Male twin’s hormones may affect sister’s life

Women who have a male twin are less likely to marry and have children, perhaps because of being exposed to their brother’s testosterone for nine months in the womb, researchers have reported.

A study of Finnish twins showed that women were 25% less likely to have children if their twin was a male. Those who did have children gave birth to an average of two fewer babies than women who had a twin sister.

And an analysis of 18th and 19th century data showed that women who had a male twin were 15% less likely to get married, Virpi Lummaa of the University of Sheffield in Britain and Finland’s University of Turku and colleagues reported.

Researchers have long known that fetuses are influenced by hormones in the womb. Because male and female fetuses have similar levels of the “female” hormone estrogen, girl twins are more likely to be affected by testosterone in the womb.

The report was published June 18 in the Proceedings of the National Academy of Sciences.