A negative relationship between income and fertility has persisted for so long that its existence is often taken for granted. One economic theory builds on this relationship and argues that rising inequality leads to greater differential fertility between rich and poor. We show that the relationship between income and fertility has flattened between 1980 and 2010 in the US, a time of increasing inequality, as high income families increased their fertility. These facts challenge the standard theory. We propose that marketization of parental time costs can explain the changing relationship between income and fertility. We show this result both theoretically and quantitatively, after disciplining the model on US data. We explore implications of changing differential fertility for aggregate human capital. Additionally, policies, such as the minimum wage, that affect the cost of marketization, have a negative effect on the fertility and labor supply of high income women. We end by discussing the insights of this theory to the economics of marital sorting.

中文翻译：

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为什么富裕的家庭增加了他们的生育能力？育儿的不平等和市场化

收入与生育率之间的负面关系一直持续了很长时间，以至于人们常常认为收入的存在是理所当然的。一种经济理论建立在这种关系的基础上，认为不平等加剧会导致贫富差距扩大。我们显示，在1980年至2010年之间，美国收入与生育率之间的关系趋于平缓，这是不平等加剧的时期，因为高收入家庭增加了他们的生育率。这些事实挑战了标准理论。我们认为，父母时间成本的市场化可以解释收入与生育率之间关系的变化。在根据美国数据对模型进行判别后，我们将在理论上和定量上显示此结果。我们探讨了改变生育力对总人力资本的影响。此外，政策，高收入妇女。最后，我们讨论该理论对婚姻分类经济学的见解。