A persistent anomaly in the social dilemmas literature is the surprisingly high level of cooperation observed in experimental investigations of the one-shot Prisoners' Dilemma (PD). The exchange heuristic hypothesis and related approaches explain this finding by arguing that actors subjectively transform PD into the Assurance Dilemma. A tendency to transform PD into Assurance is expected to increase cooperation because defection is the dominant strategy in PD, but not in Assurance. Although existing arguments explain the processes through which these subjective transformations occur, they do not predict who actually makes the transformations. This study incorporates arguments from the social values literature and proposes four hypotheses about who transforms PD and how this affects cooperation. The hypotheses are tested against the results of a new experiment. The results strongly support the hypotheses and the overarching argument that "prosocials" transform PD into Assurance, while "individualists" play PD "as is." (Contains 4 tables and 8 footnotes.)