The equilibrium relationship between the number of siblings and their education plays a key role in the theory of fertility and its interaction with economic growth. Using data from 40 developing countries, this paper tracks how the association between childhood family size and educational attainment evolved across successive cohorts of women born between the 1940s and 1980s. In most countries, the association began positive but ended negative. Supplementary data suggest a similar reversal in the association for men and a concomitant reversal in the relationship between parental economic status and fertility, also from positive to negative. Roughly half of the reversal can be attributed to the rising aggregate education levels of the previous generation. The results challenge existing analyses of the aggregate consequences of fertility differences between the rich and poor. Because large families switched from being more educated than small families to being less educated, heterogeneity in fertility across families initially increased but now largely decreases average educational attainment.