An increase in physical capital input raises the marginal product of mental labor proportionally more than it raises the marginal product of physical labor.#
While women and men have equal quantities of brains, men have more brawn. And, the more developed is an economy, the higher the rewards of brains relative to brawn.#
If all childrearing is done by women, an increase in men’s wages will have such a pure income effect. Increases in women’s wages raise both household income and the price of children, and so have offsetting income and substitution effects on the demand for children.#
Higher wages for women raise the cost of children relatively more than they raise household income, and lead to a reduction in the number of children that couples choose to have#