 They argue, based on a mixture of historical, ethnographic and theoretical work, for a scenario that begins with what they also call ‘tribes’—they consider these to be ethnolinguistic units of around 500 to 1500 people—competing against each other in ways that are aided by culturally transmissible differences in group properties. This competitive process eventually produces “culturally transmitted cooperative, group-oriented norms, and systems of rewards and punishments to ensure that such norms are obeyed” (Richerson and Boyd 2005). Once cultural transmission has established this social environment, natural selection acting on genetic variation then favours an innate psychology that is suited to this new, socially-inherited set of environmental problems.