Economic explanations for the fertility transition focus on the role of returns to schooling, especially for women, which have encouraged women to obtain more education and facilitated the rise in women's wages relative to men's. The private opportunity costs of children have therefore increased, and parents have been motivated to substitute child schooling for additional births Declines in fertility have proceeded unevenly, first across the high income countries, and more recently across the low income countries. The cross sectional differentials in fertility are also frequently analyzed in household surveys, suggesting parallels with the cross-country comparisons. At an aggregate level, states have simultaneously legislated socialized support for the consumption of the elderly, which has eroded the incentives for childbearing, and subsidized child human capital through schools and public health programs, which has encouraged parents to demand fewer, higher quality, children.