Cooperation and competition between twins: Findings from a Prisoner's Dilemma game

Attention to factors influencing cooperation and competition during human s ocial interaction has increased within recent years. This study tested the hypothesis that higher levels of cooperation would be associated with incre ased genetic relatedness between interactants, and explored questions conce rning the expression of cooperative behavior over time. A Prisoner's Dilemm a game, in which participants elect to display cooperative, competitive, or exploitative behaviors relative to a partner, was administered to 59 monoz ygotic and 37 dizygotic twin pairs, between 10.92 and 82.67 years of age. R esults from multivariate analysis of variance procedures, cross-lag sequent ial analyses, and hierarchical linear modeling supported associations betwe en zygosity, and frequency and continuity of cooperation. Mechanisms by whi ch zygosity may affect cooperation were explored by examining relationships between response combinations, and twins' IQ similarity and social closene ss. The findings are considered with reference to an evolutionary perspecti ve on behavior that offers a theoretical basis for considering how the rela tive genetic relatedness of social partners affects their social-interactio nal processes and outcomes. This report is the first in a series of studies designed to address mechanisms underlying differences in cooperation among pairs who vary in average genetic commonality. (C) 1999 Elsevier Science I nc.