Many people are conditionally cooperative: they cooperate if others do so as well. Conditional cooperation is usually investigated in experiments where the choices of others are known. In many circumstances, however, there is uncertainty about the cooperativeness of others. Using a novel experimental protocol, we manipulate the information subjects receive regarding the likelihood that their partner cooperates in a Prisoner's Dilemma, and whether this likelihood is described unambiguously or learned through experience and thus ambiguous. In all treatments, subjects' cooperation rate increases monotonically with the likelihood that their partner cooperates. Comparing decisions made under description to those made under experience, we observe a description-experience gap in which rare events appear to be more influential under experience than under description. This contrasts with earlier results from the individual choice literature, which typically finds the opposite pattern. Additional measures reveal that the gap is driven by conditional cooperators, who seek and respond to social information more than other types. We argue that stronger priors under social than individual uncertainty can account for this reversal and, in a second experiment, confirm that priors are indeed stronger under social uncertainty.