  Cooperation in one-shot anonymous interactions is a widely documented aspect
of human behaviour. Here we shed light on the motivations behind this behaviour
by experimentally exploring cooperation in a one-shot continuous-strategy
Prisoner's Dilemma (i.e. one-shot two-player Public Goods Game). We examine the
distribution of cooperation amounts, and how that distribution varies based on
the benefit-to-cost ratio of cooperation (b/c). Interestingly, we find a
trimodal distribution at all b/c values investigated. Increasing b/c decreases
the fraction of participants engaging in zero cooperation and increases the
fraction engaging in maximal cooperation, suggesting a role for efficiency
concerns. However, a substantial fraction of participants consistently engage
in 50% cooperation regardless of b/c. The presence of these persistent 50%
cooperators is surprising, and not easily explained by standard models of
social preferences. We present evidence that this behaviour is a result of
social preferences guided by simple decision heuristics, rather than the
rational examination of payoffs assumed by most social preference models. We
also find a strong correlation between play in the Prisoner's Dilemma and in a
subsequent Dictator Game, confirming previous findings suggesting a common
prosocial motivation underlying altruism and cooperation.
