These results suggest a general role for kin selection and that the relative importance of kin selection varies across species, as predicted by Hamilton's rule.

The estimated mean effect size of kinship on helping behavior was rkin = 0.33 and was significantly larger than zero [P < 0.01 (Table 1 and Fig. 1)]. This is a relatively strong effect when compared with the average effect size of r {approx} 0.19 for ecological and evolutionary variables (28) and suggests that, on average, approximately 10% of the variation in helping rates can be explained by relatedness