Experimental evidence indicates that human cooperation is an important force in social interactions. People cooperate even in anonymous interactions, where there are no future gains to reap from cooperation. This behavior is in line with what has been termed “strong reciprocity”, that is, the tendency to reward others for cooperative acts and punish others for non-cooperative acts at potentially high costs, even when there is no future interaction. However, it is also consistent with the mistaken use of repeated-interaction strategies in one-shot encounters (“maladaptive reciprocal altruism”). We report the results from an experiment designed to unambiguously identify maladaptive reciprocal altruism as a behavioral force separate from strong reciprocity. The experiment shows that maladaptive reciprocal altruism exists and explains a substantial part of the observed cooperation.