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The Bizarre Side Effect Of Having A Male Twin

Image by Alison Winterroth / Stocksy
March 19, 2019

Ever wondered how being a twin actually influences your life? Well, besides the obvious fact that you develop with another human in the womb, a new study suggests that what happens between conception and birth could have long-term implications for females with a male twin. 

In utero twins are exposed to hormones such as testosterone, which we know are critical for development and the healthy functioning of the body. A new study finds that exposure to the sex hormone testosterone could leave negative lasting effects on females.

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The robust study published in the Proceedings of the National Academy of Sciences (PNAS) on Monday found females who were exposed to a male twin in utero were less likely to graduate from school, earned less money by their 30s, and had lower fertility and marriage success versus twins who were both females.

If you're thinking it could have to do with what happened after birth, they also accounted for that by tracking data on female twins who had lost their male twin within the first year of life and found similar results.

The research led by Northwestern University and the Norwegian School of Economics was prompted by the "testosterone-transfer hypothesis1," a belief that females with a male twin are exposed to higher levels of testosterone in the womb leading to behavioral changes after birth.

This study supports the hypothesis of testosterone transfer between twins, but the authors caution that these outcomes cannot be generalized to all female-male twins. "Not everyone will be affected in the same way, and some female twins may not be affected at all," said study corresponding author and economist and research associate at Northwestern University's Institute for Policy Research, Krzysztof Karbownik, in a statement.

The researchers reiterate that they are not suggesting women base their fertility decisions on these findings, as "these effects are highly unlikely to result from any individual fertility decision made by a woman or couple, given that twins are a small subset of births, and male-female twin pairs even rarer yet."

Instead, the study offers insight into the potential long-term behavioral changes that could result from practices like in-vitro fertilization. With more women conceiving later in life2, we could see a greater reliance on fertility treatments as well as a rise in the number of twin births3, making this knowledge particularly important.

At mbg, we know that hormones play a crucial role in our health, for better or for worse, and this is really just one more reason to believe it.

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