Health research has failed to adequately explore the combination of social and biological sources of differences in men's and women's health. Consequently, scientific explanations often proceed from reductionist assumptions that differences are either purely biological or purely social. Such assumptions and the models that are built on them have consequences for research, health care and policy. Although biological factors such as genetics, prenatal hormone exposure and natural hormonal exposure as adults may contribute to differences in men's and women's health, a wide range of social processes can create, maintain or exacerbate underlying biological health differences. Researchers, clinicians and policy makers would understand and address both sex-specific and non-sex-specific health problems differently if the social as well as biological sources of differences in men's and women's health were better understood.