Mean values and standard deviations (in brackets) of the first round, the first 10 rounds, the last 10 rounds, and overall contributions by condition.

INTRA: intra-group comparison only. INTER: intra- and inter-group comparison. COOP+: public good with n = 3 and individual return from cooperation = 0.7. COOP–: public good with n = 3 and individual return from cooperation = 0.4. Reported are mean values on the level of independent observations. Thus, values of first round contributions are mean values on the individual level. Values of contributions in round 1–10, round 11–20, and overall are mean values on the level of intra-groups (INTRA) or on the level of matched groups (INTER).