One of the most important decisions American women make about their careers has nothing to do with raises, internships, or degrees — it’s about who they choose to spend their life with. The one piece of advice that young ambitious women are never told but could make all the difference: The unequal division of labor that often comes with marriage and motherhood has the potential to derail their professional ambitions and personal passions. So if you get married — particularly if you’re going to marry a man — make sure it’s someone who will take your career as seriously as his own.

The difference between a partner who supports your ambitions and one who doesn’t is so stark that Avivah Wittenberg-Cox, an expert in workplace gender issues, wrote in the Harvard Business Review last year, that women were better off staying single than marrying an unsupportive partner.

It’s harsh, but in a country where the default assumption is that a man’s work should take priority over that of his female partner’s, women taking a hardline approach to who they marry is not such a terrible idea.

Wittenberg-Cox cites a 2014 study, for example, of Harvard Business School graduates that showed the majority of men surveyed expected their careers to take priority over their wife’s work, while most of the women questioned expected that their marriages would be equal.

One person’s career taking precedence over another’s isn’t just about big work-life decisions — consider the common scenario where one parent stops working to take care of children. (According to the Department of Labor, 43 percent of women with small children have passed up a promotion or asked for less responsibility at work because they needed to care for a family member.) It’s also about the everyday choices that can have a snowball effect on your ability to progress professionally. If you’re the person who takes off work when a child gets sick, for example, or if you have less mental space for work because you’re responsible for the majority of domestic labor — that impacts how well you can do your job.

This is especially true for women who freelance, work from home, or have otherwise flexible jobs: The assumption is that they can be the point person for all things domestic. I say this from experience; I have a wonderful, supportive husband, but it is very easy to fall into to traditional gender roles when one person is working from home. You become the go-to for childcare and domestic work, even if your job brings in as much money as your spouse’s.

This is not just about “getting ahead” or lucrative boardroom positions — it’s about who gets to be dedicated to their work or passions, and who doesn’t.

The only way to counteract falling into this trap is to be proactive about having an equal relationship. There’s a difference between having a partner who says they support your career and one who is willing to do the work to make that statement true. I’ve found that even among the most “progressive” men, there is a tremendous gap in those who support women’s equality politically and those who are willing to do it in tangible ways in their personal lives.

Take those Harvard Business School graduates who were interviewed about their expectations on work and family. The way their futures played out won’t surprise you: They went on to live the lives that the men wanted. Nearly three quarters of men surveyed reported that their jobs had taken priority, while women’s hopes for egalitarian marriages were dashed.

The truth is that most American women support their husbands’ careers by default. Men, on the other hand, need to learn how to do the same for their wives — and understand that it may come at some cost to their own happiness and ambition. Why should women be the only ones doing the sacrificing?

That said, women entering the workforce can’t count on a new generation of men suddenly deciding to upend the status quo. So, as they’re learning about mentors and resumes, another important lesson needs to make its way in: Who you marry matters, in more ways than you can imagine.