Differential psychologists rightly identified evolutionary theory as a unifying framework for
explaining the origins and persistence of individual differences in a wide array of human
psychological characteristics. Psychological diversity occurs on multiple levels, including between
species, populations, generations, and individuals. Each level reveals the outcome of
evolutionary processes at different temporal scales. I embrace a range of methods and results
from quantitative and population genetics, developmental evolution, and phylogenetically grounded
comparative psychology to explore how personality evolves in humans and nonhuman
primates. At the level of species, I compared personality structure derived from
rater assessments for four species of macaques and found a consistent, core set of personality
dimensions (Dominance, Confidence, and Friendliness) describing these species. At the
population level, I studied the relationship in humans between fertility/longevity trade-offs
and the average personality of a country and found that Neuroticism and Agreeableness
exhibit adaptively plasticity to life-history conditions. At the level of families, I estimated
the quantitative genetic structure of personality in orang-utans and found that, like humans,
a large portion of the phenotypic variance was explained by non-additive genetic effects. I
examined between generation changes in personality by testing whether personality traits
in humans are genetically correlated with fitness and found that in modern environments
personality evolves very slowly. Finally, I translated current conceptual models of biological
reactivity and stress response into mathematical models of developmental evolution and
determined that evolution would select highly resilient phenotypes but that variation could
be maintained by skew in the distribution of underlying genetic factors. From these results
I broadly conclude that primate personality structure is generally conserved among species,
mean personality levels change only very slowly between human generations, and that this
evolution results in a genetic basis of personality that is characterized by epistasis. The evolution
of individual differences has much to gain from the rigorous application of evolutionary
methodology