Human cooperation in social dilemmas: comparing the Snowdrift game with the Prisoner's Dilemma.

Explaining the evolution of cooperation among non-relatives is one of the major challenges for evolutionary biology. In this study, we experimentally examined human cooperation in the iterated Snowdrift game (ISD), which has received little attention so far, and compared it with human cooperation in the iterated Prisoner's Dilemma (IPD), which has become the paradigm for the evolution of cooperation. We show that iteration in the ISD leads to consistently higher levels of cooperation than in the IPD. We further demonstrate that the most successful strategies known for the IPD (generous Tit-for-Tat and Pavlov) were also successfully used in the ISD. Interestingly, we found that female players cooperated significantly more often than male players in the IPD but not in the ISD. Moreover, female players in the IPD applied Tit-for-Tat-like or Pavlovian strategies significantly more often than male players, thereby achieving significantly higher pay-offs than male players did. These data demonstrate that the willingness to cooperate does not only depend on the type of the social dilemma, but also on the class of individuals involved. Altogether, our study shows that the ISD can potentially explain high levels of cooperation among non-relatives in humans. In addition, the ISD seems to reflect the social dilemma more realistically than the IPD because individuals obtain immediate direct benefits from the cooperative acts they perform and costs of cooperation are shared between cooperators.