Researchers at the University of Sheffield in Britain and Finland’s University of Turku have been researching Finnish records of twin boys and girls to see how their twin brothers’ testosterone affects the lives of twin girls. They dug into Finnish medical records between the years of 1734 to 1888. Of 754 twins, females with a twin brother were 25% less likely to have children than females with a twin sister. They were also 15% less likely to marry. These life choices may have in part to do with female exposure to male testosterone in the womb.

In the womb, a female twin fetus is exposed to her brother’s testosterone, and he is exposed to her estrogen. However, since male and female fetuses have similar estrogen levels, the female is more likely to be affected by her brother’s hormones. Because of this, the female can have more masculine facial features as well as masculine traits and behaviors. This could affect their conscious decision to get married or be attracted to a male or have a male be attracted to them. Testosterone also has the ability to damage female fertility, making it more difficult for twin women to conceive. Animal work has supported this theory, but more research with humans is necessary. Also, social factors may affect this as well.



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