children. Parents respond to this increased productivity in a way consistent with the theoretical model outlined above—they choose higher levels of child quality and reduce the number of children. As argued by Becker (1991), one of the interesting implications of a model of quality-quantity interactions is that an increase in income or a reduction in the unit price of quality can lead to large responses, with large increases in quality and large decreases in quantity. Increases in women’s schooling also lead to increased labor market productivity, as evidenced by the large rise in wages. In making labor supply decisions, however, women are faced with simultaneous increases in home productivity and labor market productivity. Over a large range of schooling it appears that increases in home productivity are large enough to offset even large increases in wages. Until about 8 years of schooling we do not see women responding to rising wages with significant increases in labor supply. The driving mechanism through which schooling reduces fertility, then, does not appear to be rising opportunity cost of time due to rising wages. Rather, couples respond to their increased productivity in producing child quality by reducing the number of children and investing more resources in each child.
While the cross-sectional relationships between education and the quality and quantity of children shown in Figures 11-1 to 11-4 do not necessarily have implications for the time series changes observed across populations, there would seem to be at least a rough correspondence between the cross-sectional and time series patterns over the broad sweep of historical fertility declines. Declining fertility has almost universally gone hand in hand with increased investments in children, as indicated by improvements in child health, survival, and education. While the specific causal mechanisms are difficult to identify, it is virtually impossible to separate declining numbers of children from the increased investments in those children in any population in which fertility has declined. Indeed, it is the almost universal increase in the “cost” of children that has caused many observers to see this higher cost as a mechanism inducing parents to have fewer children. In the context of the economic model of quality-quantity trade-offs, however, this higher cost is itself a matter of choice, with the simultaneous decisions about quantity and quality impossible to separate from each other.
The trade-off between the quality and quantity of offspring plays an important role in evolutionary biology, including applications of evolutionary biology to human fertility. As argued by Kaplan and Lancaster else-