The Reciprocity Controversy

Abstract

Reciprocity (or -reciprocal altruism) was once considered an important and widespread evolutionary explanation for cooperation, yet many reviews now conclude that it is rare or absent outside of humans. Here, I show that nonhuman reciprocity seems rare mainly because its meaning has changed over time. The original broad concept of reciprocity is well supported by evidence, but subsequent divergent uses of the term have relied on various translations of the strategy tit-for-tat' in the repeated Prisoner's Dilemma game. This model has resulted in four problematic approaches to defining and testing reciprocity. Authors that deny evidence of nonhuman reciprocity tend to (1) assume that it requires sophisticated cognition, (2) focus exclusively on short-term contingency with a single partner, (3) require paradoxical evidence for a temporary lifetime fitness cost, and (4) assume that responses to investments are fixed. While these restrictions basically define reciprocity out of existence, evidence shows that fungi, plants, fish, birds, rats, and primates enforce mutual benefit by contingently altering their cooperative investments based on the cooperative returns, just as predicted by the original reciprocity theory.