Evolutionary Personality Psychology

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CHAPTER 30

Evolutionary Personality Psychology AURELIO JOSÉ FIGUEREDO, JON A. SEFCEK, GENEVA VASQUEZ, BARBARA H. BRUMBACH, JAMES E. KING, and W. JAKE JACOBS

T

divided into four principal sections. The first reviews several major current evolutionary psychological theories of personality. The next two cover empirical evidence for these theories in human and nonhuman animal personality research. Because little empirical research examining personality has been done within evolutionary psychology, the data go outside the evolutionary literature for empirical confirmation or disconfirmation of evolutionary theories. The final section reviews the methodological merits and limitations of the research with a focus on methodological approaches that might best test evolutionary theories of personality explicitly. The material presented is neither exhaustive nor conclusive. Instead, we offer a template of a systematic approach needed to bring theory and data in evolutionary personality psychology together. To that end, we apply the method of multiple working hypotheses (Chamberlin, 1897) and strong inference (Platt, 1964). This procedure specifies plausible alternative hypotheses and then obtains empirical data to help decide among them. HIS CHAPTER IS

A R E V I E W O F C U R R E N T E VO L U T I O NA RY T H E OR I E S O F PE R S O NA L I T Y What follows is a brief review of several of the major evolutionary theories of personality to be critically evaluated. THEORIES

OF

SELECTIVE NEUTRALITY

Tooby and Cosmides (1990) suggest that heritable personality differences in humans do not result from unique personality adaptations. Using an evolutionary psychological framework, they suggest that personality is a product of environmental (situational) differences (e.g., Mischel, 1968) in humans. They propose 851

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EVOLUTIONIZING TRADITIONAL DISCIPLINES OF PSYCHOLOGY

several ways that variation in personality could occur, including reactive heritability, frequency-dependent strategies, and nonadaptive developmental amplification of traits. By their account, psychological differences between individuals and cultures are the product of different “manifest psychologies” based on an innate, underlying, and universally evolved psychology. The innate psychology, developing in different environments, produces these manifest differences. In the environment of evolutionary adaptedness (EEA), psychological adaptations were necessarily complex and coordinated to solve adaptive problems. The system’s “uniform, regular, and predictable” parts (Tooby & Cosmides, 1990, p. 28) interact in a coordinated fashion. “It is this interdependence among subcomponents that requires a monomorphism of integrated functional design” (p. 27). Hence, most heritable psychological traits are not likely the result of complex adaptations. Tooby and Cosmides (1990) assert that only traits with zero heritability and no variation are likely to be adaptations produced by natural selection. Individual genetic variation is “generally limited to quantitative variation in the components of . . . species typical psychological mechanisms” (p. 24). Individual variation exists, but not enough to interfere with the functioning of the cognitive system. “Thus, personality variation is not likely to consist of an alternative, wholly different, coordinated design that differs ‘from the ground up.’ ” (p. 30). Tooby and Cosmides (1990) describe six ways adaptively coordinated personality traits may arise from differential activation of mental organs: (1) stable activation of a mental organ in stable and enduring situations, leading to stable differences between individuals; (2) a stable individual-environment relationship regulating differential thresholds of activation; (3) early environmental cues that differentially adjust threshold of activation, leading to stable, lifelong developmental paths; (4) frequency-dependent personality traits that exist because the trait is rare and produces an advantage; (5) nonadaptive stable differences when more or less irrelevant aspects of human psychology arise; and (6) reactive heritability accounts for individual differences when a genetic predisposition to a certain trait triggers the adoption of one strategy over another. THEORIES

OF

ADAPTIVE SIGNIFICANCE

Diverging from Tooby and Cosmides’s (1990) views, Buss (1991, p. 471) proposed that personality, more specifically, the five-factor model (FFM) of personality, is a central aspect of the adaptive landscape in which humans evolved. “Perceiving, attending to and acting upon differences in others has been . . . crucial for solving adaptive problems” (Buss, 1997, p. 334). Buss (1991, p. 473; see also Buss & Greiling, 1999) suggested that if, instead of arising due to noise or by-products of other adaptations, personality differences reflect distinct adaptive strategies, then there are only four explanations for individual differences in humans: (1) Personality differences are heritable alternative strategies; (2) personality differences are “heritable calibrations of psychological mechanisms” arising through fluctuation of optimal strategies over time and place; (3) individual differences are due to situation-dependent adaptive strategies, implying that each human could develop any personality traits or degree of personality traits; and (4) individual differences arise through ontogenetic threshold calibration.

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Evolutionary psychology helps clarify the role of personality by separating evolved mechanisms from actual behavior (Buss, 1991). Many context-dependent acts may represent adaptive functioning of a single psychological mechanism. Alternately, many psychological mechanisms may combine to perform a single behavior or suite of behaviors. These principles of personality remove the problematic distinction between consistency of personality and situation specificity. A species occupying niches that require individuals to engage in different optimal strategies might produce individual differences (Figueredo & King, 2001; MacDonald, 1998). Thus, individual differences may be due to “. . . differences in ability or morphology [that] produce differences in the effectiveness with which alternative strategies can be adopted or carried out” (Buss, 1991, p. 479). Asserting that personality is central to social interactions, Buss (1991) notes that most of the terms associated with the FFM personality traits are evaluative adjectives. Hogan (cited in Buss, 1991, p. 471) argued these trait terms “reflect observer evaluations of others as potential contributors to, or exploiters of, the group’s resources.” Buss suggests that Extraversion has repeatedly been characterized as one of the first two factors in personality studies because human groups are generally hierarchical—group members at the top of the hierarchy often experience mating advantages. Reciprocal relationships also characterize human groups. Buss suggests this is why Agreeableness is the second factor found in most personality-descriptive taxonomies. Agreeable individuals are unlikely exploiters of reciprocal alliances. Buss (1997) also proposed that mean sex differences in personality arose due to different adaptive problems faced by men and women. This is consistent with the finding that men tend to be higher in traits such as dominance and women, in traits such as nurturance. Miller (2000) proposed an alternative view of human mental capabilities. He suggested that both natural and sexual selection will illuminate why there is substantial variability in behavior among individuals. Sexually selected fitness indicators are traits that all animals use to display individual fitness. Advertisement of these traits (fitness indicators) communicates an individual’s ability to deter predators, quality as a potential mate, and ability to overcome intrasexual rivals. The makeup of fitness indicators, however, appears at odds with trait adaptations formed by natural selection. Reliable fitness indicators differentiate among individuals on the basis of mate quality. Therefore, rather than exhibiting low genotypic variance and heritability, fitness indicators must exhibit high genotypic variance and variability because invariant traits do not indicate differential fitness by reliably discriminating between high- and low-fitness individuals. Weiss, King, and Enns (2002) further developed this idea, suggesting that genetic correlations among positive fitness-enhancing characteristics and conspicuous indicators of that fitness be named covitality. Thus, happiness in chimpanzees, and humans, may indicate overall fitness and should be genetically correlated with fitness-enhancing characteristics such as health, immune system vigor, body symmetry, and reproductive success. MacDonald (1998, p. 142) pointed to two extant camps of evolutionary psychologybased theories of personality, one consisting of “universal psychological mechanisms as a set of adaptations” and the other of “the world of individual differences as a continuous distribution of viable alternate strategies.” MacDonald, who assumed that “personality variation represents a continuous distribution of phenotypes that matches a continuous distribution of viable strategies”

