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In evolutionary biology, reciprocal altruism is a behaviour whereby an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, with the expectation that the other organism will act in a similar manner at a later time. The concept was initially developed by Robert Trivers to explain the evolution of cooperation as instances of mutually altruistic acts. The concept is close to the strategy of "tit for tat" used in game theory. ==Theory== The concept of "reciprocal altruism", as introduced by Trivers, suggests that altruism, defined as an act of helping someone else although incurring some cost for this act, could have evolved since it might be beneficial to incur this cost if there is a chance of being in a reverse situation where the person whom I helped before may perform an altruistic act towards me. Putting this into the form of a strategy in a repeated prisoner’s dilemma would mean to cooperate unconditionally in the first period and behave cooperatively (altruistically) as long as the other agent does as well.〔 If chances of meeting another reciprocal altruist are high enough or the game is repeated for a long enough amount of time, this form of altruism can evolve within a population. This is close to the notion of "tit for tat" introduced by Anatol Rapoport,〔Dawkins, Richard (2006). The Selfish Gene (30th Anniversary ed.). United Kingdom: Oxford University Press. ISBN 0-19-929114-4.〕 although there still seems a slight distinction in that "tit for tat" cooperates in the first period and from thereon always replicates an opponent’s previous action, whereas “reciprocal altruists” stop cooperation in the first instance of non-cooperation by an opponent and stay non-cooperative from thereon. This distinction leads to the fact that in contrast to reciprocal altruism, tit for tat may be able to restore cooperation under certain conditions despite cooperation having broken down. Stephens shows a set of necessary and jointly sufficient conditions “… for an instance of reciprocal altruism: # the behaviour must reduce a donor's fitness relative to a selfish alternative;〔 # the fitness of the recipient must be elevated relative to non-recipients;〔 # the performance of the behaviour must not depend on the receipt of an immediate benefit;〔 # conditions 1, 2, and 3 must apply to both individuals engaging in reciprocal helping.〔 There are two additional conditions necessary "…for reciprocal altruism to evolve:"〔 * A mechanism for detecting 'cheaters' must exist.〔 * A large (indefinite) number of opportunities to exchange aid must exist.〔 The first two conditions are necessary for altruism as such, while the third is distinguishing reciprocal altruism from simple mutualism and the fourth makes the interaction reciprocal. Condition number five is required as otherwise non-altruists may always exploit altruistic behaviour without any consequences and therefore evolution of reciprocal altruism would not be possible. However, it is pointed out that this “conditioning device” does not need to be conscious. Condition number six is required to avoid cooperation breakdown through backwards induction—a possibility suggested by game theoretical models.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「reciprocal altruism」の詳細全文を読む スポンサード リンク
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