Evolutionary personality psychology suggests that human personality variation is a meaningful and relevant source of human diversity, and that different combinations of heritable personality traits biologically prepare individuals to exploit the different social niches they inhabit. Human personality variation relates to numerous important life outcomes, including life history traits such as sexuality, sociality, fertility, morbidity, and mortality. The effects of personality variation upon these fitness-relevant demographic parameters renders such variation subject to both natural and sexual selection. Furthermore, the adaptive value of any given personality trait is relative to that circumscribed portion of the environment which the individual habitually inhabits (i.e., the individual’s physical and social niche), and in which the individual is functioning. Any given personality trait can be evaluated as an adaptive function or harmful dysfunction by determining its match to the particular social niche a person occupies within a particular social environment.
This entry explains the complex mix of genetic and environmental influences on human personality variation by applying a combination of the predictions of Brunswikian evolutionary developmental theory and developmental plasticity theory. This entry also considers the significance of personality disorders from the perspectives of the mismatch hypothesis and the harmful dysfunction hypothesis.
Personality Variation
Most evolutionary personality psychologists have concluded that individual differences in personality traits are adaptive in nature and therefore the result of natural and sexual selection. Differences in personality traits allow people within human social groups to differ in the effectiveness with which they can play different roles within human societies. Personality differences are of utmost importance because humans must detect and react to the personality traits of others to successfully navigate the social landscapes in which humans reside. The individual differences seen in personality are the result of adaptation to different niches available within human social groups. This reduces competition among group members. Although some members who are similar in personality may compete for certain roles within a society, their degree of specialization means that they will not be required to compete against all group members.
For example, in our modern society, certain people who possess higher levels of intelligence, conscientiousness, and motivation (e.g., willingness to spend many years studying) may be good candidates for careers as neurosurgeons. Other individuals who possess higher levels of impulsive sensation-seeking and lower levels of neuroticism (and are thus less prone to fearfulness) might be better suited than others to become deep-sea welders. While both occupations could be considered highly stressful and highly specialized, they require different types of personality characteristics. Furthermore, even within fairly narrowly defined types of careers, different individuals will gravitate toward distinct roles and physical locations within those careers. Some people will strive to become brain surgeons in extremely prestigious hospitals, whereas others will be satisfied to hold less prestigious positions in order to be able to pursue other interests, such as spending time with family and friends. The same could be said for deep-sea welders. Although all deep-sea welders might be viewed as more risk taking than the average person in any population, some will naturally gravitate toward more dangerous jobs than others. Those more reckless and impulsive individuals might also engage in such dangerous practices as smoking cigarettes within oxygen-filled decompression chambers upon resurfacing. These are modern-day examples from Western industrial society, used here for purposes of illustration, but different roles that best suit different types of individuals can be found in any human society.
The evolution of a diversity of individual traits to fit a variety of different social niches might be ultimately due to frequency-dependent selection. Frequency-dependent selection occurs when the fitness value of any particular trait is dependent on its prevalence in the population. For example, as the number of individuals possessing a certain phenotype (i.e., the observable physical, biochemical, and behavioral characteristics of an organism) increases, the adaptive value of possessing an alternative phenotype increases. At the Thanksgiving table, the more people who prefer the white meat of the turkey, the better it is for those who prefer the dark meat, because there will be more of that available. Thus, social competition drives individuals into different social niches. Filling these alternative niches offers some release from social competition.
The variation in personality documented in nonhuman animals appears to be almost entirely confined to social species. Niche-splitting (i.e., the fragmentation of the ecological space into more specialized niches) within a species leads to character displacement (i.e., the differentiation of individual traits to adapt to these different niches). The cost of deviating from the species-typical optimum (e.g., eating the dark meat when the white meat is actually better) is compensated by the benefit of release from competition (e.g., getting all the dark meat). The result for the population is an “ideal free distribution” of alternative behavioral phenotypes in which the balance of costs and benefits are equalized among competing individuals. For example, there is a tradeoff between territory size and territory quality, where resource-rich territories are generally smaller than resource-poor territories (e.g., the same amount of money will buy less land in New York than in Arizona). The resulting dispersion of individual phenotypes away from the modal norm of response for the species will thus create bell-shaped curves along different dimensions of personality.
Evolutionary Developmental Theory
Brunswikian evolutionary developmental theory proposes that when ecological conditions are variable over evolutionary time, natural selection favors organisms that are phenotypically flexible enough to adapt by means of learning from environmental contingencies (i.e., the potentially different consequences of the same behavior in different ecological contexts). However, according to developmental plasticity theory, this ability depends critically on the presence of reliable and valid cues that signal which alternative phenotype it is best to adopt in that particular time, space, and ecology. When reliable and valid cues are absent, natural selection favors the production of genetically diverse individuals that possess a wider variety of the different phenotypes that might be required to fit the locally and temporally varying conditions. Because many cues are often neither completely reliable and valid nor unreliable and invalid, Brunswikian evolutionary developmental theory describes these cues as possessing an ecological validity coefficient ranging between 0 and 1.
