Read: Colleges pledge to send more students abroad

And while women account for 56 percent of the 19.3 million undergraduates enrolled at U.S. institutions, up from 42 percent in the 1970s, administrators who run study-abroad programs say the gender gap in overseas study predates the shift in overall enrollment. Part of that is historical. In the early 1900s, women in college went abroad for art and culture before they got married; parents also perceived study abroad as a safe way for their daughters to travel.

Today, even in colleges that enroll a majority of men, those who study abroad are disproportionately women. Take Purdue University: Men account for 57 percent of the student body but only 41 percent of undergraduates who go abroad, according to a university spokesperson. Since 2013, the number of Purdue undergraduates studying overseas annually has doubled, to about 2,600, as part of a university-wide effort to increase global awareness among its students. The percentage of men going abroad, however, has remained the same.

What’s puzzling to campus administrators is that the numbers aren’t budging, even as they pitch study abroad as necessary for future employment to students who, more so than in past years, are worried about their job prospects. “Having a significant experience abroad is critical to understanding the world they’re going to work in after graduation,” says Michael Brzezinski, Purdue’s dean of international programs.

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That focus on employability is one reason the composition of majors studying abroad has shifted toward STEM and business—just not the men in those majors. As a result, study-abroad leaders have started to focus on another explanation for why men don’t go abroad: complacency. Simply put, they don’t want to leave their friends and their comfort zone. A study of 2,800 students at two- and four-year colleges found that the more male students interacted with their peers—for example, the more they spent time with a friend or dorm hall mate—the less likely they were to go abroad. But peer interaction did not have such an impact on women, the researchers found.

Samantha Brandauer, who runs Dickinson College’s study-abroad office, told me she has experienced this firsthand. In her past job at Gettysburg College, she teamed up with a colleague to convene student focus groups on why men didn’t go abroad and what the college could do about it. What she discovered was a “bro mentality” among men in college—a culture in which male students don’t want to leave their friends to study abroad and are heavily influenced by their classmates in making choices about what to do in college. “Part of this is a messaging problem, because the way we talk about study abroad as a transformative experience just doesn’t resonate with college-age men,” Brandauer says. “They don’t want to be transformed.”