Table 1. Descriptive statistics of Study 1. Public Goods Game with 4 players (Condition S) versus PGG with 40 players (Condition L). In both conditions, the maximum possible contribution was $0.10 and the marginal return was γ = 0.5. The results show clearly that the larger group is much more cooperative than the smaller group and this is confirmed by the statistical analysis, showing that the means are signifcantly different (Rank-sum, p = 0.0002).

Condition	% free-riders	% contributors	Mean contribution	SEM
S	48.38	30.64	3.92	0.56
L	21.21	60.60	6.91	0.51