Conditional cooperation and voluntary contributions to public goods

We compare a partners condition, where the same small group of subjects pla ys a repeated public good game, to a strangers condition, where subjects pl ay this game in changing group formations. From the first period onward, su bjects in the partners condition contribute significantly more to the publi c good than subjects in the strangers condition. Strangers' contributions s how continual decay, while partners' contributions fluctuate on a high leve l prior to decreasing in the final periods. We interpret subjects' behaviou r in terms of conditional cooperation which is characterized by both future -oriented and reactive behaviour.