Having a male twin is bad for girls, study finds

Having a male twin is bad for girls, study finds

Having a male twin is bad for girls, study finds

So, the theory is that, in order to support the development the boy she's sharing a womb with, a developing twin girl's amniotic fluid and bloodstream may get flushed with more male hormone than it would have otherwise.

What's to blame? Testosterone, of course.

The research found that girls who have or had a twin brother were more likely to display traits like rule-breaking, asocial and aggressive behaviour - traits consistent with exposure to the male hormones, androgens.

A 30-year of its kind, scientists examined data on all twin births in Norway over a 12-year period to find that females exposed in utero to a male twin experienced adverse educational and labor outcomes along with altered patterns of marriage and fertility as adults.

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. 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. They tested the life outcomes of 13,717 twins from Norway born between 1967 and 1978.

"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.

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

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When babies are inside the womb, ovaries and testes generate steroids such as testosterone, which create distinct differences between males and females.

The study was conceived on animals, showing slight differences in the litter females with a male twin compared to all female-litters. The results were unchanged in this sample, providing strong evidence that the long-term effects that the study documents are due to prenatal exposure, rather than postnatal socialization.

Scientists also looked for examples of when the girls' twin brother died in infancy to see the effects.

The long-term effects of having a male twin are partially explained by changes in behavior over time.

Co-author Christopher Kuzawa, professor of anthropology and an IPR fellow said, "It is important to emphasize that our findings apply to Norwegian society during the timeframe of the study, but may not apply equally across other societies or cultural settings".

According to the Australian Bureau of Statistics' latest figures, 4,469 pairs of twins were born in Australia alone in 2017.

But twins share a rare influence on one another, beginning in the womb - and it's becoming increasingly clear that even between twins, the female may be at a disadvantage.

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