Animal social groups often consist of non–relatives, a condition that arises in many cases because of group merging. Although indirect fitness contributions are reduced in such groups compared with those in groups composed of close kin, the genetic–heterogeneity hypothesis suggests that these groups may benefit from increased intracolony genetic variation, which may boost group performance through increased task efficiency or parasite resistance. We confirm one prediction of the task–efficiency explanation by demonstrating a genetic basis for task thresholds of socially important behaviours in eastern tent caterpillars. However, we found no evidence that the expanded range of task thresholds in mixed colonies translates into improved individual or colony performance in the field. By contrast, increased group size, a less commonly considered correlate of group mixing, was found to enhance individual fitness through its effects on larval growth. We conclude that fitness benefits offsetting the dilution of relatedness in heterogeneous social groups may often stem from augmented group size rather than increased genotypic diversity.