What happens when people of different backgrounds meet in the pursuit of work? Ideally, progress.

It’s a well-documented fact that lesbians, on average, make more than straight women—what economists have taken to calling the “lesbian premium.” Equally established is that gay men earn less than straight men, dubbed the “gay penalty.”

Until recently.

Two Vanderbilt economists, Christopher Carpenter and Samuel Eppink, linked self-reported sexual orientation with earnings for employed US adults in the 2013-15 National Health Interview Surveys. They found that gay, full-time employed men made, on average, 10% more than similarly employed straight men. The difference in pay persisted even when they controlled for a variety of factors: presence of a partner, the sexual orientation of a partner, race, age, ethnicity, and the size of one’s firm and family.

Their analysis showed a lesbian pay premium of about 9%, too, similar to earlier studies. Bisexual men and women, however, were not so lucky, earning less than both their gay and straight counterparts.

What changed? The truth is, Carpenter and Eppink aren’t really sure why the earnings gap has flipped for gay and straight men. But they have some theories:

It gets better. It’s possible a change in attitudes is translating into a change in fortunes, lifting old prejudices that once hampered salary negotiations for gay men. But Carpenter and Eppink have a hard time squaring that with the little change they see in the lesbian premium and the earnings penalty for bisexuals.

What’s more, the overall employment of gay men is still low. It’s hard to explain why a change in attitudes towards homosexuality would boost gay men’s paychecks but still keep many of them out of the workforce. What’s stranger, Carpenter and Eppink found that the pay gap between gay and straight men is wider for older men, who have likely faced worse discrimination over their lifetimes.

Richer people feel better coming out. It’s possible that gay men earned similar or more money than heterosexual men all along—it’s just that most of them wouldn’t out themselves to researchers. If improving attitudes towards homosexuality make people more comfortable self-identifying as gay, it’s possible that wealthier workers who once might have assumed it was too risky to come out now identify as gay in surveys. (Still, some argue it has always been less risky for well-educated high earners to come out.)

So why haven’t earnings improved for lesbians too?

It could be that legalizing gay marriage affected gay and lesbian couples differently. Carpenter and Eppink investigated whether gay couples might now be more likely to raise children, opting for a more traditional division of labor.

Gay men’s significantly lower employment rates would make sense if many male partners stayed home to take care of kids. Indeed, the researchers found that the share of partnered gay men (45%) is now higher than it was pre-legalization. Lesbian partnership rates, however, remained about the same (66%) before and after. Prior research shows that many lesbians partnered up and had children even in the absence of same-sex marriage.