  For reciprocal altruism to work, there is no need for the two individuals to be relatives, nor even to be members of the same species. However, it is necessary that individuals should interact with each more than once, and have the ability to recognize other individuals with whom they have interacted in the  past.[1]   If individuals interact only once in their lifetimes and never meet again, there is obviously no possibility of return benefit, so there is nothing to be gained by helping another. However, if individuals encounter each other frequently, and are capable of identifying and punishing ‘cheaters’ who have refused to help in the past, then the helping behaviour can evolve.  A ‘cheat’ who refuses to help will ultimately sabotage his own interests, for although he does not incur the cost of helping others, he forfeits the return benefits too—others will not help him in the future. This evolutionary mechanism is most likely to work where animals live in relatively small groups, increasing the likelihood of multiple encounters.