The maternal dominance hypothesis has been derived from work with humans which shows that women who are more dominant than other women are more likely to conceive sons. In both animals and humans dominance is a characteristic or personality trait, underpinned by testosterone and responsive to a range of environmental changes: physical, social and psychological. Studies of the sex ratio in the social sciences and animal behaviour either support or are compatible with the idea that the sex-determining role of X- and Y-chromosome bearing spermatozoa may be preceded by factors under maternal control which provide for differential access of spermatozoa. Findings in reproductive physiology and physiological psychology suggest that follicular testosterone or a related hormone may play a critical role. Reproductive physiologists have already identified maternal mechanisms which could provide the context for such a model.