Some groups of people are healthier than others. Overwhelmingly, for almost all kinds of morbidity and mortality, groups at the bottom of the social scale are less healthy than those at the top. But this simple observation describes a complex phenomenon that has become a major focus of research, teaching, intervention, and public policy and has led to recognition of the stark power of social determinants of population health. Why are poorer, less educated, lower-class groups less healthy than others? Historically, and indeed today, this has been a question that has polarized...