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.

In the new study, behavioral economist Krzysztof Karbownik at Emory University in Atlanta, biological anthropologist Christopher Kuzawa at Northwestern University in Evanston, Illinois, and colleagues turned to birth data from Norway. They looked at nearly 730,000 births between 1967 and 1978, including 13,800 twins.

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 .

The team also looked at the outcomes for women whose male twin had died either during childbirth or very shortly after. The thinking goes that these women would have been exposed to testosterone in utero, but otherwise raised as a nontwin. The 583 women who met that criterion had virtually the same life outcomes as the women with living male twins, suggesting it’s the prenatal testosterone causing these long-lasting effects, not just the circumstance of being raised with a male twin.

The study doesn’t explain why testosterone would have any of these effects. Previous work has found that, compared with female nontwins, female twins who shared a womb with brothers tend to develop more masculine bone structure and brains that appear more like average male brains, including having a larger left hippocampus and amygdala. These regions are involved in memory and emotional processing, respectively. Based on findings from prior research, all of this may cause women to act more aggressively, engage in more risk taking, and act in other ways that are traditionally seen as “male typical,” the authors speculate.

Karbownik further speculates that all of this might lead to lower socioeconomic success for these women; an assertive, rule-breaking woman might be ostracized for breaking with traditional gender norms. “If you think these kinds of behaviors may be penalized by society, then that would be part of the effect we’re measuring.” As society’s views on gender norms have shifted in many places in recent decades, he’d like to repeat the study with data on younger twins.

“For so long, we kind of ignored” testosterone in women and girls, Melber says. “We’re really behind the curve in terms of understanding … its long-term effects, and how it creates variation in how we behave and who we are as people.” Still, she says, the idea that prenatal testosterone exposure can cause specific life outcomes for women is premature. “You don’t want to take it to its extreme and say that there’s a foregone conclusion because there’s not.”