Null results for the effect of the behavioral immune system on in-group cooperation.

The behavioral immunes system (BIS) is an evolutionary motivational mechanism that facilitates disease-avoidance behavior, and recent studies suggested that the activation of the system would influence various forms of intergroup attitudes. However, existing research tended to focus on attitudinal variables, and the association between the system and intergroup behavior has been understudied. Thus, the present research aimed to elucidate the influence of the activation of the BIS on in-group cooperation, one of the key areas of research on intergroup behavior. As in-group cooperation is often explained by another evolutionary mechanism, the reputation system, we orthogonally manipulated the activation of these two independent systems and investigated in-group cooperation in a prisoners' dilemma. Contrary to previous findings, we did not detect in-group favoring pattern, and it was found that neither of the BIS and reputation system significantly influenced in-group cooperation. We briefly discussed the implications of the null results and called for future research to further qualify our findings.