  The theory of reciprocal altruism was originally developed by Trivers (1971), as an attempt to explain cases of (apparent) altruism among unrelated organisms, including members of different species. (Clearly, kin selection cannot help explain altruism among non-relatives.) Trivers' basic idea was straightforward: it may pay an organism to help another, if there is an expectation of the favour being returned in the future. (‘If you scratch my back, I'll scratch yours’.) The cost of helping is offset by the likelihood of the return benefit, permitting the behaviour to evolve by natural selection. Trivers termed with evolutionary mechanism ‘reciprocal altruism’.