In this paper, we address the issues of whether reductions in fertility increase human capital investments per child and whether twinning can identify the quantity—quality (Q-Q) trade-off. We show that estimates of the effects of twinning at higher parities on the outcomes of older children in prior studies do not identify family-size effects but are confounded by inter-child allocation effects because of the endowment deficit and close spacing of twins. However, examining the effects of twinning by birth order, net of the effects stemming from the endowment deficit of twins, can provide upper and lower bounds on the trade-off between the family size and average child quality. Our estimates, based on data from China, indicate that an extra child at parity one or at parity two, net of one component of birth-endowment effects associated with birth weight, significantly decreases the schooling progress, the expected college enrolment, grades in school and the assessed health of all children in the family. Despite the evident significant trade-off between number of children and child quality in China, the findings suggest that the contribution of the one-child policy in China to the development of its human capital was modest.

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