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In evolutionary biology, reciprocal altruism is a form of altruism in which one organism provides a benefit to another in the expectation of future reciprocation. This is equivalent to the Tit for tat strategy in game theory. It would only be expected to evolve in the presence of a mechanism to identify and punish "cheaters". An example of reciprocal altruism is blood-sharing in the vampire bat, in which bats feed regurgitated blood to those who have not collected much blood themselves knowing that they themselves may someday benefit from this same donation; cheaters are remembered by the colony and ousted from this collaboration.

In a series of ground-breaking contributions to biology in the early 1970s Robert Trivers introduced the theories of reciprocal altruism (1971), parental investment (1972), and parent-offspring conflict (1974). Trivers' paper "The Evolution of Reciprocal Altruism" (1971) elaborates the mathematics of reciprocal altruism and includes human reciprocal altruism as one of the three examples used to illustrate the model, arguing that "it can be shown that the details of the psychological system that regulates this altruism can be explained by this model." In particular, Trivers argues for the following characteristics as functional processes subserving reciprocal altruism:

A complex regulating system

The system subserving reciprocal altruism will be sensitive and unstable because it will often pay to cheat. For reciprocal altruism to function, therefore, "natural selection will rapidly favour a complex psychological mechanism in each individual regulating both his own altruistic and cheating tendencies and his responses to these tendencies in others".

Friendship and the emotions of liking and disliking

The immediate emotional rewards motivating altruistic behaviour and partnerships will be the tendency to like others, to form friendships, and to act altruistically towards friends and likeable acquaintances. "Selection will favour liking those who are themselves altruistic" and for reproductive purposes.

Moralistic aggression

As cheaters will take advantage of any positive emotions motivating altruistic behaviour there will be selection for a protective mechanism. Moralistic aggression will "counteract the tendency of the altruist, in the absence of any reciprocity, to continue to perform altruistic acts for his own emotional rewards". It will also educate the unreciprocating individual, and in extreme cases "select directly against the unreciprocating individual by injuring… killing, or exiling him

Gratitude, sympathy, and the cost/benefit ratio of an altruistic act

Gratitude regulates the "human response to altruistic acts" and is sensitive to the cost/benefit ratio of such acts. In addition, sympathy "has been selected to motivate altruistic behaviour as a function of the plight of the recipient".

Guilt and reparative altruism

If cheating is detected then reciprocity will end, at considerable cost to the cheater, therefore "the cheater should be selected to make up for his misdeed and to show convincing evidence that he does not plan to continue his cheating sometime in the future". In order to motivate a reparative gesture "guilt has been selected for in humans partly in order to motivate the cheater to compensate his misdeed and to behave reciprocally in the future, and thus to prevent the rupture of reciprocal relationships".

Subtle cheating: the evolution of mimics

Selection will favour the mimicking of all traits subserving reciprocal altruism "in order to influence the behaviour of others to one's own advantage". Subtle cheating may involve sham moralistic aggression, sham guilt, sham sympathy, and "the hypocrisy of pretending one is in dire circumstances in order to induce sympathy-motivated altruistic behavior". Controversially, the resulting possibility of a stable evolutionary equilibrium with a low percentage of mimics is used to argue for the adaptivity of sociopathy.

Detection of the subtle cheater: trust-worthiness, trust, and suspicion

Selection will favour the detection of moralistic aggression and "distrusting those who perform altruistic acts without the emotional basis of generosity or guilt because the altruistic tendencies of such individuals may be less reliable in the future".

Setting up altruistic partnerships

Because "humans respond to acts of altruism with feelings of friendship that lead to reciprocity" selection will favour the strategy "do unto others as you would have them do unto you". Altruistic acts towards strangers and enemies may induce friendship.

Multiparty interactions

Particularly in ancestral times humans would have lived in small, close-knit groups where "selection may favour learning from the altruistic and cheating experiences of others, helping others coerce cheaters, forming multiparty exchange systems, and formulating rules for regulated exchanges in such multiparty systems".

Developmental plasticity

As the conditions under which reciprocal altruism can operate will vary widely according to ecological and social conditions, and will vary through time for the same population "one would expect selection to favour developmental plasticity of those traits regulating altruistic and cheating tendencies and responses to these tendencies in others". No simple developmental system would be expected to meet the requirements to be adaptive because "altruistic behaviour must be dispensed with regard to many characteristics of the recipient (including his degree of relationship, emotional makeup, past behaviour, friendships, and kin relations) of other members of the group, of the situation in which the altruistic behaviour takes place, and of many other parameters". Such a system could only function effectively through the developmental plasticity that would accommodate education about the appropriate response, especially from kin. For example, education of the sense of guilt could permit "those forms of cheating that local conditions make adaptive and to discourage those with more dangerous consequences".


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