Among many nonhuman animals, individuals differ consistently in their response tendencies (e.g., shy vs bold) across multiple contexts. Researchers have tested evolutionary hypotheses accounting for these phenomena, and have also begun exploring evolutionary explanations for human personality variation. For evolutionary biologists, a trait's significance lies in its effects on
fitness
, that is, the lifetime reproductive success of individuals who bear the trait, including indirect effects through the reproductive success of genetic relatives. Recent evolutionary personality research has pursued several alternative theoretical lines of inquiry:
balancing selection
models explore whether optimal levels of personality traits vary across time, space, or trait frequency distribution;
mutation‐selection balance
models propose that selection for a single optimum personality configuration is undermined by mutations at multiple genetic loci; and
facultative calibration models
hold that personality trait levels are adjusted, during individual development, to other characteristics that affect social bargaining power. A promising general approach links personality variation to variation in life history strategy, that is, the allocation of effort among the competing demands of growth, somatic maintenance, mate acquisition, and parental investment. Emerging areas of research include relationships between personality variation and biological fitness in humans and other primates; the extent to which personality trait levels are adjusted based on individual condition; the degree to which situational flexibility varies among individuals; and whether proposed structural models of personality, such as the human five‐factor model (extraversion, neuroticism, agreeableness, conscientiousness, and openness to experience) are species‐typical or are affected by variable ecological and social conditions.