We study the effects of China’s One Child Policy on the education and labour market outcomes of women and men born in its wake. We present evidence from a difference-in-difference design that the One Child Policy led to a significant rise in the fraction of both boys and girls who grew up as only-children in the late 1970s and 80s. We then use the policy-induced increase in only-children to estimate the causal effect of growing up as an only-child on education, earnings and occupational choice. Our results indicate substantial positive effects of only-child status for both men and women. In contrast to the typically small quantity-quality trade-offs identified in prior literature, the only-child premium we estimate is large and economically meaningful. We argue that this is consistent with the theory, which allows for non-linearity of the effect of child quantity on quality. We conclude that the One Child Policy contributed considerably to the rise of human capital in urban China since the 1980s.