Joel E. Cohen is professor of populations and head of the Laboratory of Populations at the Rockefeller University and Columbia University. He is an applied mathematician and author of “How Many People Can the Earth Support?”

Most people care about education and childbearing because both affect women's well-being and prospects in life. Economists, policy makers and politicians care because education and childbearing shape economic development and population growth.

Around 1979, demographers discovered that women with more schooling had fewer children and their children had better survival rates. A woman's childbearing was statistically more closely correlated with her schooling than with the socioeconomic status of her husband or family.

The fall in a country's childbearing may have little to do with schooling or literacy of women or men.

For a decade or so, many demographers and economists assumed that a woman's schooling caused her to have fewer children. International organizations like the World Bank or the United Nations Population Fund began to advocate for better schooling for girls as a means of lowering birth and death rates, accelerating economic growth and improving women's opportunities.

Demographers now know that the differences in childbearing between more and less educated women are much greater in some regions than in others. Within a country, over time, less educated women come to resemble more educated women in having fewer children. The fall in a country's childbearing may have little to do with schooling or literacy of women or men.

For example, in Niger, one of the world's poorest countries, more educated women in 2006 had far fewer children (4.8 per lifetime) than women with no schooling (who had an average 7.2 children per lifetime). But unschooled women in 2006 were having fewer children than unschooled women in 1998 (who had 7.8 children per lifetime), obviously for reasons that had nothing to do with the women's (lack of) schooling.

John Cleland, a British demographer, warned in 2002: "Empirical support for the view that the enhancement of women's schooling is critical for fertility reduction is neither as strong nor as universal as is often implied, and the links between schooling and fertility are not clearly understood."

We found that childbearing interfered with education much more than education reduced childbearing.

Nevertheless, nearly everywhere, more educated women have fewer children. Economists and demographers are still trying to understand what causes what. Changes in compulsory schooling provide natural experiments.

Increasing the legally required number of years of schooling in the United States, Norway, Great Britain and Northern Ireland have lowered the probability that teenage women would have children. In the U.S. and Norway, the "incarceration effect" (keeping girls in school, giving them less time for sex) explained partially but not entirely why additional schooling lowered teenagers' childbearing.

Norwegian women who were required to have more schooling deferred giving birth to their 20's or later. They did not reduce their average number of children by age 39 or increase their likelihood of remaining childless. The claim that education inevitably leads to the birth of fewer children is therefore not supported by our data.

To compare how much education reduces childbearing with how much childbearing interferes with education, Øystein Kravdal and Nico Keilman at the University of Oslo and I analyzed the complete history of childbearing, school enrollment and level of education of every woman born in Norway in 1964.

We found that childbearing interfered with education much more than education reduced childbearing. Norwegian women with advanced degrees had fewer children by age 39 principally because women who had children early were more likely to leave or not enter long programs of education required for higher degrees.

No, Norway is not Niger. We don't know whether education and childbearing influence each other in Niger as in Norway. We should find out. While we continue to gather data, we should enable the 215 million women who need contraception to get it, and we should ensure that all women have the opportunity for an education.