Almost a decade ago, the writer Linda Hirshman exhorted ambitious women to marry men with less money or social capital than they had. In articles and her book, Get to Work, she told women that they should avoid ever taking on more than half of the housework or child care. How to do it? Either marry a man who is extremely committed to equality, or do what she says is the easier route and “marry down.” Hirshman explained in the American Prospect that such a choice is not “brutally strategic,” it’s just smart. “If you are devoted to your career goals and would like a man who will support that, you’re just doing what men throughout the ages have done: placing a safe bet.”

This was a highly controversial piece of advice at the time, but Hirshman might have been right. A new study of Harvard Business School graduates from HBS’s Robin Ely and Colleen Ammerman and Hunter College sociologist Pamela Stone shows that high-achieving women are not meeting the career goals they set for themselves in their 20s. It’s not because they’re “opting out” of the workforce when they have kids, but because they’re allowing their partners’ careers to take precedence over their own.

The study’s authors interviewed 25,000 men and women who graduated from Harvard Business School over the past several decades. The male graduates were much more likely to be in senior management positions and have more responsibility and more direct reports than their female peers. But why? It’s not because women are leaving the workforce en masse. The authors found, definitively, that the “opt-out” explanation is a myth. Among Gen X and baby boomers they surveyed, only 11 percent of women left the workforce to be full-time moms. That figure is lower for women of color—only 7 percent stopped working. The vast majority (74 percent) of Gen Xers, women who are currently 32-48 and in the prime of their child-rearing years, work full time, an average of 52 hours a week.

But while these women are still working, they are also making more unexpected sacrifices than their male classmates are. When they graduated, more than half of male HBS grads said they expected their careers would take precedence over their partners’. Only 7 percent of Gen X women and 3 percent of baby boomer women said they expected their careers to take precedence. Here’s what they did expect: The majority of women said they assumed they would have egalitarian marriages in which both spouses’ careers were taken equally seriously.

A lot of those women were wrong. About 40 percent of Gen X and boomer women said their spouses’ careers took priority over theirs, while only about 20 percent of them had planned on their careers taking a back seat. Compare that with the men: More than 70 percent of Gen X and boomer men say their careers are more important than their wives’. When you look at child care responsibilities, the numbers are starker. A full 86 percent of Gen X and boomer men said their wives take primary responsibility for child care, and the women agree: 65 percent of Gen X women and 72 percent of boomer women—all HBS grads, most of whom work—say they’re the ones who do most of the child care in their relationships.

Of course, marital arrangements aren’t the only force holding women back. Part of the reason these women aren’t advancing at the same rate as their male counterparts is that after they have kids, they get “mommy-tracked.” In many ways, they’re not considered management candidates anymore. “They may have been stigmatized for taking advantage of flex options or reduced schedules, passed over for high-profile assignments, or removed from projects they once led,” the authors note. Other studies support these findings, as they have shown that there is a real, substantial motherhood penalty that involves lower pay and fewer promotions for women with kids, because employers assume they will be less dedicated to their jobs (as do, we now know, their husbands).

But the personal piece of the female achievement gap puzzle is important, and it’s something that’s very difficult to shift. The study’s authors note that while millennial HBS grads are a little more egalitarian than their older peers, half of the youngest men still assume that their careers will take precedence, and two-thirds of them assume their spouses will do the majority of child care.

Based on these numbers, Hirshman suddenly seems prescient. Take a look at the current crop of female CEOs: A lot of them have husbands who don’t work. Xerox CEO Ursula Burns took a page out of Hirshman’s book and joked at a 2013 conference, “The secret [to success] is to marry someone 20 years older.” Her husband retired as she was hitting her career stride, allowing him to take primary responsibility for their kids. If becoming a CEO and having a family is what you desire, you might want to take that advice.