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(p. 139), proposed the genetic variation underlying individual differences allows species with differentiated personalities to occupy a diverse range of social and environmental niches. Natural selection may render different parts of the personality distribution and the strategies associated with different personality traits optimal under different conditions. MacDonald’s theory requires that the relative fitness of individuals possessing differing levels of personality traits be about equal. MacDonald (1998) suggested individual differences in personality are central to the adaptive landscape in which humans evolved (Buss, 1991). He noted several areas of focus in personality theory that can be used to interpret individual differences from an evolutionary perspective. Parental investment theory (Trivers, 1972), for example, predicts male tendencies to devote more energy to mating effort and female tendencies to devote more resources to parental investment. Thus, it is unsurprising that males rate higher in personality domains subsumed under the behavioral approach system, for example, social dominance, sensation seeking, extraversion, and risk taking. These characteristics presumably provided survival and reproductive advantages to males in our evolutionary past. Females rate higher on scales of Nurturance/Love. Presumably, these characteristics provided reproductive and survival advantages to females and their offspring (MacDonald, 1998). MacDonald also argues that developmental patterns in personality characteristics are consistent with evolutionary theory. Sex differences in behavioral activation systems are, for example, maximal during late adolescence and early adulthood, declining throughout adulthood. Late adolescence is precisely the time when individual reproductive potential peaks. Conversely, aspects of temperament such as sensitivity to reward, which emerges in infancy or behavioral inhibition characteristics, which emerge toward the end of the first year of life, remain stable throughout life (p. 135). THEORIES

OF

FREQUENCY DEPENDENCE

Wilson (1994) and Figueredo and King (2001) suggest that frequency-dependent selection offers the best framework for understanding individual differences in personality. Wilson identified three conditions under which behavioral polymorphisms might occur: (1) trophic (same species utilizes different resources in an environment), (2) reproductive (sexual dimorphism where the different sexes adapt to different circumstances), and (3) life history (differences are genetically maintained in a population when reproductive benefits change with ecological niches). He also identified three mechanisms from which polymorphism can arise: (1) genetic differences producing genetic polymorphism directly; (2) a single genotype, through phenotypic plasticity and epigenetic influences, producing different adaptive phenotypes; and (3) ontogenetic shifts throughout each individual life cycle producing sequential differences in phenotypic expression. If the fitness of one genotype depends on coextant genotypes (frequency dependent), then adaptive genetic differences can be maintained and stabilized in the population. Even with sexual recombination, this can occur because assortative mating eliminates maladaptive combinations. Wilson’s model has four basic components. First, individuals can be generalists (moderately adapted to different niches) or specialists (specifically adapted to do very well in a single niche). Generalists or specialists receive the greater

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adaptive benefits depending on the nature of the available niches. Second, individuals evaluate and choose niches. The third and fourth components involve negative density and frequency-dependence. The fitness of the individual depends on the number and traits exhibited by other individuals in the niche. In any given population, Wilson predicts there will be a combination of “adaptive genetic variation . . . and phenotypic plasticity. . .” (p. 232) with a “mixture of ‘developmental generalists’ that can match their phenotype to local conditions and ‘developmental specialists’ that cannot” (p. 230). Using the bold-shy dimension in human personality as an illustration, Wilson suggests there could have been two ancestral niches, one for risk takers and one for risk avoiders (likely to be reflected in the shy-bold continuum). To support the idea, Wilson describes the risk-taker, risk-avoider continuum existing in nonhuman species. Additionally, Wilson uses Kagan, Reznick, and Suidman’s (1988, cited in Wilson, 1994) work on the heritability of shyness and boldness in infants as tentative support of the generalist-specialist model. Generally, children at either personality extreme demonstrate shyness and boldness traits stably. Those in the middle do not. Clarke and Boinski (1995), based on the relevant primate literature, concluded that the bold-shy dimension in humans also characterizes traits in nonhuman primates. A heritable basis for temperament appears to exist, but experience modifies temperament. They also suggest that differences in the life-history strategy of species relate to temperament. Some evidence suggests that species with “active and instrumental foraging strategies” (p. 118) are bolder than species without those strategies. Figueredo (1995) suggested a sociality hypothesis of individual differences, assuming: (1) Personality differences in individuals are characteristic of social species, and (2) individual variation on personality dimensions might be adaptive in social competition. Using the FFM, Figueredo and King (2001) suggested that more types of within-group social relations increase the sophistication of those relationships. Frequency-dependent selection may explain how different and fragmented personality dimensions provide a balance for individuals adopting alternative or conditional adaptive strategies. These authors predicted that three phenomena will increase personality variation in social, but not nonsocial, species during evolution. First, each of the FFM dimensions have a pole (e.g., high Agreeableness) that appears to enhance fitness relative to the opposite pole, thereby fostering directional selection toward “ideal” humans. Frequency-dependent selection, however, may disrupt directional selection. For example, a preponderance of ambitious, assertive, and dominant extraverts could create a niche for low extraversion individuals benefiting from a cautious, silent, secretive approach to life. Some balance between the extremes is the most likely result. Second, variation of frequency-dependent selection pressures across different personality dimensions may increase personality variation. For example, highly Extraverted individuals might achieve higher fitness in populations with a high proportion of Agreeable (tolerant, even gullible) individuals. Once the proportion of high Extraversion individuals exceeded a threshold, however, selection may favor low Agreeableness individuals with wily, deceitful, and “street-smart” proclivities. A large number of cross-dimension interactions and consequent selection are possible.

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Third, interactions across the FFM dimensions within individuals may affect individual differences in personality traits. The literature examining the FFM and personality disorders (e.g., Costa & Widiger, 1994) shows that the severity of psychopathology is not a simple function of the number and extent of suboptimal values on one or more personality factors. High scores on Agreeableness and Emotional Stability, for example, ordinarily associated with high fitness and successful social adjustment, combined with low scores on some facets of Extraversion and Conscientiousness, predict high scores on schizoid scales. Figueredo and King (2001) predict that three phenomena will increase individual differences only in species that are social and engage in extended and intense interactions. Individual differences will be greatest when the social structure of the species supports frequent social interaction across many individuals within relatively stable groups. Furthermore, greater densities of free social interaction between males and females occupying different dominance positions will produce greater individual differences. In species with small groups and rigidly enforced dominance hierarchies, individual difference will be muted. The large variety of social structures within nonhuman primates permits tests of this hypothesis. The theory predicts, for example, smaller individual differences in solitary species and species that form monogamous family units or single-male harems, but larger individual differences in species with age-graded or multimale social structures. Hence, chimpanzees should display greater individual differences than orangutans or gorillas. Likewise, macaque monkeys should display greater individual differences than baboons or langur monkeys. Documented selective pressures can be viewed as broadly analogous to the forces proposed for the social evolution of individuation (Figueredo & King, 2001). One of these is the competitive exclusion principle, in which no two species can permanently share the same ecological niche. Evolutionary responses to this situation include the local extermination of one species by the other (competitive exclusion). Others are niche-splitting and character displacement, which reduce competition by the species diverging ecologically. These strategies are not intentionally implemented in the interests of peaceful coexistence. They are automatic consequences of depressed fitness experienced by individuals in the zone of greatest competition between species—and the consequent relative fitness bonus experienced by individuals at the outer extremes of the overlapping population distributions. Another ecological analogy is mixed-species flocks in birds. Flocking birds gain fitness benefits through enhanced antipredator vigilance and “selfish herd” effects. Those flocking with their own species, however, must bear the cost of greater social competition (e.g., for food). In contrast, birds flocking with other species may experience the benefits of increased antipredator vigilance in the context of a relative “competitive release” from conspecifics. Thus, mixed-species flocking with noncompeting allospecifics might constitute a form of optimal foraging under risk of predation. Similarly, ecological analogies are found in theories of optimal territory size. Some explain the unequal sizes of territories in hummingbirds as a consequence of social competition, where a balance between the costs and benefits of territorial defense produces a distribution of roughly equal numbers of flowers in adjacent hummingbird territories. An implicit trade-off between the quantity (size) and the quality (resources) of the alternative territories available to any individual produces this outcome, the ideal free distribution.

