UC Santa Barbara

Despite the importance of human cooperation, how humans choose their cooperative partners and how they divide the spoils of cooperation are not yet fully understood. Mutual partner choice creates competitive conditions for cooperative relationship formation, in which individuals compete to form cooperative relationships with the most valuable available partners. Biological market theory therefore predicts that the spoils of cooperation will be divided as a function of each party’s cooperative partner value. This dissertation reports research designed to test two interrelated hypotheses: (1) humans will possess psychological mechanisms designed by natural selection to assess the value of an individual as a long-term cooperative partner, and (2) human intuitions about how the spoils of cooperation should be divided will track cues of cooperative partner value.

Six studies were conducted to test these hypotheses. Two commonly-used economic games (the ultimatum game and the trust game) were used to test cooperative partner preferences and resource division intuitions (studies 1-5), and a friend choice task was used to test cooperative partner preferences (study 6). Cooperative partner traits were represented by facial photographs (studies 1, 2 and 6), or were experimentally manipulated with verbal information and in-game monetary consequences (studies 3-5).

Results supported the hypotheses. Participants exhibited preferences for partners who appeared more valuable as long-term cooperative partners, and these preferences appeared to be specialized for ancestral forms of cooperation: participants were more sensitive to cues of ancestral productivity than contemporary productivity, participants were more sensitive to cues of productivity when they revealed stable skills rather than luck, and men relied relatively more on a partner’s productivity (versus their generosity) than did women. In addition, intuitions about resource divisions were sensitive to cues of partner value: partners who appeared more valuable received more favorable divisions in the ultimatum game, and more productive partners were judged as more fair in the trust game, regardless of their actual generosity. The discussion of results focuses on future directions for the application of biological market theory and the logic of cooperative partner choice to problems in social psychology.