“The Left Hand of Darkness”, a classic science-fiction novel by Ursula K. Le Guin, imagines a faraway planet called Winter on which all people are “ambisexual”. Each month adults undergo “kemmer”, a few days in which they develop sexual characteristics determined at random: either male or female. Anyone, in other words, might fall pregnant. After the kemmer, all sexual characteristics fade. The dualism-the protected and protector; the dominant and submissive-that “pervades human thinking”, writes Le Guin, is almost entirely absent on Winter.

Such a planet would help answer an enduring question: why do men earn more than women? Perhaps male “dominators” succeed where female “submissives” do not. Yet the weight of research suggests that, after accounting for the constraints women face from bearing and rearing children, there is little left to explain. The most effective economist in the field, Claudia Goldin of Harvard University, who won a Nobel prize in 2023, seemed to settle the debate. Motherhood, her work suggested, explains basically all of the wage gap.

A handful of papers published over the past two years have reignited the debate. They were based on powerful and novel datasets, which matched health records with income data in Scandinavian countries. This new evidence allowed economists to exploit the powerful natural experiment provided by variation in women’s fertility. Researchers took women undergoing in vitro fertilisation (IVF)-who clearly wanted children-and examined the difference in long-term wages between those who fell pregnant and those who did not. At first the mothers earned much less, but this gap shrank over time. Around 10-15 years after the children were born, the mothers even earned a small premium.

Now this approach of exploiting natural variation in fertility has been used in a new study, by Camille Landais of the London School of Economics and others. It looks at women with Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, a rare condition in which a girl is born without a uterus but otherwise develops normally. These women know early in life that they will not bear children, and so differ from those who discover this after failing to conceive naturally or through IVF. This could influence their future wages, since women who plan to conceive may make different investments in their human capital. They might, for instance, spend less on education, knowing they might step back from their careers after giving birth.

Such early knowledge seems to make a big difference. The study of women with MRKH found they earn much the same as other women and men in early adulthood. Then, in their 30s and 40s, as the wage gap between men and women opens up, the women with MRKH follow a different path. Their wage trajectory is almost identical to that of their male peers. In other words, remove both motherhood and any decisions women might make while anticipating it, and the wage gap seems to vanish. It is hard to imagine a better way to isolate the effects of childbearing from other female characteristics, and study their impact on earnings. At least on this planet.