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Assume there is, for any given species in a given situation, an optimal norm of response or optimal response disposition (ORD). Deviations from this norm are not selectively neutral. Selective pressure for a species-typical monomorphism exactly at this optimum creates a centripetal force against substantial individuation. On the other hand, clustering of the entire population at the ORD is likely to produce intense social competition in the “hump” of the distribution, with the ORD setting the central tendency. This would, relatively speaking, reduce social competition at the “tails” of the same distribution, to the extent that random forces of mutation and recombination produced any variability. Reduced social competition creates disruptive selection for individuation as a centrifugal force perhaps partially counteracting selective pressure toward the ORD. Under certain circumstances, then, competitive release experienced by individuals in the tails of the distribution could compensate for the cost of deviation from speciestypical norm of response. This creates an ideal free distribution of alternative behavioral phenotypes in the population by the progressive expansion of tails of distribution around the optimal central tendency. Dispersion of individuals will create bell-shaped curves along different dimensions of personality. PE R S O NA L I T Y I N NO N H U M A N A N I M A L S : E M P I R I CA L E V I DE N C E What follows is a review of the empirical evidence supporting the theories of personality in nonhuman animals. NONHUMAN ANIMAL PERSONALITY PAST

TO

PRESENT

A chronic omission of animal models from personality theory is one reason that we have only begun to understand contributions of biological, genetic, and environmental factors to individual differences in humans. Likewise, a chronic omission of evolutionary-based models of human personality from nonhuman personality theory limits our ability to test evolutionary theories of personality. A comparative approach to personality—and the inclusion of an evolutionary perspective—permits us to account for forces shaping behavior. The study of personality in nonhuman species is an amalgamation of research drawn from disparate scientific areas (Gosling, 2001). A review of such research requires examination of data from agricultural sciences, animal behavior, biology, psychology, veterinary sciences, and zoology, incorporating research ranging from naturalistic and uncontrolled anecdotal reports (e.g., Darwin, 1859/1998; de Waal, 1998; Goodall, 1986) to laboratory-based, highly controlled observations (e.g., Capitanio, Mendoza, & Baroncelli, 1999). Recent resurgence of interest in nonhuman personality has increased studies from a handful during the 1970s to hundreds during the past three decades (see Gosling, 2001). Although each endeavor attempted to identify species-specific personality dimensions, dimensions strikingly similar to the human FFM have been identified across many species (Gosling & John, 1999), including chimpanzees (Figueredo & King, 2001). Extraversion (E), Neuroticism (N), and Agreeableness (A) appeared across many species: E in 10, N in 9, and A in another 10. Consistent with King and Figueredo (1997), Dominance (D) appeared in 9 of the species studied—but not in humans. Moreover, Conscientiousness (C) appeared in humans

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and chimpanzees. Other researchers detected a learned and retained D hierarchy in octopuses, permitting the generalization of D to 10 of the species studied (Cigliano, 1993).

METHODOLOGICAL PROBLEMS

IN THE

STUDY OF ANIMAL PERSONALITY

The generalizability of personality factors across species supports the construct of personality in nonhuman animals and demands an evolutionary account of personality. Unfortunately, methodological problems constrain conclusions based on these studies. Some of these limitations (see Mischel, 1968; Mischel & Peake, 1982) relative to human personality research were addressed and, in many minds, refuted by Kenrick and Funder (1988). Gosling and Vazire (2002) examined the research in animal personalities and argued that, although earlier studies were restricted by at least one of these limitations, the whole body of literature offers some evidence of improvement. One of the most hotly debated topics in human personality research has, for example, been whether consistent personality traits can be detected across time and situations. Many studies have explored this topic using a variety of species (see Gosling & John, 1999). Notably, Stevenson-Hinde, Stillwell-Barnes, and Zunz (1980a, 1980b) showed temporal and cross-situational stability in two personality traits (Fearful and Activity) for rhesus monkeys over a 3-year period. King and Landau (2003) showed cross-situational stability in ratings on Subjective WellBeing (SWB) in zoo chimpanzees. Others have examined short-term temporal stability of personality variables (see Gosling & John, 1999). Although the research is not definitive, it supports the notion that temporal and cross-situational stabilities in animal personality exist. One promising development is the application of generalizability theory (GT) to the problem. Figueredo, Cox, and Rhine (1995), for example, used GT to test concurrently the convergent validity, temporal stability, and interrater reliability of personality factors in stumptail macaques and zebra finches. The application of GT is promising because it is more suitable than conventional factor analysis for small sample sizes. Nevertheless, comparative animal personality is in measurement disarray. Although a few well-established and cross-culturally validated personality scales within the human domain exist, nonhuman personality theory enjoys no such luxury. Since 1936, for example, 11 studies examining chimpanzee personality traits were published. None used the same rating scales. For an evolutionary account of personality to make theoretical sense, comparable trait measurements must be accessible within and across species. Despite this, King and Figueredo (1997) described similarities in personality factor structures between chimpanzees and humans, and Lilienfeld, Gershon, Duke, Marino, and de Waal (1999) reported positive relationships between the chimpanzee factor structures they describe and those described by King and Figueredo. A simple comparative approach that ignores evolutionary theory is scientifically inadequate. Unfortunately, most nonhuman personality literature ignores the roles selection plays in shaping behavior. We now turn to a literature that examines two evolutionary hypotheses of personality, covitality (Weiss et al., 2002) and the sociality hypothesis (Figueredo & King, 2001).

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Evolutionary Personality Psychology EVOLUTION

OF

859

HAPPINESS

A largely unasked question in evolutionary psychology is, “Why did happiness evolve?” ( but see Grinde, 2002). Proximate causes of happiness have been intensively studied (Diener, Suh, & Lucas, 1999), and a large literature indicates that happiness, or SWB, correlates with potentially fitness-enhancing qualities including longevity (Danner, Snowdon, & Friesen, 2001) and health (Watson, 1988). Nevertheless, we could ask if happiness contributes to fitness beyond its phenotypic correlation with traits reflecting fitness-enhancing properties. Weiss et al. (2002) proposed an evolutionary basis for happiness based on personality studies of zoo-housed chimpanzees. Chimpanzee SWB (King & Landau, 2003) correlates highly with a broadly defined personality factor related to dominance and social prowess (King & Figueredo, 1997). Weiss et al. found a genetic correlation approaching unity between happiness and the Dominance factor, indicating a substantial overlap between the heritable components of Dominance and SWB. They concluded SWB serves as a conspicuous fitness indicator based on fitness-enhancing properties such as access to mates, allies, and food resources. The genetic correlation between happiness and Dominance makes it a genetically enforced “honest signal” of Dominance. According to fitness indicator theory, measures correlated with fitness such as number of surviving offspring, body symmetry, and overall health are not merely phenotypically, but genetically, correlated with happiness. Weiss et al. named the predicted genetic correlations among happiness and various measures of positive fitness covitality—as the converse of comorbidity.

