Conditional cooperation and voluntary contributions to public goods

We compare a partners condition, where the same small group of subjects plays a repeated public good game, to a strangers condition, where subjects play this game in changing group formations. From the first period onward, subjects in the partners condition contribute significantly more to the public good than subjects in the strangers condition. Strangers' contributions show continual decay, while partners' contributions fluctuate on a high level prior to decreasing in the final periods. We interpret subjects' behaviour in terms of conditional cooperation which is characterized by both future-oriented and reactive behaviour.