of species-typical problemsprovides another promising focus for this integration.Because the majority of humans enter into maritalrelationships, mate selection comes close to being aspecies-typical problem. In this context, exogamy(mating with individuals who are not closely related)appears to be a species tendency that roughly delin-eates the boundaries for mate choice. Within (or wemight say outside) these boundaries, however, indi-vidual differences become crucial in the specificselections that are made.

Individual selections may be governed by thespecies-typical search for the similar, or what Thies-sen (1979) has called "assortative narcissism" (p.102). Empirically, individuals do tend to select mateswho are similar with respect to age, ethnicity, religion,intelligence, attitudes, and personality characteristics(Buss, 1984a, 1984b; Eckland, 1968; Vandenberg,1972). Individual differences in decisions (e.g., eachindividual tends to choose a mate who possesses asimilar constellation of traits) are driven in part bythe species-typical solution (e.g., assortative narcis-sism) to a common human problem (e.g., mateselection).

These individual choices, however, may be fun-damentally mediated by existing hierarchies andindividual placement within them. Genetic differ-ences can create phenotypic differences (e.g., indominance) that in turn lead to group structure(hierarchy), which because of its close link withassortative mating leads back full circle to increasegenotypic variability in subsequent generations. Thishighlights the intriguing ways in which causal linksbetween species-typical traits and individual differ-ences can be identified across levels of analysis.

Beyond the tangible benefits derived by person-ality psychology, integrative efforts of this sort canmodify the strongly typological themes that dominatesome (although not all) evolutionary approaches toour species. Typological statements that "the traitsof human nature" include altruism and aggression(Wilson, 1978, p. 34) are inadequate as they standwithout specifying (a) the level of analysis to whichthey apply (group structure, phenotypic invariant,genotypic invariant), (b) the environmental contin-gencies that could yield statements in the form ofinvariant hypothetical propositions, and (c) the rangeof individual variation (both genotypic and pheno-typic) commonly found with respect to each hypoth-esized species-typical trait.

From this perspective, species-typical character-ization is incomplete without accompanying state-ments about within-species dispersion. The basictenets of evolutionary biology have required bothlines of thinking since Darwin's initial 1859 for-mulation that emphasized both variation and selec-tive character retention. Current conceptions in psy-chology should require no less.

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