Sharing the belly with a twin brother seems to have a huge detrimental effect on girls


Nobody ever said that it was easy to have a brother, but for girls who start their lives sharing their uterus with a twin, the consequences can dramatically change their lives, according to scientists .

According to the largest study of its kind, women who share their mother's womb with a twin brother are less likely to finish high school or university than twins who share with a sister – but that does not stop there .

In addition to being 15.2% less likely to complete high school and 3.9% less likely to finish university, they are also 11.7% less likely to get married.

Similarly, their earning capacity is on average lower (by 8.6%), as is their entry rate into the labor market (3.2% less) and their fertility rates are lower, producing on average 5.8% fewer children than girl-girl twins.

At least, this is what figures suggest from data on twins born in Norway between 1967 and 1978, some 728,842 births, including 13,800 twins – and taking into account factors such as month and year birth, maternal education and age at birth. the time of birth and the weight of the child at birth.

Although the experience of binoculars born in different countries and at different times may be different in itself, the findings nonetheless strongly suggest the hypothesis that belly sharing with a sibling may have long-term adverse effects. about girls.

"No one has been able to study the impact of male twins on their twin sisters on such a large scale," says economist Krzysztof Karbownik of Northwestern University.

"This study is the first to follow people for more than 30 years, from birth to schooling, to adulthood, and shows that being exposed in utero to a twin of male influences the significant results of the twin sister, including graduation, wages, and fertility. "

To determine whether these changes in the girl's life were due to nature (fetal testosterone exposure of the twin brother) or to feeding effects (postnatal socialization in the experience of growing up with a twin brother), the The team filtered the data.

When they isolated the cohort – looking only at girls whose twin brother died shortly after birth or during the first year of life – the same results were presented, virtually unchanged.

In other words, even when girls have been raised not as twins, but as single children, their life prospects in terms of success and family life have always been affected.

In the evaluation of the researchers, this means that the hypothesis of an in utero transfer of testosterone between twins is real and probably concerns a small but growing number of women in the world, because of the greater number of twins born now. because of the increased number of IVF procedures.

Essentially, by being exposed to the testosterone of their twin brother in utero via the amniotic fluid or by the mother's blood, the idea is that the girls end up being changed measurably.

"We do not show that exposed women are necessarily more" masculine ", says one of the team members, economist David Figlio.

"But our findings are consistent with the idea that passive exposure to prenatal testosterone alters the outcomes of education, labor market and female fertility."

This is a remarkable and worrying trend. it's also a one-way effect. Boys, it seems, do not suffer any lasting impact by sharing the uterus with a twin.

"We found that men exposed in utero to a twin had similar long-term results to men exposed in utero to a twin," the authors write in their article.

"Testosterone levels in women are much lower than in men and the passive exposure to testosterone of a male twin therefore represents a relatively greater increase in exposure to the testosterone. ;hormone."

There are many things to unwrap here, and we are far from having all the answers. But the problem, to the extent that the researchers have detected it, only worsens.

In Norway at least, since the birth of the cohort of this study, about twice as many girls have started their lives after sharing the belly with their twin brother for nine months.

What is happening inside this place remains a mystery, and no one says that the effects of socialization will not occur either: a study published less than a fortnight ago revealed that a younger brother in the United States had reduced the income potential of adult women by seven percent.

The results are reported in PNAS.

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