Reciprocal altruism is the name given to the provision of goods and/or services by one organism to another, when the cost incurred by the donor in such provision will later (or sometimes, immediately) be repaid in the form of a reciprocated act—a returned favor, of sorts. By "cost" in the preceding sentence, I mean "cost to overall fitness for survival and/or reproduction." Reciprocal altruism is seen throughout the animal kingdom, most notably in human beings.

In a landmark 1971 paper, titled "The Evolution of Reciprocal Altruism," purporting to reconcile altruism and evolution, Robert Trivers1 introduced the concept of reciprocal altruism to academia. He examined the ways in which altruistic acts can indirectly benefit the altruist herself, in the form of reciprocated behaviors—quid pro quo.

Trivers' most detailed work is on reciprocal altruism in humans. He writes, "The strongest argument for the operation of reciprocal-altruistic selection in man is the psychological system controlling some forms of human altruism" (46). But he starts by giving several examples in other species, including fish and birds. In these examples, individual organisms use their faculty of identifying specific other individual organisms, to cultivate one-on-one relationships in which altruistic behaviors are reciprocated; favors are exchanged.

Most intriguing is Trivers' reverse-engineering of the human moralistic emotions, examining several emotions and setting out an explanation of the evolutionary advantage there would be for an organism with the capacity for such an emotion, especially in light of that emotion's potential role in causing selectivity in altruistic behavior. Emotions like "liking," anger, jealousy, and so on, help to regulate the favor-exchange between humans, making sure that favors which are doled out are also repaid.

Trivers' model is designed to show how certain classes of behavior denoted as "altruistic" can be selected for even when the recipient is so distantly related to the organism performing the altruistic act that another evolution-derived morality theory, kin selection, can be ruled out. The model of reciprocal altruism applies, for example, to altruistic behavior between members of different species (35).
1. The "tri" is pronounced as in "trivia."

Page numbers above refer to the Trivers paper itself. The complete citation for that paper is:
Trivers, Robert L. "The Evolution of Reciprocal Altruism." The Quarterly Review of Biology 46:1 (Mar 1971), 35-57.