EVOLUTION AND VARIABILITY IN PERSONALITY The recent appearance of animal personality studies opens a set of evolutionary questions about the role of individual differences within evolution. One set asks the range of behaviors and traits that reliably vary across individuals within any species. Sponges, animals so passive and listless they were assumed to be plants until the eighteenth century, must lie close to absolute zero on a scale of personality diversity. The evolution of freely moving animals equipped with true brains increased the possible range of personality expression. The first general dimensions probably included traits related to activity levels, perhaps a primitive version of Extraversion. Another early personality dimension may have included reactivity or aggression to novel, potentially threatening stimuli, a possible precursor to Emotionality or Neuroticism. A third early personality dimension may have included exploratory (curiosity) tendencies. Gosling (2001), reviewing individual differences in behavior across a wide variety of vertebrate species, found that among reptiles, fish, and invertebrates, the only measured dimensions fell into the three categories noted previously. Extant evidence indicates the magnitude of individual differences in personality within species is predictable across taxa. Intensely social species, for example, are predicted to display magnified and systematic individual differences. This point, suggested by Clarke and Boinski (1995), was made explicit by Figueredo and King (2001), who described a sociality hypothesis for individual variation in personality. The emergence of social groups of intensively interacting conspecifics was, undoubtedly, the greatest impetus to diversity of personality traits. Indeed,

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most of the individual items (usually adjectives) used to assess human personality reflect qualities of social interactions. A purely solitary species would have a personality structure constrained to narrowly defined versions of the three dimensions noted earlier. Personality becomes evident in a social species. As a preliminary test of this hypothesis, we counted the social and nonsocial species in which personality has been documented. A “social” species was defined as one in which individuals gather for more than mating. Of the 64 species reviewed, 59 were classified as social. The remaining five—cheetahs, hedgehogs, minks, octopuses, and orangutans—deserve attention to both evolutionary histories and intellectual prowess. Orangutans appear to have evolved from a social to a solitary species in recent evolutionary time. Octopuses are intelligent organisms that show social dominance hierarchies when artificially grouped together. Likewise, although they do not seek social groups, hedgehogs and minks den together in the wild and form hierarchies when socially housed. Nevertheless, an adequate test of the sociality hypothesis entails rating species with differing degrees of sociality on identical sets of personality items. As a second test, we compared personality ratings of zoo-housed chimpanzees and orangutans. Chimpanzees live in large, promiscuous groups; orangutans are largely solitary. Ratings on 43 personality-descriptive adjectives encompassing the FFM were obtained from 145 chimpanzees and 127 orangutans (see King & Figueredo, 1997; Weiss, King, Perkins, & Blanke, 2003). The chimpanzees displayed no greater personality variance than the solitary orangutans. Male orangutans had greater variance than male chimpanzees on 24 of the 43 items. Female orangutans had greater variance on 30 of the 43 items. This unexpected result could indicate that the mechanisms enhancing interindividual variability arise later in hominid evolution. Alternatively, the orangutan could represent an ambiguous case because the species was ancestrally social. Finally, proper application of the comparative method requires more than two species compared because a single pair of species might differ for reasons other than that specifically hypothesized. Therefore, more comparative tests are needed before the sociality hypothesis of the evolution of personality is conclusively supported or disproved.

PE R S O NA L I T Y A N D S E L E C T I O N I N H U M A N S : E M P I R I CA L E V I DE N C E What follows is a review of the empirical evidence supporting evolutionary theories of personality in humans with specific emphasis on evidence for natural, sexual, and frequency dependent selection of personality traits.

EVIDENCE

FOR

NATURAL SELECTION

OF

PERSONALITY TRAITS

We divided our literature search into two main components of fitness: (1) survivorship and (2) fecundity. In population dynamics, these quantities, when multiplied and integrated across time, predict the estimated population growth. At the individual level, this product translates into lifetime reproductive success. Survivorship is expected longevity or life expectancy. Fecundity is expected fertility or production of offspring. Our review of relations among personality variation, survivorship, and fecundity was subdivided into two divisions for each

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concept: (1) personality correlates of actual or completed survivorship or fecundity and (2) personality correlates of expected or predicted survivorship or fecundity. The latter category included personality correlates of likely predictors of either survivorship or fecundity, such as health status or reproductive behavior, and is one step removed from documented differences in actual survivorship or fecundity. Personality Correlates of Completed Longevity Denollet et al. (1996) reported that Type D personality, the tendency to suppress emotional distress (high negative affectivity and high social inhibition), is positively correlated to long-term risk of mortality in patients with coronary heart disease, independent of biomedical risk factors. Several studies on relationships between personality and longevity used data from the Terman Life Cycle Study of Children. Tucker and Friedman (1996) reported that children who were rated by parents and teachers as cheerful and having a sense of humor were at higher mortality risk across the life span than less optimistic peers. Martin et al. (2002) suggested this is because cheerful children are more careless about their health throughout life. Conscientiousness also correlated positively with longevity (Friedman, 2000; Friedman et al., 1993; Schwartz et al., 1995), more in males than females, possibly due to a higher likelihood of health-promoting behavior and avoidance of health risks in conscientious individuals. Males with mood instability (higher Neuroticism), however, were at higher mortality risk across the life span than more emotionally stable males, possibly because of the externalizing behaviors (i.e., volatility, aggressiveness, and/or hyperactivity) associated with male Neuroticism. Personality Correlates of Predictors of Longevity Individuals scoring higher on the Life Orientation Test (LOT), a commonly used measure of optimism, report fewer physical symptoms and quicker and better recovery after surgery (Tucker & Friedman, 1996). Ebert, Tucker, and Roth (2002), however, found no relation between a revised version of the LOT and general health status or physical symptoms. Nevertheless, a Sense of Coherence (SOC) trait contributed uniquely to self-reported physical and mental health. Individuals high on SOC report life is understandable, manageable, and meaningful, whereas those low on SOC report life is chaotic and out of control. Neuroticism and Extraversion were positively related to reports of physical symptoms (implying a negative effect on health). Ryan and Frederick (1997) reported that Subjective Vitality (SV), a trait positively correlated to Positive Affect and negatively correlated to Negative Affect, is negatively associated with fewer physical problems (e.g., headaches, shortness of breath). SV, defined as reflecting general organismic well-being, was expected to “covary with both psychological and somatic factors that impact the energy available to the self ” (p. 529). SV was negatively associated with Neuroticism, but positively associated with Extraversion and Conscientiousness. Type A personality, frequently criticized as factorially complex, contains a “toxic” component (hostility) negatively associated with Agreeableness (Dembroski & Costa, 1988). German managers, for example, with Type A personality and external locus of control reported greater perceived levels of stress (especially interpersonal sources of stress) and poorer physical and mental health than those with Type B personality and internal locus of control (Kirkcaldy, Shephard, & Furnham, 2002).

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Hostile Type A personality is a risk factor for cardiovascular disease (Friedmen, Hawley, & Tucker, 1994). Several psychophysiological mechanisms have been proposed to account for this relationship: (1) The association of Type A personality with sympathetic and parasympathetic activation might be linked to factors such as increased arterial stress, changes in lipid metabolism, and platelet aggregation, possibly leading to manifestations such as atherosclerosis; (2) a possible association between Type A personality and immunosuppression might be mediated by increased stress; (3) hostile or neurotic individuals are also more likely than others to abuse dangerous substances (e.g., tobacco, alcohol, and other drugs); and (4) the tendency of hostile and cynical people to have interpersonal disputes may lead to ill health through loss of social support, increasing physiological hyperreactivity, and increased number of stressful situations. Many studies (Tucker & Friedman, 1996) report Neuroticism is weakly but reliably associated with diseases such as coronary heart disease. A meta-analysis of more than 100 studies (Friedman & Booth-Kewley, 1987) reported that coronary heart disease, asthma, peptic ulcers, rheumatoid arthritis, and headaches were associated with depression and anxiety ( both manifestations of trait Neuroticism) and, to a lesser extent, with anger and hostility (associated with low Agreeableness). One longitudinal study found positive relations between individuals with physical health trajectories of high and increasing symptoms and hostility and anxiety (Aldwin, Spiro, Levenson, & Cupertino, 2001). A general mechanism proposed to account for relations between personality and health (Kiecolt-Glaser, McGuire, Robles, & Glaser, 2002) invokes a role for negative emotionality in producing distress-related immune dysregulation by stimulating proinflammatory cytokines, factors implicated in a spectrum of age-related conditions. Personality Correlates of Completed Fertility A retrospective twin study examining completed fertility in more than 1,000 postmenopausal women (Eaves, Martin, Heath, Hewitt, & Neale, 1990) found an interactive relationship between Neuroticism and Extraversion. Highest reproductive success was associated with women with either: (1) high Neuroticism and low Extraversion, or (2) low Neuroticism and high Extraversion. A study comparing fertile to organically and functionally infertile women (Singh, Srivastava, & Nigam, 1992) found that organically infertile women scored higher on measures of Anxiety, Stress, Fatigue, and Guilt and lower on measures of Depression, Regression, Extraversion, and Arousal than fertile women. Functionally infertile women scored higher on Fatigue and Guilt, but lower on Anxiety, Stress, Depression, Regression, Extraversion, and Arousal. Fasino et al. (2002) reported functionally infertile people scored higher on measures of Harm Avoidance than organically infertile people or controls. They suggested that high Harm Avoidance might decrease fertility because it is correlated with lower serotonergic tone (responsible for the modulation of sexual behavior); might decrease the frequency, length, emotional involvement, and satisfaction of sexual intercourse; and might alter hormonal levels in response to stressful environmental or relational situations. Infertile women also scored lower on Cooperativeness than controls. In addition, functionally infertile women scored lower on Cooperativeness and Self-Directedness than organically infertile women, and functionally infertile men scored lower in Novelty Seeking than organically infertile men. Wischmann, Stammer, Scherg, Gerhad, and Verres (2001)

