Research into health inequalities shows that socio-economic position (SEP) is strongly associated with differentials in health. Yet at present there is no adequate model to explain the causal link between the social and the biological variables: in particular why the association between SEP and health is found in all societies; and why inequalities are not simply a reflection of poverty causing sickness, but are also seen among the wealthiest classes in the wealthiest societies. In this paper we propose a model which brings together many diverse strands of research, and helps to explain how stratification by socio-economic position may be seen in terms of differentials in the margin of resources. Resource differentials lead to stratified health outcomes owing to the differential capacity of individuals and groups to realise 'universal' psychological aspirations of a broadly 'health promoting' nature. The margin model builds upon a previously described salutogenic theory of health. On average, the relative size of the margin will predict differential health outcome. This prediction is empirically testable and opens up a new agenda for research on health differentials; it also provides a framework for understanding, planning and evaluating health promotion.