Combining these two theories leads to the prediction that individuals will manifest both sufficient developmental plasticity and sufficient genetic diversity to collectively fill all the available niches. Human personality traits such as the Big Five personality traits (i.e., Openness to Experience, Conscientiousness, Extra-version, Agreeableness, and Neuroticism) have heritability coefficients ranging from 30% to 50%. Heritability coefficients indicate the proportion of the phenotypic variation in a population that is attributable to genetic variation among individuals. The remainder of the variance is attributable to environmental influences and measurement error. Therefore, the combination of partial heritability and partial environmentality observed in human personality variation conforms precisely to the predictions of the combination of these two theories. Where environmental cues are only partially reliable and valid, as might very often be the case, a combination of genetic diversity and developmental plasticity should become the optimal adaptive strategy.
Behavioral Flexibility
An inability to adapt one’s behavior to the different situations that one encounters in life can be considered the very definition of a personality disorder. For example, in the absence of a personality disorder, an extraverted person would still be much more likely to behave in a more introverted manner at a funeral than they would in a bar. In contrast, a personality-disordered individual would not be able to adjust his or her behavior to be situationally appropriate.
Nevertheless, personality traits are somewhat static in nature, even in normal (nondisordered) individuals. This is variously called the temporal stability and the cross-situational consistency of personality traits. This means that, although personality differences are adaptive, an individual’s behavioral flexibility may be constrained by the combination of personality traits that he or she possesses. Personality traits differ in their potential to deal effectively with particular environmental contingencies. For this reason, individuals vary in their ability to exploit the range of possibilities inherent in the situations that they might encounter.
Although some individuals might be more biologically prepared than others to behave in a particular way, they will still demonstrate some adaptive flexibility in their behavior by selecting situations that are more favorable to the expression of their genetically constrained behavioral dispositions. For example, individuals tend to gravitate toward the types of environmental situations that are most suitable for their personalities. Extraverts are more likely to be found at parties, while introverts are more likely to avoid such social situations. Thus, the cost of any constraints imposed by the possession of individual personality traits can be overcome by the benefits entailed in the selection of suitable social and ecological niches.
Personality Disorders
These theories of the adaptive significance of traits within circumscribed social niches provide an explanation of the normal human range of variation in personality traits. Two somewhat overlapping frameworks have been offered to explain the existence of personality disorders. One view is that disorders are the result of a mismatch between inherited characteristics and current environmental conditions. The mismatch hypothesis suggests that the rapid rate of change in the human physical environment (e.g., from rural agricultural to high population density industrial) and social environment (e.g., from extreme social stratification represented by slavery and male superiority to more equalitarian norms) may not have allowed for a good fit between evolved personalities and current conditions, even within the lifetime of an individual. Natural selection simply has not had enough time to build up mechanisms that allow adaptation to these rapid changes occurring within historical time. However, this argument seems to be flawed in many respects. It implies that personality disorders did not exist before the rapid social change began. Furthermore, it ignores the overwhelming evidence that some personality disorders are due to biochemical imbalances.
An alternative perspective is that some variations are ill suited for particular environments. The harmful dysfunction hypothesis suggests that personality disorders may be due to a mismatch between organism and physical environment, or the result of sociocultural constraints that limit the effective functioning of otherwise effective mechanisms. For example, specific traits that are deemed unacceptable by society due to cultural norms or traditions may be labeled “abnormal,” “disordered,” or “dysfunctional” even if the traits are functioning as they were “designed” to function by natural or sexual selection. Universal standards have been shown to exist across cultures for disgust, fear, landscape preference, mate choice, and cheater detection. Hence, it is not a stretch to imagine that the same mechanisms may be at work in both the production of personality variation and in the manner in which personality traits are interpreted by others.
The psychiatric diagnosis of antisocial personality disorder (APD) can be used to illustrate these perspectives. APD is a disorder characterized by egocentric grandiosity and Machiavellianism (a tendency to deceive and manipulate others for personal gain); a smooth, glib capability to lie; shallow emotional affect; dominance; impulsivity; insensitivity to risk; goal directedness, sometimes with a lack of social emotions; and an overall lack of regard for the moral or legal standards in the local culture. The pervasive pattern of disregard for the rights of others that characterizes APD raises the question of whether this is in fact a real disorder or a group of interrelated behaviors that may be valued or devalued based on their environmental or social utility. For example, a “cheater phenotype” may increase an individuals overall reproductive success within certain ecological niches. Many “psychopaths” are known philanderers, with a history of several failed marriages and offspring by multiple partners. It is only when a mismatch between cultural constraints (e.g., norms, customs, and laws) interacts with the behaviors associated with APD that any harm or dysfunction is manifest.
The connection between the mismatch hypothesis and harmful dysfunction models of personality disorders is important in determining which physical or behavioral characteristics are truly abnormal from an evolutionary perspective. Thus, an evolutionary psychological perspective not only bridges the perceived gap between biology and culture, but it also gives a potentially workable etiology (i.e., a theory of causation) that connects physical and behavioral disorders within a broader theoretical framework.
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