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reported functionally infertile women scored higher on Depression and Anxiety than controls, suggesting a role for negative emotionality and stress in the etiology of functional infertility. Personality Correlates of Predictors of Fertility Hellhammer, Hubert, Phil, Frieschem, and Nieschlag (1985) reported that high Self-Confidence, Extraversion, and Social Assertiveness correlate negatively with male fertility. Men from infertile couples scored lower than controls on Critique Anxiety, Contact Anxiety, Inability to Resist Demands, and Feelings of Guilt, but higher on Ability to Demand. Hypophyseal gonadotropins (e.g., LH, FSH) correlated positively with Sociability and Extraversion; whereas levels of sex steroids (testosterone and estradiol) correlated positively with Feelings of Guilt and Norm Orientation, but negatively with Ability to Demand. Accessory gland function (e.g., fructose, volume) also correlated positively with Critique Anxiety and Ability to Demand. Alexithymia, a difficulty recognizing, identifying, and communicating emotions, reduced fantasy capacity, and externally oriented cognitive style was inversely associated with women’s frequency of penile-vaginal intercourse (Brody, 2003). Lower alexithymia and more frequent intercourse were also associated with better indicators of physical and mental health. Eysenck (1976) and Wilson (1997) examined responses from over 1,000 unmarried students and identified 14 sexuality factors. Extraversion correlated positively with Promiscuity and negatively with Nervousness and Prudishness. Students high on Introversion were more puritanical and downplayed the importance of physical sex. Neuroticism correlated positively with Excitement, Nervousness, Guilt, and Inhibition, and negatively with Satisfaction. Students high on Psychoticism were high on Curiosity, Premarital Sex, Promiscuity, and Hostility. Those high on Eysenck’s Lie Scale (a measure of social desirability) were unadventurous and conventional, similar to those low on Psychoticism. The authors found two orthogonal factors for Libido and Satisfaction. Extraversion correlated with permissiveness, strong libido, and desire for a variety in both partners and sexual activities. Neuroticism correlated with a range of sexual difficulties and with Excitement. Psychoticism was associated with tough, adventurous, and impersonal approaches to sexuality. The authors reported that men liked pornography and impersonal sex more than women did, whereas women expressed more contentment with their sex lives. Men were generally higher on Psychoticism than women. Other research showed that sex offenders were higher in Psychoticism than controls as well as other imprisoned men. Similarly, prostitutes scored higher in Psychoticism than other women. EVIDENCE

FOR

SEXUAL SELECTION

OF

PERSONALITY TRAITS

A precondition for reproductive success in sexually reproducing species is obtaining and sometimes retaining a mate. The special adaptive problems posed by obtaining and retaining a mate, however, fall within the domain of sexual selection. Sexual selection can be subdivided into intrasexual competition and epigamic selection, or mate choice. Here we concentrate on documented relations between personality and mate choice. The dearth of information on relations between personality and intrasexual competition, except as it relates to selection between alternative sexual partners, precludes its discussion.

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There are two dominant approaches to the study of relations between personality and mate choice. The first studies absolute preferences for one personality trait over another, called consensual preferences because they are similar across all individuals. The second studies relative preferences for a sexual partner’s personality in relation to an individual’s own personality. This type of preference presumes assortative mating according to personality and comes in two varieties: (1) positive assortative mating, based on attraction to similarity in personality with an individual’s sexual partner, and (2) negative assortative mating, or disassortative mating, based on attraction to dissimilarity (or “complementarity”) in personality with an individual’s sexual partner. A distinction often made between the personality correlates of actual or completed mate choice is the study of extant sexual partners and the personality correlates of likely predictors of mate choice, such as expressed preferences for imaginary or idealized mates. Absolute or Consensual Preferences Using national and international samples, Buss (1985, 1989) reported men and women rate kind and understanding (Agreeableness?) and intelligent as the two most desired characteristics in a partner. Buss and Barnes (1986) found these two traits were joined by exciting personality (Extraversion?) on the list of consensual traits most desired by men and women in a partner. In another sample, the authors reported the 10 most highly valued mate characteristics were: good companion, considerate, honest, affectionate, dependable, intelligent, kind, understanding, interesting to talk to, and loyal. They suggest that these consensual cross-gender preferences maximize chances for marital satisfaction and marital survival by contributing to compatibility with the partner. Alternatively, these personality characteristics might serve as proximate cues to reproductive investment from potential mates, including parental investment—important for males and females in a species with biparental care. Smith (1996) found that inner-city African American high school students ranked honest and caring as the most important characteristics in a partner. These were closely followed by fun to talk to and humor, perhaps analogous to the exciting personality mentioned earlier. Green and Kenrick (1994) showed male and female respondents expressed a preference for partners with both masculine and feminine personality characteristics. This combination is often called androgyny. Respondents expressed the same partner preferences for several hypothetical relationships, including a date, a one-night stand, and marriage. Furthermore, men and women reported that feminine or expressive characteristics are more important than masculine or instrumental characteristics. These results are consistent with the marital satisfaction/survival and reproductive/parental investment explanation. Although cross-sex similarities are undeniable, sex-specific partner preferences also exist. Buss and Barnes (1986), for example, reported women ranked college graduate and good earning capacity higher than did men, and men ranked physically attractive higher than did women. Women ranked considerate, honest, dependable, kind, understanding, fond of children, well-liked by others, good earning capacity, ambitious and career oriented, good family background, and tall higher than did men. In contrast, men ranked physically attractive, good looking, good cook, and frugal higher than did women. Although these preferences are not all personality characteristics, the results indicate that men and women have distinct priorities in mate selection.

