Besides simply knowing who your friends are, sociality depends on a sense of mutual trust and obligation. In traditional societies, community cohesion is in large part generated by a sense of shared common purpose, of being signed up to the same grand project that entails obligations towards, and expectations of appropriate behaviour from, fellow community members in the interests of mutual survival and success. Moral dilemmas provide us with a simple assay of this. Virtually all the work on moral dilemmas has focussed on identifying moral universals – the rules that we all follow (or should follow) and that should be impartial and non-discriminatory. However, everyday experience tells us that we do not always abide by these moral injunctions: instead, we operate a partial morality in which we privilege some individuals over others (e.g. in terms of ethnicity, sex, social class, or when favouring our own children over other people’s, or even one child over another within a family, but most of all when we favour family over friends, and our friends and family over other people’s). The transition from small scale communities to large scale societies may be especially critical in influencing both how moral judgments are made and how partially people behave in favouring others or excusing their behaviour.