By Sola Ogundipe

Women who shared their mother’s womb with a male twin (boy/girl twins) are less likely to graduate from high school or college, have earned less by their early 30s, and have lower fertility and marriage rates when compared with twins who are both female, according to new research.

Boy/girl twins are always fraternal because they can only form from two separate eggs that are fertilized by two separate sperm.

In the largest and most rigorous study of its kind, Northwestern University and Norwegian School of Economics researchers 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 labour outcomes along with altered patterns of marriage and fertility as adults.

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“Nobody has been able to study how male twins impact their twin sisters at such a large scale,” said study corresponding author Krzysztof Karbownik, an economist and research associate at Northwestern University’s Institute for Policy Research (IPR).

“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, including school graduation, wages and fertility rates.”

The study, “Evidence that prenatal testosterone transfer from male twins reduces the fertility and socioeconomic success of their female co-twins,” was published this week in Proceedings of the National Academy of Sciences (PNAS).

The researchers used data on 13,800 twin births between 1967 and 1978 to show that females exposed in utero to a male twin are less likely to graduate from high school (-15.2 percent), to complete college (-3.9 percent) or to get married (-11.7 percent). They also have lower fertility rates (-5.8 percent) and life-cycle earnings (-8.6 percent.)

The study supports the “twin testosterone-transfer hypothesis,” which posits that females in male-female twin pairs are exposed to more testosterone in utero via the amniotic fluid or through the mother’s bloodstream that they share with their twin brother.

One explanation for the long-term effects the researchers discovered is changes in behavior, which have previously been demonstrated in girls with male twins.

Unlike the females, the researchers found that male twins do not experience long-term consequences of being exposed to a female twin in utero.

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“This is a story about the biology of sex differences,” said co-author David Figlio, Dean of Northwestern’s School of Education and Social Policy and IPR fellow.

“We are not showing that exposed females are necessarily more ‘male-like,’ but our findings are consistent with the idea that passive exposure to prenatal testosterone changes women’s education, labor market, and fertility outcomes.”

To separate the effects of fetal testosterone from postnatal socialization, the research team repeated their analyses focusing only on female twins whose twin sibling — either twin sister or twin brother — died shortly after birth, and thus they were raised as singletons. 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.