Since the turn of the millennium, highly educated women in the United States have had higher fertility rates than women with intermediate levels of education. What''s more, while fertility among American women with some college education has been nearly stagnant over the last 30 years, fertility among women with advanced degrees has increased by 0.7 children or more than 50%. These are the central findings of research by Moshe Hazan and Hosny Zoabi, published in the September 2015 Economic Journal.

Highly educated women have always had fewer children, but the new study finds that while this was true in the United States until the 1990s, it is no longer true today. What can explain the rise in fertility among highly educated women during a period that saw the largest increase in women''s labour supply?

The researchers argue that the growing divide between rich and poor in American society has created two groups of women: one who can afford to buy services that help them raise their children and run their homes; and another who are willing to supply these services cheaply. For women with college and advanced degrees, the labour cost in the childcare industry relative to their own wage decreased by 10% and 16%, respectively, over the past 30 years. This change accounts for about one-third of the increase in the fertility of highly educated women.

Other commentators might suggest alternative explanations. For example, perhaps there has been a rise in fertility because partners of highly educated women share more of the burden of raising children compared with partners of less educated women. Or perhaps highly educated women are benefitting from recent advances in ''assisted reproductive technology'' (ART), which enable them to spend long years at university without reducing their chances of future parenthood.

But the new study shows that these alternatives explain only a very small amount of the increase, if any. For example, partners of college graduate women spend only 30 minutes per week more with their children compared with partners of women with lesser education. And partners of women with advanced degrees spend less time with their children compared with those married to women with a college degree, even though women with advanced degrees have higher fertility rates.

Similarly, ART accounts for less than 1% of births in the 2000s. What''s more, comparing 15 US states with infertility insurance laws and the remaining states, which do not have such coverage, shows no difference in fertility rates during the 2000s.

The results of the new study have several implications. For public policy, they highlight potential benefits from pro-immigration policies. Unskilled immigrants can potentially have a positive effect on fertility by increasing the supply of cheap home production substitutes. For many developed countries, which are facing the challenges of an ageing and shrinking population, this may be something to consider.

There are also potential consequences for economic growth. Given the strong correlation between parents'' education and children''s education, an increase in the relative representation of children coming from highly educated families means that the next generation is going to be relatively more educated. This is good news for growth.

''Do Highly Educated Women Choose Smaller Families?'' by Moshe Hazan and Hosny Zoabi is published in the September 2015 issue of the Economic Journal. Moshe Hazan is at Tel Aviv University. Hosny Zoabi is at the New Economic School.