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

Grooming and agonistic support are 2 common primate behaviors that have been hypothesized to constitute examples of reciprocal altruism. In particular, because primates often direct their grooming up the dominance hierarchy, it has been suggested that they may exchange grooming for agonistic support. Empirical tests of this hypothesis have resulted in highly inconsistent findings. I synthesized the published literature on the relation between grooming and agonistic support in primates using modern meta-analytical techniques. A meta-analysis of 36 studies carried out on 14 different species showed that a significant positive relation exists between grooming and agonistic support (weighted average r = 0.154, corrected for publication bias). These findings suggest that grooming and agonistic support may have evolved as part of a system of low-cost reciprocal altruism. They also highlight the potential of meta-analysis in tackling the study of behavioral phenomena characterized by low overall frequency and small effect sizes.