Past research has revealed that people have a strong tendency to respond to others' cooperative behavior with cooperation and to others' noncooperative behavior with noncooperation. Yet it is unclear whether or not this tendency still holds when people have only incomplete information about their interaction partner's past behavior. To address this question, we designed a new paradigm - a coin allocation paradigm - in which participants were provided with incomplete information about another person's degree of cooperative behavior (i.e., coin allocations to the participant). Consistent with our hypotheses, two experiments revealed that incompleteness of information undermines both expectations about another person's cooperation as well as one's own cooperation. Moreover, complementary analyses indicated that the detrimental effects of incompleteness of information on cooperation were mediated by expectations of other's cooperation. We suggest that a relatively strong belief in self-interest serves to fill in the blanks when information is incomplete, which undermines expectations of other's cooperation as well as one's own cooperative behavior. © 2012 Elsevier B.V..