Few baby boomer women out-earn their partners, but new data from Insider and Morning Consult suggests that things may be changing for younger women.

Of baby boomer respondents who said they make more money than their partners, about 18% were women. Of millennials who reported making more than their partners, 35% were women — almost double.

But, men make more money as they advance in their careers — meaning many millennial women who currently out-earn their partners may eventually find themselves making less than their husbands and boyfriends.

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It's becoming more common for millennial women to make more than their partners. Compared to their mothers' generation, new data shows they're twice as likely to earn more.

Insider and Morning Consult surveyed 2,096 Americans, 1,068 of whom identified as women. Overall, about 25% of people who identified as women said that they made more than their spouse or partner, while 75% of those who identified as men said the same.

The data on pay looks very different for millennial women (defined as age 23 to 38 for this study) than it does for baby boomers (ages 55 to 73).

Only 18% of baby boomer women said they make more money than their partner or spouse, while 35% of millennial women reported making more money than their partner or spouse. By that measure, millennial women are twice as likely as their mothers to earn more than their partners.

How the gender pay gap factors in

Survey respondents didn't identify whether they were in same- or different-sex relationships, but it's worth looking at how things might change financially between partners in different-sex relationships because of the gender pay gap. Will millennial women who currently out-earn male partners continue to make more? It's unlikely.

In America today, women make about 80.7% of what men do on average, according to data from the US Census Bureau. That gap is even wider for women of color. In every industry and in every region of the US, women are paid less than their male coworkers, according to data from the Bureau of Labor Statistics.

While more than a third of millennial women are currently out-earning their partners, it's predicted that the pay gap will widen for that generation as millennials age and advance in their careers — which means it's likely that some of our women survey respondents will someday earn less than their partners. And there's a clear reason for that.

Quentin Fottrell writes for MarketWatch that bigger pay changes often come later in a person's career. "[As men] climb the ladder and get bigger jobs, men are paid exponentially more." Fottrell reports that millennial women executives earn 4.9% less than men who hold similar roles, and that figure increases to 6.2% for baby boomer women employees.

Parenthood also disproportionately affects women, with women still responsible for most family caregiving. The National Partnership for Women and Families calculates that when accounting for unpaid time off and time taken out of the workforce to care for families, women will make $0.49 for a man's $1 over a 15-year period.

A study by Cornell University's Francine Blau and Lawrence Kahn concludes breaks and maternity leave "may also leave women less likely to be considered for high-level positions."

Women also see fewer opportunities to make more: Research from the Harvard Business Review confirms that women get fewer promotions than their male coworkers over the course of their careers. Similarly, a 2019 report from McKinsey and Company shows that women are promoted less frequently. The study found that for every 100 men who are promoted to manager, just 72 women are promoted to the same role.

There's no way to predict the future, but considering the pay-gap statistics, high-earning millennial women in heterosexual relationships might not always make more than their male partners.