Women Who Shared Womb With Male Twin Less Likely to Graduate High School and Get Married

Women who have a male twin are less likely to graduate from high school than those born with a female twin, according to a study. Scientists studied all 13,717 twins born in Norway between 1967 and 1978 to understand whether being a woman born with a twin brother affected the ability to hit traditional markers of success in adulthood.

The research was inspired by what is known as the twin testosterone-transfer hypothesis. It is believed that females in opposite-sex twins are exposed to higher than normal levels of testosterone in the womb, which could affect their behavior. Scientists haven't noted similar problems in men born with a female twin. Equally, the difference could be due to girls being raised alongside twin brothers or because males grow faster and thus take up more nutrients in the womb.

The research involved 583 females whose twin brother died in his first year of life and 239 whose twin died within 28 days after he was born. The team hoped this would provide a clearer picture of the effects of socialization and prenatal exposure to testosterone. They were not, however, able to measure how much testosterone each fetus was exposed to in the womb.

The data revealed that females with a male twin were less likely to graduate from high school or college, by 15.2 percent and 3.9 percent, respectively. They also had lower fertility rates (by 5.8 percent) and earnings (by 8.6 percent). These women were also less likely to get married, by 11.7 percent. A similar trend was found in women whose sibling had died, suggesting the difference wasn't socialized.

Partly because of in vitro fertilization and partly because people are having children later in life, the rate of twins born has doubled since 1980 in many countries, including Brazil and China, as well as nations in Europe and North America. Uncovering the potential negative effect of testosterone-transfer is therefore important, the authors said.

Study co-author David Figlio, dean of Northwestern's School of Education and Social Policy and the university's Institute for Policy Research (IPR) fellow, said in a statement that "this is a story about the biology of sex differences."

"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," he said.

Dr. Krzysztof Karbownik, an economist and a research associate at the IPR and a corresponding author for the study, told Newsweek it was not possible to pin down why the differences appear to arise and whether the cause was biological or cultural and societal norms.

"It most likely is the combination of both factors, but it is not clear how much each of the factors contribute to our long-run effects," he said. "Paired with other psychological and behavioral studies, it would suggest that societal gender norms are important for female economic well-being and that perhaps gender non-conforming-females are penalized for their behaviors."

Karbownik stressed that "we want to emphasize that this paper is not about assisted reproductive technology or IVF, and we do not advocate against it."