In an experiment using 60 undergraduates as subjects, members of five-person groups decided whether they would contribute their resource money to the prvision of a public good. Expectation of other members' cooperativeness (or the likelihood that thay would contribute to the provision of a public good) was manipulated by letting subjects believe that other members' cost for cooperation (the amount of resource money they were required to contribute) was high (low expectation conditon) or low (high expectation condition). Two types of functions (conjunctive and disjunctive) were used to relate the number of contributors to the amount of public good provided. In the conjunctive condition, an additional contributor increased the amount of public good (bonus money given to all members) more as the number of contributors increased, and the additional increase in the bonus was less as the number of contributors increased in the disjunctive condition. Both trust in other members and the motivation of fre