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When factors such as risk of pregnancy, detection, and disease were eliminated from consideration, female university students described themselves as more “willing” to engage in sexual intercourse with hypothetical males described as possessing positive personality traits (e.g., bright, generous, sense of humor, and successful architect), parental traits, and safe traits (Surbey & Conohan, 2000). The male students’ willingness to engage in causal sex did not change with the positive, parental, or safe trait manipulations. The male students’ willingness to engage in casual sex did, however, decline as the potential partners’ physical attractiveness declined. Berry and Miller (2001) reported the quality of videotaped interactions between previously unacquainted opposite-sex pairs relates to personality and physical attractiveness. Only Extraversion predicted the rated interaction quality of men, whereas only physical attractiveness predicted the rated interaction quality of women. Jensen-Campbell, Graziano, and West (1995) found that females preferred males high on Altruism/Agreeableness to those low on Altruism/Agreeableness. The highest physical attractiveness, social attractiveness, social desirability, and dating desirability were reported for Agreeable and Dominant men. An interaction between Agreeableness and Dominance seemed to contribute to females’ perception of male Agreeableness. Females rated Agreeable and Dominant men as wealthier. In contrast, male respondents did not find female dominance important in attraction. Sadalla, Kenrick, and Vershure (1987) reported that Dominance increased the perceived attractiveness of males, but had no effect on female attractiveness. Attractiveness of Dominant males was unrelated to the sex of the rater or the sex of the individual the male was interacting with. Although Dominance was rated as attractive, constructs such as aggression or being domineering were not. Manipulated Dominance was related to the male’s sexual attractiveness, but not general likeability. Relative Preferences or Assortative Mating The majority of studies on assortative mating for personality have reported significant positive correlations across romantic partners around .2 (Buss, 1985). Even higher positive correlations (.4 to .6) have been reported for sensation seeking (Zuckerman, 1994). Buss (1984) generally supported these findings but also found a single negative correlation between spouses for dominance and submissiveness. This negative correlation appeared in ratings made by interviewers and by the spouses themselves. Overall, however, the weight of the evidence favors romantic partner similarity (positive assortative mating) over complementarity (negative assortative mating or disassortative mating). One evolutionary explanation of these results is genetic similarity theory (Rushton, 1989). By this theory, individuals seek out genetically similar romantic or social partners by phenotypic matching. Among the converging lines of supporting evidence is the high degree of phenotypic similarity on a variety of traits shown by friends and romantic partners. Moreover, friends and lovers seem to assort more strongly on characteristics with a high degree of genetic heritability and are thus more closely linked to shared genes. The function of this mechanism in mate choice is presumably to preserve the coherence of coadapted genomes that work well together, as well as to increase the coefficients of relatedness of parents to offspring, promoting parental investment. This mechanism presumably enhances

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inclusive fitness in nonsexual social partners by promoting altruism toward genetically similar others, enhancing the survival of genes shared with conspecifics that are not necessarily close kin by recent common descent. Although this theory remains controversial, it produces interesting and testable predictions. A body of social psychology literature examines interpersonal attraction in social partners, not all of it specifically in relation to mating. Posavac (1971) reported that fraternity brothers have similar personality characteristics on a variety of measures. Posavac and Pasko (1974) also reported greater social attraction to individuals similar to college students even when they controlled for consensual preferences, although respondents reported greater social attraction to individuals who possessed popular characteristics. Hendrick and Brown (1971) found that Introverts rated other Introverts as a more reliable friend and more honest and ethical, although both Introverts and Extraverts preferred Extraverts on various measures of social attraction (e.g., liking, interesting at party, ideal personality, and prefer as leader), partially supporting the principle of consensual preferences. Suman and Sethi (1985) computed an index measuring the proportion of similarity between both Introverted or Extraverted individuals and a hypothetical stranger—and found the degree of social attraction between two persons increased in direct proportion to the rise of this index. Returning to assortative mating among romantic partners, females but not males in a computer dating situation (Lum & Curran, 1975) preferred opposite sex partners that were moderately to highly similar to themselves on Extraversion. No significant relations were found for matching on Neuroticism. Keller, Thiessen, and Young (1996) compared the similarity of dating and married couples in both physical and psychological traits. Although dating and married couples assortatively mated on physical characteristics, assortative mating was higher for married couples on psychological traits. Furthermore, other studies (reviewed by MascieTaylor, 1988) report that spousal similarity is not attributable to married couples becoming more similar over time. In fact, Buss (1984) found older married couples tended to be less rather than more similar to each other. Eysenck and Wakefield (1981) reported similarity predicts general marital satisfaction. Although typical spousal personality correlation was .20, some were higher, including .73 for marital satisfaction, .41 for sexual satisfaction, .43 for libido, .51 for radicalism, and .56 for tender-mindedness. Marital satisfaction was higher for men higher on Psychoticism and lower on Neuroticism than women. Men had higher marital satisfaction if their wives were more tender-minded than they, and women had higher marital satisfaction if their husbands were more toughminded than they. Marital satisfaction was lower when men had high libido, but was unrelated to women’s libido (men had generally higher libido than women in this study). There was no evidence of increasing similarity with length of marriage. Dissimilarity predicted divorce. Evidence for assortative mating on undesirable traits most directly contradicts the principle of consensual preferences. Arguably, the opposite of many of the prosocial qualities that people find consensually desirable is Machiavellianism. A Machiavellian mate is cold, detached, manipulative, and exploitative. Novgorodoff (1974) found men, especially low-Machiavellian men, preferred low-Machiavellian women as romantic partners. High-Machiavellian women preferred high-Machiavellian men as romantic partners. Touhey (1977), however, found high Machiavellian individuals showed little attraction to similar

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others. Instead, individuals high in anxiety and social desirability were more attracted to similar others than those who were not. Depression is another apparently undesirable trait in a mate. Nevertheless, Rosenblatt and Greenberg (1988) found that only nondepressed individuals preferred nondepressed social partners. Depressed individuals neither preferred nor dispreferred depressed over nondepressed social partners. This pattern, however, may not carry over into mating. Lewak, Wakefield, and Briggs (1985) found that similarity on the Depression (D) Scale of the Minnesota Multiphasic Personality Inventory (MMPI) predicted increased marital satisfaction in a nonclinical sample. This same study reported assortative mating on the Psychopathic Deviate (Pd) Scale of the MMPI in clinical and nonclinical samples; similarity in Pd also predicted marital satisfaction. An Additional Empirical Test Ample evidence supports both absolute (consensual) and relative (assortative) preferences for romantic partner personality in human mate choice. Of the absolute (consensual) preferences, some are sexually dimorphic and others sexually monomorphic. In the spirit of strong inference, however, we designed a study to pit these hypotheses against each other (Figueredo, Sefcek, & Jones, 2004). We created the NEO-MATE, a translation of the NEO-FFI items from self-reports to desiderata in an ideal romantic partner, to assess romantic partner preferences. We administered the NEO-MATE and the NEO-FFI to 104 University of Arizona undergraduates. We emphasized the participants should use the NEO-MATE to rate their own ideal romantic partner, rather than one they believe others might value. We administered the NEO-FFI after the NEO-MATE to prevent priming participants to match ideal romantic partners’ personalities to their own. The bivariate correlations between self- and ideal partner ratings on these factors were significant and substantial, indicating a tendency toward positive assortative mating on all personality factors, at least in the desired imaginary romantic partners. These correlations were .81 for Openness to Experience, .36 for Conscientiousness, .60 for Extraversion, .73 for Agreeableness, and .38 for Neuroticism. These data permitted us to perform a more stringent test of absolute preferences in ideal romantic partner personalities. By subtracting the factor scores on self-rated personality factors from those of the ideal romantic partner factors, we obtained difference scores indicating discrepancies between ratings of self and of ideal romantic partners. The mean difference scores for each factor, with the exception of Openness to Experience, differed significantly from zero. Respondents rated ideal romantic partners higher than themselves on Conscientiousness, Extraversion, and Agreeableness, and lower than themselves on Neuroticism. The patterns observed in ideal romantic partner preferences are not merely indirect effects of preferences for a romantic partner personality similar to their own. The difference scores indicate preferences above mere matching to an individual’s self-reported personality scores. The test of the hypotheses is reasonably clean because these different scores showed no statistically significant effects of respondent age or sex. Thus, the main conclusion of this study is, as declared by the Dodo Bird in Alice in Wonderland, “Everybody wins, and we all must get prizes!” Although the results support at least aspirational positive assortative mating for all FFM factors, they also indicate a relatively invariant preference (across

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age and sex) for romantic partners with more Conscientiousness, Extraversion, Agreeableness, and less Neuroticism than oneself. EVIDENCE

