For 25 years, Duncan played a major role in the Panel Study of Income Dynamics (PSID), one of the world’s longest-running and most influential studies of human development. The study has been interviewing a representative sample of American families and their children since 1968, following children into the fourth decade of their lives. The study’s data have allowed Duncan and his colleagues to find correlations between income in early life and life circumstances in adulthood: Children from poor families are less likely to finish school, and they work and earn less than their more fortunate peers. Duncan has also shown that low family income has a stronger association with circumstances in adulthood when it occurs during the first five years of life than when it occurs later on.