Prepared as a chapter for the Annual Review of Psychology, 1995.
For more information on personality theory and research, go to the Personality Project

ALL PEOPLE ARE THE SAME: THE STUDY OF SPECIES TYPICAL BEHAVIOR

It is easy to forget, when considering human behavior, how similar we all are to each other. Demonstrations of this similarity include the compelling "Barnum effect" observed when judging the accuracy of self descriptions based upon human universals. That one experiences some anxiety when meeting an attractive stranger, or sometimes thinks about things that other people might find peculiar is not a sign of uniqueness but rather something one shares with everyone. Rather than dismissing these similarities, evolutionary theorists, psychodynamicists, sociologists and others hope to find an understanding of the universalities and general laws of human nature.

Evolutionary Personality Psychology

Evolutionary personality theory focuses on the why of behavior, rather than the how of biological models, or the what of descriptive taxonomies. It is "best regarded as a theory about the origins, rather than the content of human nature" (D. Buss 1991, p 463). It has been described as providing a grand framework that "links the field with what is known about the processes that govern all forms of life [and identifies] the central human goals and the psychological and behavioral strategic means deployed to obtain these goals." (p 486). "Evolutionary psychology is simply psychology that is informed by the additional knowledge that evolutionary biology has to offer, in the expectation that understanding the process that designed the human mind will advance the discovery of its architecture." (Cosmides et al 1992, p 3.)

Evolutionary causes for individual differences

Although focusing on general laws, evolutionary theory tries to explain individual differences. The problem of reconciling genetic diversity within species with principles of evolutionary adaptation is complex: "Both the psychological universals that constitute human nature and the genetic differences that contribute to individual variation are the product of the evolutionary process [... . Personality is from an] evolutionary perspective, analyzable as either (a) an adaptation, (b) an incidental by-product of an adaptation, (c) the product of noise in the system, or (d) some combination of these." (Tooby & Cosimides 1990, p 19).

Evolutionary theorists ask why there are genetically based individual differences. Individual differences might result from frequency dependent selection pressures that can lead to complex polymorphisms and maintain a stable mix of genotypes. In an environment with many potential niches, individuals, by being different, can select the niches that maximize their own fitness, and thus the population is a mix of multiple genotypes each searching for and creating optimal environments (D. Wilson, in press, D. Wilson et al 1993; D. Wilson et al in press).

Another intriguing hypothesis for the adaptive significance of individual differences, for sexual reproduction, as well as for much greater genetic diversity within rather than between racial groups, is that variation and recombination is a response to parasites. "Large, complex, long-lived organisms constitute ecological environments for immense numbers of short-lived, rapidly evolving parasites—disease causing microorganisms. ... parasites and hosts are locked in an antagonistic coevolutionary race." (Tooby & Cosimides 1990, p 32). Sexual reproduction, although genetically costly (without assortative mating, sexual reproduction assesses a 50% "inheritance tax" at each generation), produces offspring with a genetic makeup that one's parasites have never before encountered. The function of individual differences and sexual rather than asexual reproduction might be to survive this constant onslaught of parasitic infestation. In humans, the importance of physical appearance (a sign of pathogen resistance) in mate selection may be associated with pathogen prevalence (Gangestad & Buss 1993).

Sexual strategies

Survival and reproduction are the two fundamental challenges of evolution. This general principle leads to individual differences between the sexes in terms of reproductive strategies. Males and females differ in the costs associated with reproduction and use different strategies to maximize their fitness. Although males are potentially almost unlimited in their number of offspring, females are not. Females can be certain about motherhood, but males can never be certain of paternity. From these biological realities, several interesting predictions have been tested. Male swallows, dunnocks, and humans "take a proprietary view of women's sexuality and reproductive capacity." (M. Wilson & Daly 1992, p 289). Males, thought to be concerned with paternity certainty, are more upset by sexual infidelity of their partners, while females, thought to be concerned with the long term emotional investment of their partners, are more concerned about emotional infidelity (D. Buss et al 1992). In general (but see Gangsted & Simpson 1990), females are more choosy about sexual partners than are males, even though males and females do not differ in their preferences for long term relationships (Kenrick et al 1990). In a powerful example of the theoretical possibilities, D. Buss & Schmitt (1993) formalized the predictions of evolutionary personality with nine hypotheses about human mating patterns.

Sociology of generational effects

Sociological approaches to personality are strikingly different from evolutionary personality theory in terms of level of analysis, but they are similar with respect to the level of generality. For example, it is easy for personality theorists to forget that different generations have experienced significantly different challenges and opportunities throughout their life span. The experience of war, national economic collapse, or the threat of nuclear extinction have had profound effects upon those who have experienced them. Although these are universal experiences for all alive at the time, only generational cohorts share both the experience, as well as the timing at the same stage in their lives. Detection of potential generational effects requires many waves of longitudinal data for people of different age cohorts. A single longitudinal study that focuses on the experiences of a particular cohort will show impressive consistencies and coherencies over large parts of the life span but will fail to detect the effect on personality of the timing of major life events. Data from several of the classic longitudinal studies have been used to address such generational effects on personality through the lifespan (Elder, in press). Archival data from the 12 waves (1922 through 1986) of the Terman (1925) study show cohort effects on later career achievement, transitions and trajectories (Elder & Pavalko 1993). The age of experiencing the great depression and the disruption of career upon entry into the military during W.W.II had reliable effects on life-time accomplishment for this group.

Psychodynamic theory

Psychoanalytic approaches "take as axiomatic the importance of conflicting mental processes; unconscious processes; compromises among competing psychological tendencies that may be negotiated unconsciously; defense and self-deception; the influence of the past on current functioning; the enduring effects of interpersonal patterns laid down in childhood; and the role of sexual and aggressive wishes in consciously and unconsciously influencing thought, feeling, and behavior" (Westen 1990, p 21). With such an inclusive definition, it is not surprising that discussions of psychodynamic approaches integrate findings from more experimental areas of psychology about self (Markus & Cross 1990), unconscious awareness (Kihlstrom 1987, 1990) and even biological distinctions in memory systems.

Psychodynamic theories with an emphasis upon cognitive representations rather than biological drives (i.e., object relations theory) are more compatible with the research paradigms of social cognition (Westen 1991). Obstacles to integration of these two approaches, however, include strong differences in the data used for theory building (clinical insights versus systematic laboratory based data) and in the level of generality that the theory addresses (Westen 1990). Some psychiatric theories propose useful links of psychodynamic with psychobiological approaches to personality and the personality disorders (Siever & Davis 1991). Continue

Prepared as a chapter for the Annual Review of Psychology, 1995.
For more information on personality theory and research, go to the Personality Project