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Evolutionary explanations of anger as a commitment device hinge on two key assumptions. The first is that it is observable ex-ante whether someone will get angry when feeling badly treated. The second is that anger is associated with destructive behavior. We test the validity of these assumptions by studying whether observers are able to detect who rejected a low offer in an ultimatum game. We collected photos and videos of responders in an ultimatum game before they were informed about the game that they would be playing. We showed pairs of photos or videos, consisting of one responder who rejected a low offer and one responder who accepted a low offer, to an independent group of observers. We find support for the two assumptions. Observers do better than chance at detecting who rejected the low offer, especially for rejecters who get angry at low offers.
