Longitudinal studies support the proposition that the extent and severity of chronic conditions in middle and late ages are to a large extent the outcome of environmental insults at early ages, including in utero. Data from the Early Indicators Project, undertaken at the Center for Population Economics, suggest that the range of differences in exposure to disease has narrowed greatly over the course of the 20th century, that age-specific prevalence rates of chronic diseases were much lower at the end of the 20th century than they were at the beginning of the last century or during the last half of the 19th century, and that there has been a significant delay in the onset of chronic diseases over the course of the 20th century. These trends appear to be related to changes in levels of environmental hazards and in body size. These findings have led investigators to posit a synergism between technological and physiological improvements. This synergism has contributed to reductions in inequality in real income, body size, and life expectancy during the 20th century.