FOR

FREQUENCY-DEPENDENT SELECTION

OF

PERSONALITY TRAITS

Tooby and Cosmides (1990) specified several criteria that must be met to show evidence for frequency-dependent selection of personality traits. To qualify as alternative adaptive strategies, alternative phenotypes must exhibit heritable individual differences, and component personality traits must covary in a predictable, adaptive manner and exhibit evidence of functional design as logically coherent and coordinated adaptations. To support claims of frequency-dependent selection, the fitness of an alternative phenotype must vary inversely to its relative frequency in the population: The rarer a particular phenotype with respect to its alternatives, the higher its relative fitness must be. Finding direct evidence for frequency-dependent selection presents a challenge. Hence, we pursued the second-best strategy by collecting indirect evidence. If personality variation is correlated with reproductively relevant traits subject to frequency-dependent selection, then a direct investigation of the relevance of frequency-dependent selection to personality variation in humans is warranted. Fitting personality variation into an overall pattern of reproductively relevant traits fulfills one of the Tooby and Cosmides’s (1990) conditions for frequencydependent selection. A source of interindividual variation held to be subject to frequency-dependent selection is alternative reproductive strategies. An array of comparative studies describes the coexistence of alternative phenotypes (and perhaps genotypes) of both male and female conspecifics that pursue divergent sexual and reproductive strategies (e.g., Buss & Greiling, 1999; Gangestad & Simpson, 2000; Rowe, 1996). Indeed, many game-theoretical models conclude alternative or conditional strategies can be held in a perpetual state of balanced polymorphism by frequencydependent selection (Tooby & Cosmides, 1990). The K-Factor Because of potential trade-offs between the multiplicative parameters of survivorship and fecundity in generating total reproductive output, various combinations of these parameters yield identical final products. Thus, different life-history strategies that yield the same total fitness are possible. For example, a strategy based on high survivorship and low fecundity may perform as well as one based on high fecundity and low survivorship. These alternative life-history strategies are known as r- and K-strategies, respectively. Different species often have stereotypical life-history strategies not subject to substantial individual differentiation. Rabbits, for example, have relatively rapid sexual development, are highly fertile, and provide little parental care to offspring, resulting in high infant mortality. Even after reaching maturity, rabbits are short-lived. In contrast, elephants have slow and delayed sexual development, produce few offspring, and provide long-term parental care, resulting in low infant mortality. Furthermore, adult elephants are long-lived. Thus, relatively speaking, we classify rabbits as r-strategists and elephants as K-strategists. Until recently, researchers studied variations in life-history strategy by comparing different species (MacArthur & Wilson, 1967). This domain has recently been extended by measuring systematic differences in life-history strategy

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Table 30.1 Factor Pattern (Standardized Regression Coefficients) for the K-Factor K-Factor Biological Father Attachment /Investment Other Father Figure Attachment /Investment Adult Romantic Partner Attachment Mating Effort Machiavellianism Risk Taking

.36 −.36 .38 −.51 −.58 −.41

among human individuals, social classes, and ethnic or racial groupings (Rushton, 2000). Some developmental evolutionary theories and related behavioral genetic work suggests there is substantial individual variation in life-history strategy within groups (Belsky, Steinberg, & Draper, 1991; Chisholm, 1996; Rowe, 2000). The analyses of these correlational patterns have mostly consisted of univariate analyses testing specific causal hypotheses and have not attempted to describe the wider pattern of correlations implied by these theories. The generative theory, however, suggests it is possible to construct a latent variable model, specifying a single common factor (K) that underlies life-history parameters—including an assortment of sexual, reproductive, parental, and social behaviors. If so, this K-Factor is an important and underappreciated individual difference variable in human development. Furthermore, correlations relating traditional personality factors to this K-Factor might serve as indirect evidence for frequency-dependent selection of personality. To examine this hypothesis, we created a battery of measures sampling key behavioral indicators of the K-Factor and administered it to 222 University of Arizona undergraduates (Figueredo, Vasquez, et al., 2004). These instruments measured: (1) attachment to and investment from the biological father (adapted from Fine, Worley, & Schwebel, 1985), (2) attachment to and investment from father figures other than the biological father (adapted from Fine, et al., 1985), (3) adult romantic partner attachment (Brennan, Clark, & Shaver, 1998), (4) the Mating Effort Scale (Rowe, Vazsonyi, & Figueredo, 1997), (5) the short form of the Machiavellianism Scale (Christie & Geiss, 1970), and (6) a Risk-Taking Questionnaire (Eadington, 1976). Factor analysis of these measures produced a single common factor (the K-Factor) accounting for 92% of the reliable variance. Table 30.1 displays the factor pattern. The Enormous Three To correlate this K-Factor to traditional personality factors, we avoided making specific associations to particular personality inventories by administering three major personality inventories, the NEO-FFI (Costa & McCrae, 1992), the EPQ-R (Eysenck & Eysenck, 1975), and the ZKPQ (Zuckerman, Kuhlman, Joireman, Teta, & Kraft, 1993), to the same sample of undergraduates. We performed a higher order factor analysis to create common factors that cut across the particular personality inventories and obtained three common factors: Big N (for Neuroticism), Big E (for Extraversion), and Big P (for Psychoticism), that accounted for virtually 100% of the reliable variance. This was

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EVOLUTIONIZING TRADITIONAL DISCIPLINES OF PSYCHOLOGY Table 30.2 Factor Pattern (Standardized Regression Coefficients) for the Higher-Order Personality Factors and the Bivariate Correlations among Them

NEO-FFI EPQ-R ZKPQ NEO-FFI EPQ-R ZKPQ NEO-FFI NEO-FFI EPQ-R ZKPQ ZKPQ

Neuroticism Neuroticism Neuroticism/Anxiety Extraversion Extraversion Sociability Conscientiousness Agreeableness Psychoticism Impulsivity/Sensation Seeking Aggression/Hostility

Big N

Big E

Big P

.81 .87 .88 −.10 −.10 .17 −.23 .00 −.12 −.08 .16

-.07 .03 .05 .80 .79 .78 -.02 .28 −.06 .36 .04

.04 −.02 .03 −.10 .10 −.05 −.49 −.62 .66 .62 .58

1.00 −.34 .22

−.34 1.00 −.06

.22 −.06 1.00

Interfactor Correlations Big N Big E Big P

essentially a replication of results published previously by Zuckerman et al. (1993). Table 30.2 displays the rotated factor pattern under an oblique Promax rotation. Personality and Demographic Correlates of K The bivariate correlations of the KFactor with higher order personality factors were −.24 for Big N, .12 for Big E, and −.67 for Big P. The correlations were statistically significant for Big N and Big P and approached significance for Big E. The high negative correlation of the KFactor with Big P also supported Zuckerman and Brody’s (1988) prediction that Psychoticism is more relevant to K than Neuroticism or Extraversion. Furthermore, the bivariate correlation of the K-Factor was −.24 with Sex, denoting generally lower K-Factor scores for males, but was not related to Age in this restricted age-range sample. The lower mean on the K-Factor for males is consistent with the theoretically predicted and empirically well-documented sex differences in reproductive strategy (e.g., Trivers, 1972). These results confirm the idea that personality variation is relevant to reproductive life-history strategy. Moreover, if life-history strategy is subject to frequency-dependent selection, these results imply ( but do not demonstrate) that the personality correlates of reproductive strategy are subject to frequencydependent selection. CONCLUSIONS AND LIMITATIONS We have reviewed indirect empirical evidence suggesting human personality variation is related to natural selection (through correlations to longevity and fertility), to sexual selection (through correlations to consensual and assortative mate choice), and to frequency-dependent selection (through correlations to reproductive life-history strategy). The evidence is fragmentary, and exactly how each se-

