So although much of the current literature still suggests that sex mismatch impacts the success of transplants, studies suggest immunosuppressant drugs can be sufficiently tailored for it to be worth the risk.

There are other gender-related inequalities in organ donation, too. One study of 101 urban black patients found that women on dialysis were less likely to be evaluated for kidney transplantation than men on dialysis. They also were less likely to want a kidney transplant – despite receiving more offers than men. Meanwhile, a much larger study of more than 700,000 patients found an odd gender disparity in terms of body mass index: while overweight women were significantly less likely to receive transplants than their thinner counterparts, overweight men were more likely to receive transplants.

It isn’t clear what causes those disparities. But there are some solid theories about why so many more women than men donate.

One reason is simple. Spouses are often the first to volunteer to donate a kidney to their loved one. And while women are more likely to get chronic kidney disease, men are more likely to be treated for end-stage renal failure – meaning of heterosexual couples, more wives than husbands may feel compelled to step up. In a study of 631 living kidney donors in Switzerland, for example, 22% were female life partners while 8% were male partners.