* This article was prepared for presentation at the D. Gale Johnson Memorial Conference, University of Chicago, October 25, 2003, and draws on my previous papers “Changes in the Process of Aging during the Twentieth Century: Findings and Procedures of the Early Indicators Project” (NBER Working Paper 9941); “Secular Trends in Physiological Capital: Implications for Equity in Health Care,” Perspectives in Biology and Medicine 46 (2003): S24–S38; “Nutrition, Physiological Capital, and Equity,” presented at a Conference on the Right to Food and the Costs of Hunger, Rome, June 20–21, 2003; and “Nutrition, Physiological Capital, and Equity,” presented at the International Health Economics Association Third International Conference, “The Economics of Health, Within and Beyond Health Care,” University of York, July 23, 2001. The research for this article was funded in part by NIA Grant P01 AG10120. I have benefited from the suggestions of two anonymous referees.