The age at which women have babies could be a more important cause of the gender pay gap than their education level, according to a Center for Economic Studies paper on salary differences within American heterosexual marriages.

Image: Center for Economic Studies

Working mothers who have their first child before the age of 25 or after they are 35 will gradually close the salary divide with their husbands. But having a child between those two ages will significantly affect how much women earn, the study says.

It goes onto note that the findings suggest that it is "more harmful" to a woman's career progression to have children in her late 20s and early 30s. It is less disruptive to have children before her career has taken off or once she is established in her career.

The gap widens

The study looked at earnings for couples who had their first child between 1978 and 2011, using US Social Security and Census Bureau records, and included women who worked for two years before first giving birth.

“Our main results show that the spousal earnings gap doubles between two years before the birth of the first child and the year after that child is born,” the researchers write. “After the child’s first year of life the gap continues to grow for the next five years, but at a much slower rate, then tapers off and even begins to fall once the child reaches school-age.”

The pay gap between all spouses doubled immediately after the first birth as mothers’ pay fell while fathers’ wages increased. Overall, women earned $12,600 less than men before giving birth and $25,100 less afterwards.

For women who have their first child between the ages of 25 and 35 and return to work, their pay never recovers relative to their husbands’ earnings. But those who have their first baby either before 25 or after 35 -- either before their careers get started or once they are established -- eventually make up some of the difference.

Image: Center for Economic Studies

A question of education?

How well qualified women were did not seem to affect the results. “Women with less education tend to have their children younger, while women with more education tend to have their children older,” the researchers wrote.

Given that more and less-educated mothers gave birth at opposing ends of the age spectrum, the researches noted that “it still seems surprising that the earnings dynamics of the older and younger mothers show similar patterns.”

They added: “Women with less than a high school diploma have a smaller initial childbirth penalty relative to their spouses, but it grows to be equal to those with a high school diploma or college degree.”

While the main findings were reflected among all-white couples, black and mixed or other-race couples had a smaller pay-gap increase after their first child was born, but the researchers said these findings were hard to interpret and needed more investigation.

The Scandinavian conundrum

The results also reflect similar studies in Scandinavian countries that, as some commentators point out, offer generous government-backed paid parental leave, while the US offers none.

According to a Danish survey, despite social policies that enable both parents to share up to a year of paid leave between them, full-time working women still earn 15% to 20% less than men -- a gap similar to that in the US and other developed nations.

Women also tend to vary their hours more than men to take care of children, according to an OECD report, which is likely to contribute to their lower incomes.

Meanwhile, a 2013 study by Cornell University economists suggests that “family friendly” policies in some OECD countries could be the cause of the pay gap as they encourage part-time work and employment in lower-level positions.

“US women,” a report abstract says, “are more likely than women in these other countries to have full-time jobs and to work as managers or professionals. Moreover, the authors find that while more generous family-friendly policies raise women’s employment, “most of this effect comes from the expansion of part-time, rather than full-time, jobs.”

As a result, they say, employers may “engage in statistical discrimination against women,” which results in them being less likely to be considered for high-level, high-paying, positions.