Women with a twin brother paid less than those with a twin sister

Bad news for women with a twin brother -- sharing a womb with a boy can have an adverse effect on the female's success. 

Women with a male twin tend to earn lower wages than their sibling and are less likely to marry and have children than are women with a twin sister, according to a study that examined more than 13,000  twin births in Norway over a 12-year period. Indeed, the study concluded that females with male twins:

• Earn 8.6 percent less than females with female twins over the course of their lives
• Are 15.2 percent less likely to graduate from high school
• Are 3.9 percent less likely to graduate from college
• Are 11.7 percent less likely to get married
• Are 5.8 percent less likely to have children

What's to blame? Testosterone, of course. 

Researchers from Northwestern University and the Norwegian School of Economics found that exposure to a male twin in utero
can cause worse educational and labor outcomes in their twin sister in part through the male twin's prenatal transfer of testosterone to their female counterpart through the amniotic fluid in the womb.  

The study showed male twins are not negatively affected by their exposure to a female twin in utero. Researchers also cautioned that some female twins might not be affected at all by prenatal testosterone exposure, and that there could also be positive long-term effects of the testosterone exposure in girls. 

"This is a story about the biology of sex differences," said study co-author David Figlio, dean of Northwestern's School of Education and Social Policy. "We are not showing that exposed females 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."

The study, "Evidence that prenatal testosterone transfer from male twins reduces the fertility and socioeconomic success of their female co-twins," published in the Proceedings of the National Academy of Sciences (PNAS), examined 13,800 twin births between 1967 and 1978.