Why We Cooperate

A key element of human morality is prosocial behavior. Humans are unique among animals in their willingness to pay costs to benefit unrelated friends and strangers. This cooperation is a critical part of our identity as moral beings. Here, we ask why humans cooperate, and what can explain variation in cooperative behavior across social groups, individuals, and situations. We begin by defining cooperation, and discussing economic games as laboratory models for studying cooperative behavior. Next we provide an overview of several mechanisms that can make cooperation advantageous in the long run: direct reciprocity, indirect reciprocity, and institutions. We then turn to cooperative behavior that occurs beyond the reach of such mechanisms and thus is truly costly. We discuss the Social Heuristics Hypothesis, which contends that when individuals learn that cooperative strategies are generally advantageous, they become internalized as intuitive defaults. These defaults can spill over to produce “irrational” cooperative behavior even when externally imposed incentives are absent. Thus, we suggest that daily experience plays an important role in shaping culturally varying conceptions of cooperation as a virtuous behavior. Finally, we close by discussing open questions for future research on cooperation.