Once the preserve of men, today the MBA is increasingly unisex. Forty per cent of those taking the GMAT test last year were women; and women now account for some 30 per cent of all MBA places. But how do they fare after graduation? Do they rise as high and earn as much as their male counterparts?

The simple answer is no, but probably not for the reasons you think. New research sheds much needed light. The study, Dynamics of the Gender Gap for Young Professionals, was carried out by Marianne Bertrand, professor of economics at the University of Chicago Booth School of Business and two colleagues from Harvard University, Claudia Goldin and Lawrence Katz. They found that, despite their growing presence on business school campuses, women with an MBA still do not do as well in the job market as men. “Our research suggests that it is not so much about a glass ceiling as about individual choices,” says Bertrand. “People think that women are knocking on the door of the boardroom but can’t get in; our findings indicate that discrimination is not a big part of this story.”

The researchers studied the careers of 2,500 people who gained an MBA between 1990 and 2006 at Booth School of Business (formerly the University of Chicago Graduate School of Business) to see how career paths differ by gender.

When they graduate, men and women have nearly identical incomes. But five years after graduation, men earn 30 per cent more on average. That gap continues to grow, so that between 10 and 16 years after graduation, men earn 60 per cent more.

The proportion of women with MBAs not working at all also rises substantially in the decade after graduation, with 13 per cent of women not working nine years later, compared with 1 per cent of men.

So what explains the widening gap? In a word, children. “There are not just two groups here, there are really three: men, women without children and women with children,” says Bertrand. “The first two groups are very similar in terms of earnings. It is only when women become mothers that the gap starts to open.”

She identifies two job market-related factors that make the difference — career interruptions and hours worked. Overall, women take more career breaks and work shorter hours. But this is especially pronounced for mothers. In the first 15 years after graduating, women with children have about an eight-month deficit in post-MBA experience compared with men, while women without children have only a 1.5 month deficit. Mothers also seem to choose jobs that are family-friendly and avoid jobs with long hours and greater career advancement possibilities.

Bertrand’s findings are supported by evidence from the UK. Robyn Hudson received her MBA from Judge Business School in 2007, and is now strategic development manager at UCL Hospitals Foundation Trust, London. As part of her MBA dissertation project, she interviewed women MBA graduates to find out about their career experiences since leaving.

“There is a pay gap between the genders, which becomes even more pronounced as women have children,” she says. She points out that many jobs and corporate cultures are not mother-friendly. “Career choices too have an influence. There are more women in senior positions in the charity and public sectors, for example, which traditionally pay less.”