Girls With Twin Brothers Have More Testosterone, More Problems

Girls with twin brothers are less likely to graduate high school or college, and have lower earning potential, fertility rates, and marriage rates than twin girls who share a womb with their sisters, according to a new study. Researchers suspect that exposure to high levels of testosterone in utero may be partly to blame.

“This is the first study to track people for more than 30 years, from birth through schooling and adulthood, to show that being exposed in utero to a male twin influences important outcomes in their twin sister, including school graduation, wages, and fertility rates,” study coauthor Krzysztof Karbownik, an economist and research associate at Northwestern University, said in a statement.

Preliminary studies had already shown that girls exposed to high levels of testosterone in the womb, such as those with twin brothers, may be especially vulnerable to risky behaviors and increased aggression in the long-term. Although some scientists argued that this could be the result of nurture—growing up with a male peer—one telling study found that, even when girls had twin brothers who died as infants, the effects were the same. But until now, no longitudinal study had thoroughly examined the question of how male twins impact their sisters.

For the study, Karbownik and colleagues looked at data from 13,800 sets of twins born in Norway between 1967 and 1978. Females born with male twins were 15.2 percent less likely to graduate high school, 3.9 percent less likely to graduate college, 11.7 percent less likely to be married, 5.8 percent more likely to have fertility problems, and 8.6 percent more likely to earn less. Confirming what Karbownik and his colleagues already suspected, the effect was also true for girl whose twin brothers had died as newborns, indicating that the findings were a result of prenatal hormonal exposure, rather than socialization. No negative effects were noted for boys with twin sisters.

“This is a story about the biology of sex differences,” said study coauthor David Figlio, dean of Northwestern’s School of Education and Social Policy. “Our findings are consistent with the idea that passive exposure to prenatal testosterone changes women’s education, labor market, and fertility outcomes.”

The results of the study are particularly relevant now, given that increases in in vitro fertilization have bumped up the number of twin births since the 1980s. More girls are being born with twin brothers than ever before. But the authors are not discouraging IVF, or even making a judgement call as to whether the effects of testosterone exposure in the womb are inherently negative. Indeed, in a world of increased gender equality, girls with higher testosterone levels may make more money and attract better mates. It is possible that twin girls are screwed over not by biology, but by society’s reticence to cater to women with traditionally “masculine” qualities.

“Basically we find that there are some very interesting long-term biological effects of being a sister to a twin brother,” Figlio says. “Whether we view those effects as ‘positive’ or ‘negative’ may be culturally dependent.”