How context influences primates' decisions about reciprocity

Reciprocally patterned behavior is widespread in animals in the wild, but experimental evidence has been frustratingly inconsistent. Contrary to earlier contentions that this inconsistency is because reciprocity in non-human species is a rare or fragile effect, recent authors have argued that the evidence suggests that reciprocity is widespread, that it often relies on cognitive mechanisms that are common across species, and is potentially an important factor in animals' daily lives. Another possible explanation for its apparent rarity, then, is that due to experimental studies' (intentionally) structured environment, they can lack the appropriate context to promote and support reciprocity. Focusing on outcomes from experimental reciprocal tasks in non-human primates, I consider several factors that may be important, including the identity of the interactors and their relationship to one another, whether there is free choice of partners, whether the individuals are interacting directly, the timing of the interaction, the commodity involved, whether individuals have a reason to reciprocate, and the equity of the interaction. Clarifying the role of each of these factors will help improve experimental tasks and the social and ecological contexts that promote reciprocity.