What was the return to education in the United States at mid-century? In 1940, the correlation between years of schooling and earnings was relatively low. In this article, we estimate the causal return to schooling in 1940, constructing a large linked sample of twin brothers to account for differences in unobserved ability and family background. We find that each additional year of schooling increased labor earnings by approximately 4 percent, about half the return found for more recent cohorts in twins studies. These returns were evident both within and across occupations and were higher for sons from lower socio-economic status families.