Evolutionary theories on altruism among relatives, friends, and group members are considered as the regular parts of the psychological handbooks by now. However, helping to strangers - which is a unique capacity of human beings - has not satisfactorily been explained in psychological studies linked to the Darwinian framework. Why are we generous towards those who are not our relatives, whom we cannot expect to return, even whom we have not met at all? I argue that those emotional and cognitive information-processing mechanisms that predispose humans to behave altruistically towards strangers constitute an inherent part of the human psychological equipment. A question arises, how these mechanisms could have been shaped during evolution, given that altruism may impose certain costs and risks on the altruist. Another crucial question is that what specific psychological processes mediate the adaptive algorithms of prosocial behavior to the actual interpersonal relationships. I suggest four behavioral strategies and psychological mechanisms - and the underlying possible evolutionary processes - that may be responsible for shaping altruism towards strangers: 1. Similarity and sympathy (kin selection); 2. Strong reciprocity and altruistic punishment (group selection); 3. Reputation-gaining, cosily signaling and competitive altruism (individual selection); 4. Cognitive processes that have been primarily shaped for other, non-altruistic tasks, such as theory of mind, Machiavellianism, empathy (exaptation).
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