In addition to kin selection and direct reciprocity, he shows that:

It has been observed that image scoring promotes cooperative behavior in situations where direct reciprocity is unlikely.

Direct reciprocity was proposed by Robert Trivers as a mechanism for the evolution of cooperation.

The hypothesis of direct reciprocity was proposed by Trivers (1971) to explain altruistic behaviors among nonrelatives.

Direct reciprocity.

Gleeson argues this is only relevant in societies where a direct reciprocity is assumed between individual work and individual reward.

Direct reciprocity states that individuals are more likely to cooperate with individuals that they are likely to encounter again.

There have been many hypotheses set forward to explain the formation and stability of male alliances, most notably kin selection, direct reciprocity and mutualism.

All of these are similar to direct reciprocity, since the beneficiaries are identified as such in each case, and contributors know exactly what they can expect in return.

Three mechanisms have been hypothesized to reconcile the principles of natural selection and cooperation: kin selection, direct reciprocity and mutualism .

The coalitions formed by the lower ranking males have been interpreted as direct reciprocity because the cooperative subordinate males have presumably cooperated before.

Direct reciprocity and cooperation in a group can be increased by changing the focus and incentives from intra-group competition to larger scale competitions such as between groups or against the general population.

Direct reciprocity can lead to the evolution of cooperation only if the probability, w, of another encounter between the same two individuals exceeds the cost-to-benefit ratio of the altruistic act:

It is also proposed that separation from outdoor contact causes a loss of sensory and information-processing ability that was developed over the course of human evolution, which was spent in direct reciprocity with the environment.

Direct reciprocity can operate between unrelated individuals because the benefit to the receiver is greater than the cost to the altruist; therefore, when both individuals have been on the receiving end of their partnership they have both increased their fitness.

In a paper in Science in 2006 Nowak enunciated and unified the mathematical rules for the five understood bases of the evolution of cooperation (kin selection, direct reciprocity, indirect reciprocity, network reciprocity, and group selection).

Male alliances have been hypothesized to have evolved within the context of kin selection in red howler monkeys, within the context of direct reciprocity in savanna baboons and within the context of mutualism in lions.