Does the battle of the sexes begin in utero?
A new study has found that women who have shared a womb with a twin brother are less likely to graduate from high school or college, may earn less money and will have lower marriage and fertility rates than twins who were both female.
A possible culprit? Blame it on the male hormone testosterone manspreading its way into female twins’ amniotic sacks.
The researchers, who hail from Northwestern University and the Norwegian School of Economics, looked at data on about 13,800 twins born in Norway from 1967 to 1978, and tracked their lives over three decades.
“Nobody has been able to study how male twins impact their twin sisters at such a large scale,” said economist Krzysztof Karbownik, a study author and research associate at Northwestern’s Institute for Policy Research. “This is the first study to track people for more than 30 years, from birth through schooling and adulthood, to show that being exposed in utero to a male twin influences important outcomes in their twin sister.”
The findings published last week in the journal Proceedings of the National Academy of Sciences (PNAS) show that girls with a male twin are about 15 percent less likely to graduate from high school that a girl with a twin sister and almost 4 percent less likely to complete college.
The women with male twins also had on average nearly nine percent lower earnings by their early 30s than female twins with sisters, about six percent lower fertility rates and 12 percent lower marriage rates.
No long-term negative impact on the male twins was found as a result of sharing space with a female twin.
The rate of twin births in the U.S. has nearly doubled since 1980, a development attributed to women having babies later in life and increased reliance on in vitro fertilization.
Karbownik said it is known from other studies that once a male fetus’s testes start developing, there is a spike in testosterone in his amniotic sack, and some of it goes into his sister’s.
In addition, Karbownik said women pregnant with a male child have a higher level of testosterone in their blood than women carrying a girl, or female twins.
What the initial study does not get into is how much of the later-life outcomes are due to the biological effect of the testosterone, versus how much is due to how the girl is socialized, Karbownik said.
To get a better sense of the prenatal testosterone’s impact, the team repeated their study focusing on female twins whose twin siblings — either a sister or a brother — died shortly after birth. The results were the same as for the twins raised with their sibling. To the researchers, that was clear evidence that the long-term effects documented by the study were due to prenatal testosterone exposure, rather than postnatal socialization.
Karbownik said he believes the issue of prenatal exposure as opposed to postnatal socialization merits further study.
A next step, the author said, could be to look at whether the results are consistent across different cultures and time frames.
“The fact that we can distinguish between prenatal and post-natal exposure doesn’t mean post-natal channels are not responsible for (some of) what we observed,” Karbownik said. “It just means that growing up with a brother is not responsible for this.”