Multiple selective pressures maintain and increase heritable behavioral variability among individuals across both developmental and evolutionary time: (1) directional social selection favors convergent traits, promoting mutually beneficial cooperative interactions; (2) disruptive social selection favors divergent traits, providing release from within-species competition; (3) genetic diversification responds adaptively to the stochastic (random) characteristics of environmental hazards such as uncontrollable morbidity (disease) and mortality (death); (4) developmental plasticity epigenetically directs development adaptively along different alternative pathways, modifying permanent and stable behavioral dispositions to suit long-term contingencies of survival and reproduction; and (5) behavioral flexibility deploys rapid and reversible short-term adaptive behavioral responses to transient situations.
Encyclopedia of Human Behavior
Figueredo, A. J., Sefcek, J. A., Black, C. J., Garcia, R. A., & Jacobs, W. J. (2012). Evolutionary personality psychology. In V. S. Ramachandran (Ed.), Encyclopedia of human behavior (2nd ed., pp. 111-117). Oxford, England: Elsevier/Academic Press.