Having a male twin is bad for girls, damaging not only their education and job prospects but their chances of a happy family life, new research has found.
A 30-year study found girls who share a womb with a male twin go on to perform significantly less well academically and socially than girls whose twin is a girl.
The scientists behind the research believe exposure to testosterone in the womb is to blame.
They examined data on 13,800 twin births between 1967 and 1978 in Norway, following up the participants for thirty years after birth.
The figures showed that women who had a twin brother were 15.2 per cent less likely to complete high school and 3.9 per cent less likely to graduate from University.
They were also 11.7 per cent less likely to get married and showed a 5.8 per cent lower fertility.
Meanwhile their earnings were on average 8.6 per cent lower.
The scientists at Northwestern believe the differences are explained by biological factors in the womb rather than the social impact of growing up with a male twin because they repeated their analysis on female twins whose brother had died shortly after birth.
“This is a story about the biology of sex differences,” said 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.”
During sensitive developmental periods in utero, steroids produced by the ovaries and testes, including testosterone, help establish biological differences between males and females.
Previous, smaller studies have suggested that such exposure to opposite-sex hormones can lead to lasting changes in behavior and other traits. The new study is published in the Proceedings of the National Academies of Sciences.