Grooming and agonistic support: a meta-analysis of primate reciprocal altruism

Grooming and agonistic support are 2 common primate behaviorsthat have been hypothesized to constitute examples of reciprocalaltruism. In particular, because primates often direct theirgrooming up the dominance hierarchy, it has been suggested thatthey may exchange grooming for agonistic support. Empiricaltests of this hypothesis have resulted in highly inconsistentfindings. I synthesized the published literature on the relationbetween grooming and agonistic support in primates using modernmeta-analytical techniques. A meta-analysis of 36 studies carriedout on 14 different species showed that a significant positiverelation exists between grooming and agonistic support (weightedaverage r = 0.154, corrected for publication bias). These findingssuggest that grooming and agonistic support may have evolvedas part of a system of low-cost reciprocal altruism. They alsohighlight the potential of meta-analysis in tackling the studyof behavioral phenomena characterized by low overall frequencyand small effect sizes.