The workforce analytics firm Visier has published a fascinating analysis of the gender wage gap using the salaries and positions of 165,000 workers at large American companies.

It suggests one major driver of the workforce is that when women hit their 30s, they stop getting promoted into management positions at the same rate as men.

You can see what it looks like in this chart, which shows the percentage of men and women in management positions at different ages.

You can see that when women and men are in their 20s, they’re just as likely be in management positions. But things diverge when women hit their 30s — and by the time workers are in their 40s, the gender gap in management is huge.

And here’s the weird thing: Women are being promoted at the same rate as men throughout their careers. They’re just not being promoted into management positions.

This matters for the gender wage gap because managers tend to make more money than those in other roles. In the Visier data set, managers earned twice as much as non-managers.

If women were promoted into management roles at the same rate as men, Visier estimates that would close about one-third of the gender wage gap among the workers it studied.

“The gap happens precisely at the time when women are most likely to be having children, and it very much feels like a motherhood penalty during those years,” says Josie Sutcliffe, Visier’s vice president of marketing, who managed the report.

Visier works on data analysis with large companies that want to better understand their own workforce. It had permission from clients to aggregate data on their workers — their salaries, job titles, and promotions — which created a large data set on the gender wage gap.

The gender wage gap gets worse with age

This new study confirms what other researchers have found: The gender wage gap gets worse as women get older. The Visier report finds that the wage gap is largest for women between the ages of 32 and 48. This lines up with previous research findings from Claudia Goldin, an economist at Harvard and leading wage gap researcher.

As in Goldin’s research, Visier shows that the wage gap tends to shrink when women hit their late 50s and early 60s.

In other words: The wage gap is largest during the years when men and women start families and raise children. And it shrinks about 18 years later — right around when adult children are likely moving out of their parents’ house.

As I’ve written about previously, there is ample evidence that women are still responsible for the majority of child rearing and housework, even in households where both parents hold full-time jobs. That additional burden can become a significant obstacle to career advancement and higher salaries.

There is one big question this study can’t answer

One question I had for Sutcliffe when we spoke was whether they knew anything about why women start disappearing from management positions. Was it that women were applying for the jobs and not getting them? Or were they deciding not to pursue management jobs, possibly because of the responsibilities they held at home?

It’s quite possible the answer is a combination of both. Researchers have found that the job application process can discriminate against mothers. One 2007 study in the Journal of American Sociology had participants review résumés that they thought were from actual job applicants. They tended to offer higher salaries to men and women who were not mothers, and viewed applicants they knew to be mothers as less competent — even when they were looking at the exact same résumé.

At the same time, it's almost certainly the case that some women are choosing not to apply for management jobs. Positions that involve supervising other employees — being around to answer their questions and evaluate their work — can be more difficult to succeed in if you have commitments outside of work.

Understanding the root of the gender management gap is important to thinking through the best solutions. Discrimination in hiring, for example, would likely require different changes within a company than structural forces that keep women from applying for management positions in the first place. Both are problematic, and just knowing that the management gap exists is a good place to start.