This article begins with the observation that people often cooperate in social dilemmas. Specifically, they may do so either for altruistic reasons or for reasons indirectly linked to self-interest. Because these competing motivations exist in different contexts, we explore the consequences of the incorporation of altruism in the economic framework in the context of a series of prisoner's dilemma experiments. Our experiments involve student subjects in groups of three. Each student plays two rounds of a monetary prisoner's dilemma game with the other two subjects in that group. We find evidence for recognizably different levels of the altruistic behavior of cooperation due to personal attributes and specific experimental conditions. Cooperation rates are higher for female students and more senior students. Of more interest is the evidence that economics majors tend to behave more cooperatively than noneconomics majors.