LABOUR markets are hotbeds of inequality. For every dollar a white American man in full-time work earns, the average white woman earns 78 cents and the average Latina only 56 cents. Marriage is a boon for male earnings; motherhood drags female earnings down. Likewise, gay men earn about 5% less than heterosexual ones in Britain and France, and 12-16% less in Canada and America, even after controlling for things like education, skills and experience. Yet one minority appears immune to this scourge: lesbians. Marieka Klawitter of the University of Washington looked at 29 studies on wages and sexual orientation last year.* On average, they found a 9% earnings premium for lesbians over heterosexual women, compared with a penalty of 11% for gay men relative to straight men. This discrepancy has been borne out by research on America, Britain, Canada, Germany and the Netherlands. Even after adjusting for the fact that lesbians are on average more educated than straight women, and less likely to have children, the gap persists.

Research on this topic should be taken with a pinch of salt. Some studies rely on direct questions about sexual orientation. But around half of the 29 studies surveyed by Ms Klawitter were based on surveys in which respondents were not asked directly whether they were gay. Instead, they were asked who they live with and what their relationship with them is. Both methods tend to find a wage premium. But they may both miss some gay women in a way that distorts the results.

If getting good data is tough, pinning down why there might be a wage premium for lesbians and a penalty for gay men is even tougher. Perhaps lesbians who are “out” are more competitive than their heterosexual peers. After all, studies tend to find that men are more competitive than women, which could explain some of the wage gap between the sexes. But a working paper published last year found that whereas gay men behaved less competitively than straight men (accounting for roughly two-fifths of their earnings penalty), there was no such difference between lesbians and other women.

Lesbians may not need to behave differently to be treated differently. They could face positive discrimination, if employers promote them on the assumption that they will not have children and so devote more time to work than straight colleagues. A study Ms Klawitter published in 2011 found that gay men working in the public sector suffered a smaller penalty than those in the private sector, whereas lesbians enjoyed a premium in the private sector but none in the public sector. One interpretation could be that discrimination of all sorts is more fiercely policed in government offices, dampening prejudice against gay men and in favour of gay women.

Lesbians’ higher earnings could also be a function of the gender of their partner. Men earn more than women, straight or gay; lesbians, deprived of the extra earnings a male partner would bring, may work harder to compensate. At any rate, they work more hours per day and weeks per year than straight women, on average (see chart). Over time this could translate into more experience and better chances of promotion. There is a clue in a paper from Nasser Daneshvary, C. Jeffrey Waddoups and Bradley Wimmer of the University of Nevada, which finds that lesbians who have previously been married to men receive a smaller premium than those who have not.

Finally, it could be that in same-sex couples women do not feel obliged to do as much childcare or housework, giving them more freedom to fulfil their potential in the workplace. Lesbian couples tend to work more equal hours, even when they have children, and several studies find that same-sex households share chores more evenly than heterosexual ones.

Whatever the reason for lesbians’ wage premium, it does not make them a privileged group. There is evidence that they face discrimination in hiring relative to straight women, even though their pay is better. Poverty rates among lesbian couples are 7.9%, compared with 6.6% among heterosexual ones. And for boosting earnings, as in so many realms, nothing beats being a straight, white, married man.