Behaviours directly associated with reproduction but not sexual behaviour per se are also often highly sexually dimorphic, with the best example being singing by male song birds, with females singing relatively little. Elaborate courtship displays, scent marking, territorial defence, mate guarding are all related behaviours shown by males to attract and then protect females. Conversely, next building, maternal behaviour, maternal aggression are more female typical behaviours, particularly in mammals. Despite the clear distinctions in motivation and division of labour between the sexes, a strict application of the organisational/activational hypothesis to explain the differences begins to break down and we instead observe a complex interplay of early hormone and experience combined with adult context and hormonal influences. As we move further away from behaviours directly relevant to reproduction and consider stress responding, pain perception, food preferences, learning and memory, drug addiction and so on, the applicability of the hypothesis becomes even less. This is not to say that there are no neural underpinnings that direct sex differences in behaviour, but that a strict adherence to early organisational hormonal effects activated by adult hormones does not satisfactorily explain the basis of the sex difference. Additional variables, including recent evidence for a genetic contribution (31), must be considered to create a more nuanced view of how and why males and females differ. Moreover, in some cases, what is taken for a sex difference is not really a sex difference at all but rather an example of hormonally-mediated plasticity in the adult (32). Oestradiol exerts profound effects on hippocampal morphology and function, inducing a 30% increase in the density of dendritic spines, increased long-term potentiation and enhanced spatial learning (33). These and similar data are often interpreted as evidence for a sex difference that is the result of sex differentiation, but, when males are considered, they fall somewhere in between the measures for females with and without oestradiol. Is this really a sex difference? The answer is ‘yes’ and ‘no’; there are some intrinsic sex differences in the hippocampus, and responses to extrinsic variables such as stress may impact the hippocampus differently in males versus females (34), but unambiguous evidence for classic sex differentiation is lacking. The hippocampus may prove a brain region that exemplifies a mix of hormonal, genetic and experiential influences to generate a sex-specific phenotype. Discerning the relative weight of each variable and its impact on function will provide a valuable integrative view of sex differences with relevance across a wide range of functions.