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lective pressure operates on human personality variation or how they interact is unknown. We now argue, however, it is no longer reasonable to suppose, given this body of evidence, however incomplete, that human personality variation is independent of selection. It is reasonable to form a viable evolutionary personality psychology with further research to achieve a comprehensive understanding of evolved human nature. We note that most of these studies are observational, not experimental; the reported correlations may not imply causation. As always, personality factors might influence fitness consequence, fitness consequence might influence personality factors, or some unmeasured third variable may influence both, producing spurious correlations. Unmeasured third variables as diverse as pleiotropic genes that influence both personality and health status or reproductive behavior or common environmental influences might be at work. Nevertheless, this review serves as motive to initiate a more comprehensive evolutionary approach to the study of personality variation. We also note that the reported personality correlations used measures of current survival and reproduction. We do not know if these measures were valid in ancestral environments because current selective pressures may or may not be relevant to past selection. We suggest, however, that in this case the current selective pressures are the best proxies we have for past selective pressures. Betzig (1998) argued convincingly that contemporary differential reproductive success may be more reliable from an evolutionary perspective than commonly supposed and provides useful information concerning species-typical reproductive strategies. Most of the cited evidence relates to features of the immediate social and sexual environments, which may not have changed all that much from ancestral environments, rather than the physical or technological environments. In this case, empirical information from current environments is more useful than speculation regarding presumed ancestral environments. CONCLUSIONS We conclude with a discussion of selected methodological considerations and proposed future directions for research in evolutionary personality psychology. CONSTRAINING ACCOUNTS

OF

PERSONALITY

A nonadaptationist framework predicts that individuals will not have personality traits classifiable by general laws or principles. In contrast, the adaptationist framework predicts stable personality traits—that traits are classifiable by the adaptive problems they were designed to solve and that traits evolve as a function of the adaptive problems faced by the organism over evolutionary time. Many personality theorists who use this framework exploit sophisticated statistical procedures to identify personality traits, but they unintentionally ignore a problem lying at the heart of the field: the appropriate constraints to set on the data on which the statistical techniques are based. Guided by folk-psychological constructs, traditional personality theory seeks to identify universal traits that exist in various combinations in individuals. In contrast, evolutionary personality theory demands that the acceptable data and theoretical structures be severely constrained both vertically and horizontally.

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Vertical Integration Theory developed under an evolutionary framework is constrained by what is known in the fields immediately below the level of interest— in this case, physiology, anatomy, and genetics (the neurosciences)—and fields immediately above the level of interest—in this case, evolutionary biology, evolutionary anthropology, ecology, and ethology. Acceptable data are similarly constrained. A proper evolutionary psychology involves the coordination of theory about: (1) the adaptive problems facing the species, (2) available solutions to those problems, and (3) how individuals recognize and solve those problems. Horizontal Integration Taking constraints offered by vertical integration, psychological theory within an evolutionary framework is constrained by descriptions of and accounts for what the organism does in the natural world and in the laboratory. A proper empirical approach to any problem involves: (1) watching subjects in a variety of naturalistic settings and using method of agreement and concomitant variation, (2) formulating hypotheses about what is seen, (3) taking these hypotheses to the laboratory and working out the rules governing what was seen using experimental methodology, and (4) taking those rules back out to the naturalistic world and making sure the laboratory rules accurately predict events in the naturalistic setting. Hence, observations set the questions, experiments sharpen the observations, and the results of those experiments provide means for making new observations (social psychologists call this full-cycle psychology e.g., Cialdini, 1980). We have seen that the best methodological approach to theory and data gathering in evolutionary personality psychology exhibits characteristics and addresses problems unlike traditional personality psychology. Evolutionary personality psychology is a life science. This fact forces it to answer to naturalistic and laboratory-based data. It also forces it to answer to data and theory framed in the life sciences immediately above and below it. Such accountability constrains the ways theory may be structured and dramatically expands the range of data that must be considered. DATA QUALITY Acknowledging that evolutionary personality psychology is irretrievably allied to the other life sciences permits us to use the hard-won knowledge and methods of systematics found in those fields (see, e.g., Brooks & McLennan, 1991). By these standards, the operational database upon which personality theory rests appears unacceptably informal. Much of the data entering personality theory are based not only on apparently artificial categories but also on self-report provided by untrained observers. Such data necessarily come from uncalibrated observational instruments. Documented problems involving reactivity, observer drift, ipsative drift, observer decay, contextual effects, and other well-characterized problems with human observers suggest that even highly trained observers are recording instruments with unknown and variable characteristics ( Jacobs et al., 1988; Klahr & Simon, 1999; Repp, Nieminen, Olinger, & Brsca, 1988). The problem is compounded when untrained (or informally trained) observers report on the quality and quantity of their own personal experience or actions during interviews or on questionnaires. Clearly, though self-reports are a source of hypotheses, data obtained from them are at best not sufficiently constrained to serve as the primary data for an adequate taxonomy of personality traits.

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MacDonald (in press) described similar conclusions in a recent review of the personality literature from an evolutionary perspective. He argued that personality is an evolved set of mechanisms designed to solve adaptive problems. Some of these mechanisms are relatively fixed and universal; others are relatively labile and idiosyncratic. These mechanisms are not forward looking. Indicators of a specific adaptive problem activate a specific set of mechanisms controlling adaptive responses that solved the problem in the phylogenetic or ontogenetic history of the individual. MacDonald concluded that direct situational activation of those mechanisms is the best way to identify and characterize those mechanisms. He predicted that we will find homologous systems in other species serving adaptive purposes (which may or may not be modular), personality structure will mirror the ecology of the species (the adaptive problems that it faces), and the ecological approach can be used to estimate the likelihood that an identified trait evolved to solve specific adaptive problems. LIMITATIONS Despite methodological difficulties, we draw several tentative conclusions. First, theory in evolutionary personality psychology is ahead of the data. Few studies test specific evolutionary predictions. Most of the evidence we reviewed was collected atheoretically or guided by nonevolutionary theories. There is, nevertheless, a modicum of evidence supporting each of the major evolutionary theories explaining the continued existence of systematic individual differences in the face of selection. Although the existing data leave much to be desired, they suggest individual differences in personality are subject to natural selection, sexual selection, and frequency-dependent selection. The data also suggest these differences exist in a wide variety of different species and point to general accounts of why and how they evolved. We conclude that these findings serve sufficient warrant to develop a truly evolutionary personality psychology seeking to explain the observed patterns of individual variation adaptively. We are beginning this process; basic work must be done before the field matures enough to produce definitive accounts of the adaptive significance of individual personality differences. We hope this contribution helps frame basic issues and identifies major areas needing protracted attention. Such a comprehensive intellectual framework is required to perform the theoretically guided and methodologically focused empirical research that is needed. R EFER ENCE S Aldwin, C. M., Spiro, A., III, Levenson, M. R., & Cupertino, A. P. (2001). Longitudinal findings from the normative aging study: III. Personality, individual health trajectories, and mortality. Psychology and Aging, 16(3), 450– 465. Belsky, J., Steinberg, L., & Draper, P. (1991). Childhood experience, interpersonal development, and reproductive strategy: An evolutionary theory of socialization. Child Development, 62, 647–670. Berry, D. S., & Miller, K. M. (2001). When boy meets girl: Attractiveness and the five-factor model in opposite-sex interactions. Journal of Research in Personality, 35, 62–77. Betzig, L. (1998). Not whether to count babies, but which. In C. Crawford & D. Krebs (Eds.), Handbook of evolutionary psychology: Ideas, issues, and applications (pp. 265 –273). Hillsdale, NJ: Erlbaum. Brennan, K. A., Clark, C. L., & Shaver, P. R. (1998). Self-report measurement of adult attachment: An integrative overview. In J. A. Simpson & W. S. Rholes. Attachment theory and close relationships (pp. 46 –76). New York: Guilford Press.

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