Measures of biological variation have long been associated with many indices of social inequality. Data on health, nutrition, fertility, mortality, physical fitness, intellectual performance and a range of heritable biological markers show the ubiquity of such patterns across time, space and population. This volume reviews the current evidence for the strength of such linkages and the biological and social mechanisms that underlie them. A major theme is the relationship between the proximate determinants of these linkages and their longer-term significance for biologically selective social mobility. This book therefore addresses the question of how social stratification mediates processes of natural selection in human groups. Data like this pose difficult and sensitive issues for health policy and developments in this area and in eugenics are reviewed for industrialised and developing countries.