Paternity testing using genetic markers has shown that extra-pair paternity (EPP) is common in many pair-bonded species. Evolutionary theory and empirical data show that extra-pair copulations can increase the fitness of males as well as females. This can carry a significant fitness cost for the social father, who then invests in rearing offspring that biologically are not his own. In human populations, the incidence and correlates of extra-pair paternity remain highly contentious. Here, we use...