Key concepts Inclusive fitness, direct and indirect fitness, co-efficient of relatedness, kin altruism, parental investment, parent–offspring conflict, K and r selection, parental manipulation

To many people human behaviour is social behaviour. Linguistic communication is meaningless unless used between at least two people; sex by definition involves more than one person (usually, although not invariably, two); child rearing and sibling relationships are clearly social and most working practices occur in groups. Humans do engage in some solitary activities – reading a book or taking a bath, for example, are not normally social events. But even by primate standards, we are an extremely socially integrated species. So it's not surprising that some evolutionary theorists have suggested that evolutionary psychology may have the greatest impact on social psychology (Archer, 1996; Simpson and Kendrick, 1997). Social behaviour can broadly be divided into pro- and anti-social patterns of response. Both can be found frequently in interactions that involve kin. Social scientists have long sought to explain why such love–hate relationships exist in families. Evolutionary psychologists think they have the answer – to them it's all about maximising inclusive fitness.