Reciprocal altruism
In evolutionary biology and evolutionary psychology, reciprocal altruism is a form of altruism in which one organism provides a benefit to another without expecting any immediate payment or compensation. However, reciprocal altruism is not unconditional. Firstly the act of altruism must give rise to a surplus of cooperation, in the sense that the gains to the beneficiary must be perceived to be meaningfully larger than the costs to the benefactor. Secondly the act of altruism should be reciprocated by the original beneficiary if the situation is later reversed. Failure to do so will usually cause the original benefactor to withdraw future acts of altruism.
In order for the altruist not to be exploited by non-reciprocaters, it would be expected that reciprocal altruism can only exist in the co-presence of mechanisms to identify and punish "cheaters". This strategy of behaving altruistically at first, but withdrawing aid in the case of non-reciprocation is similar to the tit for tat strategy in game theory.
A potential 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.
Another potential example would be Internet file sharing communities. The ability to download a given file (albeit quite often illegally) directly depends on other people, who already possess this file, sharing (also called seeding) said file. Those who download the file and later refuse to seed it are known as leeches. There are methods to ban leeches, however.

# Robert Trivers' Theories
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 several characteristics as functional processes subserving reciprocal altruism.

# A complex regulating system
For reciprocal altruism to function, natural selection must favor a complex psychology in each individual, regulating the tendency to give, the tendency to cheat, and the response to others' acts of giving and cheating.
- Emotion: The tendency to like others, to form friendships, and to act altruistically towards likeable acquaintances will motivate altruistic behavior.
- Moralistic aggression: Because cheaters take advantage of these positive emotions toward altruistic behaviour, natural selection also develops a protective mechanism: moralistic aggression. The altruist, instead of continuing to perform selfless acts, will try to compel change in the offender. This mechanism can also educate unreciprocating individuals, and in extreme cases isolate, injure, or exile them.
- Gratitude and sympathy: Gratitude regulates the response to altruism in a way sensitive to the cost/benefit ratio of such acts. Sympathy motivates altruism in relation to the plight of the recipient.
- Guilt: If cheating is detected, then reciprocity ends, often at considerable cost to the cheater. Selection therefore pressures the cheater to make up for the misdeed, and to convince others that it will not be repeated. Guilt functions in this way to motivate people toward compensation and reciprocal behavior in the future. Guilt can thus prevent rupture in relationships. Certain psychological problems such as depression may be seen as a mechanism to enhance sincerity and reconciliation.
- Subtle cheating: By mimicking altruism, a person may influence others to one's own advantage. Subtle cheating may involve sham moralistic aggression, sham guilt, sham sympathy, and pretense for awakening sympathy. A stable evolutionary equilibrium could include a low percentage of mimics, in controversial support of adaptive sociopathy.
- Trust: Selection favors the ability to detect moralistic aggression. Those who perform altruistic acts without an emotional basis (generosity or guilt) may be less reliable in the future.
- Partnerships: Altruism can lead to feelings of friendship, which then create reciprocity. The strategy of "doing unto others as you would have them do unto you" can thereby become an advantage. Giving to strangers and enemies may induce friendship.
- Multiparty interactions: Particularly in situations where humans live in small, close-knit groups, one can learn from the altruistic and cheating experiences of others. People may act together to coerce cheaters and form exchange systems, with agreed upon rules.
- Developmental plasticity: As ecological and social conditions vary widely in place and time, the developmental plasticity of altruistic and cheating traits would also bestow an advantage. No simple system could meet the requirements of altruism because of its regard for relationships and situations. The system could function effectively only with plasticity, in learning about appropriate responses, especially from kin. Education about guilt, for example, could permit lesser forms of cheating and discourage those with more dangerous consequences.