The Co-Evolution of Love and Hate

Experimental and other evidence demonstrates that many individuals willingly give to strangers, reward good deeds and punish violations of norms by others even at a significant cost to oneself, and favor fellow group members over others. These behaviors exhibit aspects of both altruism ­ benefitting other group members at a cost to oneself and parochialism, conditioning one's behavior towards others on the degree of similarity in ascriptive characteristics. Both altruism and parochialism are puzzling from an evolutionary perspective as both would appear to reduce individual payoffs (whether fitness or material wellbeing) by comparison to other members of one's group who eschewed these behaviors. Lower payoffs, in turn, are expected to result in the elimination of these behaviors in a population governed by any dynamic in which lower relative payoffs result in a declining frequency of the behavior. The view advanced here is that altruism and parochialism co-evolved, each providing an environment favoring the evolutionary success of the other, and neither being singly capable of proliferation in human populations under conditions approximating those experienced by our Late Pleistocene ancestors. The plausibility of this view is suggested by extensive simulations with an agent-based model of individual-level and group-level selection representing the long term evolution of human behaviors. Altruistic behaviors are modeled at both the individual and group level (groups practice varying degrees of resource sharing), while parochialism is captured by small group size, within-group social segmentation, group boundary maintenance resulting in limited inter-group migration, and hostility towards out group members