Women with a twin brother don't do as well in school and earn less money than women with a twin sister, and a new study suggests testosterone is one reason. They're less likely to marry and have children, as well. Researchers see the natural exposure to their brother's testosterone in the womb as a cause, the New York Times reports. "Women exposed to testosterone have some of the educational challenges more frequently associated with men," says David Figlio of Northwestern University, who co-wrote the paper. The research, by Northwestern and the Norwegian School of Economics, covered everyone born in Norway over a 12-year period through 1978: 728,842 people, including 13,800 twins. "Nobody has been able to study how male twins impact their twin sisters at such a large scale," one researcher says in a Northwestern press release.
The study supports the idea that the female twin is exposed to more testosterone through amniotic fluid or the mother's bloodstream shared with her male twin. The males, however, don't seem to be affected by having a female twin. Testosterone exposure could be part of the reason that women do better than men in school, but not in the workforce, the Times points out. Research has shown competence is emphasized for girls and confidence for boys. Boys develop the self-control required to succeed in school later than girls do. And that's before reaching the workplace, where other inequities kick in. Compared to those with a female twin, women with a male twin were 15% more likely to drop out of school and 4% less likely to graduate from college, and they had 6% fewer children, per New Scientist. In their 30s, they earned 9% less. (Read more twins stories.)