Differentials in mortality by socioeconomic status and the nature of social relationships have been well established in a broad range of settings and time periods. A myriad of studies has investigated the many potential causal pathways linking these aspects of the social environment to health and mortality, with a current emphasis on psychosocial factors, neighborhood contexts, and biological linkages. Recent efforts have exploited longitudinal surveys, some beginning at birth, to strengthen causal inferences. Fewer researchers have focused on identifying selection mechanisms or reverse causal pathways through which, for example, poor health may lead to reduced income or exclusion from marriage.