We compare behavior in modified dictator games with and without role uncertainty. Costly surplus creating actions are most frequent with role uncertainty while selfish behavior is most frequent without role uncertainty. A classification of subjects into four different types of preferences (Selfish, Social Welfare maximizing, Inequity Averse and Competitive) shows that role uncertainty overestimates (underestimates) the prevalence of Social Welfare maximizing (Selfish and Inequity Averse) preferences in the subject population. Our results have important methodological implications for experiments used to measure the prevalence of interdependent preferences.