In modern societies, children are no longer capital goods, but luxury consumption items

Ron Bailey reports that there's an inverse relationship between a nation's economic freedom and its total fertility rate.

Economic freedom and the rule of law produce prosperity which dramatically lowers child mortality which, in turn, reduces the incentive to bear more children. In addition, along with increased prosperity comes more education for women, opening up more productive opportunities for them in the cash economy. This increases the opportunity costs for staying at home to rear children. Educating children to meet the productive challenges of growing economies also becomes more expensive and time consuming.