Similarity between partners entails positive consequences for cooperative interactions. But do people rely on this assumption to construe egocentric judgments about others? Five experiments examined the possibility that people project onto their partners because they believe that similarity to the self leads to success in cooperation. Studies 1a and 1b show that people hold an egocentric similarity belief in cooperation. Studies 2a and 2b test the existence of this belief in more indirect ways. The next three studies manipulate the applicability of the similarity belief and investigate its impact on projection. Study 3 finds that cooperation no longer leads to projection when participants expect a low probability of success. Study 4 replicates this effect in a real cooperative setting. Finally, Study 5 shows that projection occurs only when participants expect their characteristics to be responsible for the success of cooperation. The negative consequences of overestimating similarities in cooperation are discussed.