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Girls who share a womb with boys tend to make less money than those with twin sisters

Female twins who shared a womb with a brother tend to get less education, earn less money, and have fewer children than girls who shared a womb with another girl, according to an analysis of hundreds of thousands of births over more than a decade. Researchers suspect the cause is testosterone exposure during fetal development, though the exact mechanism remains a mystery.

"I think it's a really interesting look at how this really complicated system might impact females," says Talia Melber, a biological anthropologist at the University of Illinois in Urbana who wasn't involved in the study. Still, she cautions, a lot more work needs to be done to establish a causal link.

Fraternal twins, in which each of two eggs is fertilized by a different sperm cell, occur in about four of every 1000 births. About half of those result in male-female twin pairs. Typically, about 8 to 9 weeks into gestation, a male fetus begins to produce massive amounts of testosterone, which helps jump-start the development of male reproductive organs and brain architecture; female fetuses receive only modest amounts of the sex hormone. In male-female twins, though, small amounts of the male fetus's testosterone can seep into the female twin's separate amniotic sac. Scientists have known about this phenomenon for decades, and have been arguing for just as long over what effects, if any, it has on women later in life.

[...] Controlling for factors such as birth weight and maternal education, women who had a male twin were 15.2% less likely to graduate from high school, 3.9% less likely to finish college, and 11.7% less likely to be married—compared with women with a twin sister. They also had 5.8% fewer children and earned 8.6% less money, the team reports today in the Proceedings of the National Academy of Sciences.

Evidence that prenatal testosterone transfer from male twins reduces the fertility and socioeconomic success of their female co-twins (open, DOI: 10.1073/pnas.1812786116) (DX)

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