Three social science approaches—evolutionary psychology, behavioral ecology, and behavioral genetics—share the metatheory of evolution. They also suggest several mechanisms that may account for heritable individual differences in personality and motivation, including stabilizing selection, fluctuating selection, trade-offs, balancing selection, life history theory, and behavioral syndromes. These mechanisms are discussed as possible explanations for individual differences in the five-factor model of personality and in a new theory of human motivation. The theory postulates that 15 latent motive dimensions evolved in humans to facilitate behavior in five social domains. Trade-offs that, in combination with fluctuating and balancing selection, might have maintained individual differences in motive phenotypes are described. The reliability and validity of a method to assess individual differences in the strength of these motive dimensions is also discussed.