Nobody ever said it was easy to have a brother, but for girls who start life by sharing a womb with male twins, the consequences can change dramatically, scientists say.
According to the largest study of its kind, women who share their mother's womb with a twin brother end up less often in high school or college than female twins sharing a sister, but it doesn't stop there.
In addition to being 15.2 percent less likely to complete high school and 3.9 percent less likely to complete college, they are also 11.7 percent less likely to get married.
Similarly, their earning capacity is on average less (by 8.6 percent), as well as their labor participation rate (3.2 percent less), and their fertility rates are lower, which means that on average 5.8 percent fewer children are produced than girls' twins.
At least, that is what the figures suggest based on data on twin births in Norway between 1967 and 1978, with a total of approximately 728,842 births, including 13,800 twins – and after taking into account factors such as month and year of birth, maternal education and age at the time of birth and birth weight of the child.
Although the experience of twin girls born in different countries and at different times may be different in itself, the findings are strongly indicative of the hypothesis that sharing a womb with a brother can have lasting and harmful effects on girls.
"No one has been able to study how twins are affected by their twin sisters on such a large scale," says economist Krzysztof Karbownik of Northwestern University.
"This is the first study to track people for more than 30 years, from birth to school and adulthood, to show that uterine exposure to male twins affects important outcomes in their twin sisters, including graduation, wages, and fertility rates."
To isolate whether these changes in girl's life were due to nature (exposure to fetal testosterone from the twin brother) or parenting effects (postnatal socialization in the twin brother experience), the team filtered the data.
When they isolated the cohort – only looking at girls whose twin brothers died shortly after birth, or in the first year of life – the same results were presented, virtually unchanged.
In other words, even when girls were not raised as one of twins, but effectively as single children, their outlook on success and family life was still negatively affected.
In the researchers' assessment, this means that the hypothesis of in utero-testosterone transfer between twins is real, and probably affects a small but growing subset of women around the world, due to the larger number of twins born now as a result of elevated IVF procedures.
Essentially, girls are exposed in measurable ways to being exposed to testosterone from their twin brother in utero through amniotic fluid or through the mother's bloodstream.
"We don't show that exposed women are necessarily more & # 39; male & # 39;" says one of the team members, economist David Figlio.
"But our findings are consistent with the idea that passive exposure to prenatal testosterone changes the outcomes of women in education, labor market and fertility."
It is a remarkable and involved trend; it is also a one-way effect. It seems that boys do not experience a lasting impact by sharing the womb with a twin sister.
"We found that men who were exposed to a female twin sister in the womb had similar long-term results to men who were exposed to a male twin sister in the womb," the authors write in their newspaper.
"Women have much lower testosterone levels than men, and passive exposure to testosterone from male co-twins means a relatively greater increase in hormone exposure."
There is a lot to unpack here and we don't have all the answers yet. But the issue, as far as the researchers have discovered it, only becomes more urgent.
In Norway, since the cohort in this study was born, about twice as many girls start their lives after sharing the womb with their twin brothers for nine months.
What goes on inside remains a mystery and nobody says socialization effects: a study published less than fourteen days ago found that having a younger brother in the US reduced the adult earning capacity of women by seven percent.
The findings are reported in PNAS.