A growing body of feminist work is concerned with examining how the development of social science theories has been influenced by assumptions about natural sex differences. This paper examines the impact of such assumptions on the construction of theories in the psychology of aggression. It is based on a survey and critical review of psychology books and major research articles on aggression published between 1890 and the present. The paper emphasises the diverse manifestations of assumptions about natural sex differences, which are apparent even in theories which emphasise environmental determinants of behaviour. It is concluded that there is a need to systematically scrutinise psychological theories for their assumptions about natural sex differences, and that social constructionism can be an important ally in feminist efforts to challenge the 'naturalness' of sex differences.