Between the ages of 20 and 40, the median pay for women ticks up slightly every five years until topping off at $49,000, where it stays through age 65, according to data released by salary comparison site PayScale on Thursday . But men see their salaries grow all the way until they’re at least 50, when their median earnings hit $75,000.

But the reason for the divergence at middle age isn’t simply the “obvious explanation” — that women take time off from work to have kids and return lacking the necessary skills to move up — as Heidi Hartmann, the president of the Institute for Women’s Policy Research, a Washington, D.C.-based think tank, puts it. Instead there are other factors at play that result in women reaching their peak earnings potential well before men, Hartmann says.

One major explanation for the trend: Female-dominated jobs, like teaching and nursing, tend to be less specialized, so they don’t demand the string of promotions and pay premiums that firms typically use to keep their best performers around, says Hartmann. Though professions that fit this bill, such as law and engineering, have certainly become more open to women than in previous years, they’re still not as gender-integrated as they could be, Hartmann says.