Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Dr. Cynthia J. Pietras

Second Advisor

Dr. Scott Gaynor

Third Advisor

Dr. Alan Poling

Fourth Advisor

Dr. Maarten Vonhof


The present study will experimentally investigate human cooperation (sharing) in a laboratory foraging task that simulates environmental variability and resource scarcity (shortfall risk). Specifically, it investigates whether a risk-reduction model of sharing developed by evolutionary biologists (derived from a risk-sensitive optimization model known as the energy-budget rule) could predict human cooperative behavior. Participants respond to earn points exchangeable for money when point gains were unpredictable. Failures to acquire sufficient points result in a loss of accumulated earnings (a shortfall). Participants are given the choice between working alone or working with others. The difficulty of meeting the earnings requirement is manipulated across conditions to investigate the effects of economic context on sharing. The effects of social variables on sharing are also investigated to determine whether they constrain optimal decision making. Thus, the experiments investigate cooperation when: (1) participants are told the other person is a computer or another person (who is actually fictitious), (2) the correlation in earnings between the participant and partner is manipulated, (3) the earnings between the participant and partner are inequitable, and (4) the probability of reciprocity from the partner is varied. Overall, the results show that sharing is well predicted by a risk-reduction model of sharing. Sharing occurs under relatively good environmental conditions (positive budget conditions) when sharing reduces shortfall risk, but sharing does not occur under poor environmental conditions (negative budget conditions) when working alone is the only way to meet earnings requirements. Social variables have no effect on optimal decision making. These results contribute to our understanding of how economic conditions and social stimuli influence cooperation, and indicate that risk-reduction models may be useful for predicting cooperation, and failures to cooperate, among various social units, including businesses, political organizations, socio-cultural groups, and individuals. They also provide additional evidence that foraging models developed by evolutionary biologists can be useful tools for analyzing and predicting human behavior, including behavior involving monetary outcomes.

Access Setting

Dissertation-Open Access