The paradox of women's higher morbidity rates and lower mortality rates compared to men raises many questions for biologists, clinicians and social scientists. Until recently, the research and policy dialogue has focused almost exclusively on either social or biological differences. Our proposed book, Gender Matters: An Examination of Health Consequences of Constrained Choices and Social Policies, will provide a balanced analysis of how differences in men's and women's lives and in their physiology contribute to paradoxical health differences. We will focus on the following three aims: 1) Review and synthesize the biomedical and social science literature on gender differences in health; 2) Examine how constrained choices and social policies shape gender differences in health; and 3) Explore whether and how social contexts (including work, family, and community) and policies can be altered to reduce gender differences in health and improve individual and population health. We will elaborate a model of how the subtle and cumulative health effects of constrained choice in men's and women's lives affect their mental and physical health and show how social and health care policies that fail to consider findings from both social and biological research can inadvertently foster circumstances that predispose men or women to health problems. Our work will yield new insights into the paradox of gender differences in health and provide a better understanding of the wide range of social barriers to choosing health.