There’s a lot of talk about how and why it’s so difficult for women to get to the C-suite. But what happens once they’re there?

According to preliminary findings from a joint Bain & Company and LinkedIn study shared exclusively with Fortune, many senior-level women continue to question their success even after they’ve made it to the top.

The study looked closely at the aspiration, confidence, and endurance of male and female executives at different stages in their career (entry-level, midcareer and senior). Aspiration was defined as the desire to reach top management, confidence as the belief that one could reach it, and endurance as not questioning one’s ability to succeed despite bad days.

The research is based on an online survey of over 8,400 LinkedIn users, all of whom have undergraduate degrees.

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“Endurance is the reservoir on which all of us draw in order to keep going,” explains Bain partner Julie Coffman, who also chairs Bain’s Global Women’s Leadership Council.

For the most part, the study’s findings were consistent with existing research: men tend to aim higher and be more confident than women at the beginning and middle of their careers. At the senior-level, however, these gaps more or less disappeared. “The men and women in senior roles were pretty much identical in these respects,” says Coffman.

 

Yet when it comes to endurance, the gap never closes, a finding that Coffman says translates to women asking themselves: Is the day-to-day effort that I put in really worth it?

The study found that only 73% of female executives have endurance, compared to 81% of men.

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One of the enablers of endurance is how executives handle stress. A little over a third of senior-level men said they are not able to manage the stress of work with minimal impact on other activities, compared to nearly half of women.

While there are many possible causes of the “endurance gap,” Coffman has a few ideas. For one, women still tend to be primary caretakers and have partners with equally demanding careers. “If you’ve got a partner that’s in a really intense role, the endurance to keep going flags a little bit.”

Another possibility: lack of role models. When women look around and don’t see some they aspire to emulate, she says, “there’s a bit of a challenge in getting really excited about your success.”