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

Abstract

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.

Figure 1

The mean±s.e. proportion of cooperative acts in 12 interactions between two females (FF), a female and a male (FM) or two males (MM) in the IPD and the ISD. Numbers below the data points represent the sample size (i.e. the number of player pairs).

Figure 2

Fluctuation in the proportion of cooperative acts across 12 interactions in the IPD (filled circles) and the ISD (open circles; n=24 pairs of players in both games).

Figure 3

The players' strategies classified as (a) a TFT-like strategy (grey bars, n=19; white bars represent expected values for generous TFT) or (b) a Pavlovian strategy (grey bars, n=32; white bars represent expected values for Pavlov). The p-values (mean±s.e.) indicate the probability of cooperation after both players cooperated (p₁), after the focal individual cooperated and opponent defected (p₂), after the focal individual defected and the opponent cooperated (p₃) and after both players defected (p₄). As there were no significant differences in the p_1–4 values between the IPD and the ISD, we present the pooled results across both the games.

Figure 4

Comparison of the total pay-off (mean±s.e.) after 12 interactions achieved by players using a TFT-like (white bars), a Pavlovian (black bars) or an undefined (grey bars) strategy in the IPD and the ISD. Asterisks indicate significant differences in the pay-off between two strategies.