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Women with twin brothers are more likely to drop out of high school and college and earn less money than women with twin sisters, a new study found. They are also less likely to marry and have children. These effects are due in part to girls being exposed to higher-than-normal levels of testosterone while in the womb with their twin brothers, new research shows.

SALT LAKE CITY — Women who have a twin brother are more likely to drop out of high school and college and earn less money than women who have a twin sister, a new study found. They are also less likely to marry and have children.

The study, which was published Monday in the journal Proceedings of the National Academy of Sciences of the United States of America, found that these effects were largely due to the girls being exposed to higher-than-usual levels of testosterone while in the womb with their twin brothers, although social and cultural factors also play an important role.

While all girls are exposed to testosterone while in utero, having a boy in the womb at the same time exposes them to a higher level of the hormone, The New York Times reported. This can provoke more traditionally masculine behavior in girls, leading to more disruptive behavior at school, lower educational achievement, and decreased wages.

The study’s findings are particularly relevant because around the world, the rate at which women have been giving birth to twins has nearly doubled over the last 40 years. This is largely due to the fact that many women are having children later in life and using in vitro fertilization more frequently, the study found. In the United States today, 1.1 percent of baby girls have twin brothers, up from 0.6 percent in 1971, the study estimated.

"This is a story about the biology of sex differences," David Figlio, an economist and dean of Northwestern's School of Education and Social Policy and one of the study's co-authors, said in a statement to Newsweek.

"We are not showing that females (exposed to testosterone) are necessarily more 'male-like,' but our findings are consistent with the idea that passive exposure to prenatal testosterone changes women's education, labor market and fertility outcomes," he said.

What were the key findings of the study?

The study — which examined the 728,842 people born in Norway from 1967 to 1978, including 13,800 members of a set of twins — found that women who had a twin brother were around 15 percent less likely to graduate from high school and 4 percent less likely to graduate from college compared to women who had a twin sister. The study didn't examine women who weren't part of a set of twins.

Women with a twin brother were also around 12 percent less likely to get married and 6 percent less likely to have children, and they earned around 9 percent less. Around half of the women's decreased earnings could be traced back to their lower rates of graduation from high school and college, the study found. Researchers also controlled for year of birth, month of birth, maternal age, maternal level of education at time of birth and children's birth weight.

These findings were consistent not only for girls who grew up alongside their twin brothers, but also for girls whose twin brothers died within a year of birth. This suggests that the effects on women of having a twin brother are more a result of biology — in this case, being exposed to testosterone — than socialization.

Previous studies of opposite-sex twins have suggested that girls with a twin brother show more aggression and are more likely to break the rules, though it was unclear whether this was due to prenatal exposure to testosterone or to socialization.

" Women exposed to testosterone have some of the educational challenges more frequently associated with men. "
David Figlio, an economist and dean of Northwestern's School of Education and Social Policy

The study points out that ethical constraints have prevented scientists from directly measuring prenatal amounts of testosterone concentration in humans. However, in studies conducted with rats, testosterone has been shown to transfer across fetal membranes through diffusion in amniotic fluid. Other studies, conducted on monkeys, rats, and humans, have shown that circulating levels of maternal testosterone can increase as a result of being pregnant with a male child, which can also increase the amount of testosterone to which girls in opposite-sex twin sets are exposed.

The study found that boys with a female twin did not show any substantial differences from boys with a male twin.

What effect does testosterone have on women?

The Times reported that both men and women naturally produce the hormone testosterone, but the level produced and the effect it has depends on the person in question.

Typically, testosterone is associated with certain behaviors that are often seen as masculine, such as aggression, competition and risk-taking behavior. According to research cited in the Times, those behaviors might cause boys to perform less well than girls in school, which rewards self-control, organization, and obedience.

However, those same behaviors are often valued in the workplace because they are linked with success, which could help explain why men outperform women there. But women who display more masculine characteristics in the workplace are often penalized for doing so.

“Women exposed to testosterone have some of the educational challenges more frequently associated with men,” Figlio told the Times. “However, to the extent to which labor market discrimination exists in society, they don’t have the discriminatory benefits that men enjoy.”

The study doesn't suggest that women who have a twin brother are "inherently ill-equipped for success," but it could indicate that these women "are experiencing consequences from simply behaving differently," Katherine Wu of PBS argued.

As the Times reported, in utero exposure to testosterone has also been shown to affect women's behavior in relationships. In the Norway study, women with a male twin who decided to marry and those who decided not to marry both had lower fertility rates.

Nature vs. nurture

However, it isn't just testosterone levels that affect the development of women who have a twin brother.

Dr. Krzysztof Karbownik, an economist and a research associate at Northwestern's Institute for Policy Research who was a co-author for the study, told Newsweek there's no one reason that definitively explains the differences observed between women who have a twin brother and women who have a twin sister, but that the differences are due to both biological and social or cultural factors.

" You can always change the cultural and social environment. Humans are plastic. "
Bernard Crespi, an evolutionary biologist at Simon Fraser University in Canada

Social factors do play a role in personal development, research shows.

For example, a study published March 7 in Labour Economics, entitled "The Brother Earnings Penalty," showed that American women who have a younger brother earn about 7 percent less money than women who have a younger sister, partly because of how the women were socialized. The study found that "brothers reduce parents' expectations and school monitoring of female children while also increasing females' propensity to engage in more traditionally female tasks," which in turn led to those women earning less money.

“There are always interactions between biology and culture and the environment,” Bernard Crespi, an evolutionary biologist at Simon Fraser University in Canada who peer-reviewed the paper, told the Times. “You can always change the cultural and social environment. Humans are plastic.”

11 comments on this story

The Norway study isn't by any means all-encompassing, nor are its results universal.

As Claire Cain Miller pointed out in the Times, "Not all girls with twin brothers are affected, or affected in the same way, and the researchers did not study other parts of life in which females with male twins might excel."

Christopher Kuzawa, a Northwestern anthropologist who co-wrote the study, told PBS, "A lot of factors come together in different ways in different individuals. All of this is just a teeny lens into the vastness of human diversity."