Girls born with twin brothers may come with a disadvantage. Researchers find that girls who share their mother's womb with a boy twin are more likely to be unsuccessful in school, marriage, and income.

Researchers from Northwestern University and Norwegian School of Economics embark on what they say is the biggest and most extensive study of its kind. The findings of the study strengthens the premise that having exposure to a male twin in utero can result in persisting effects on females.

Data Collection On Testosterone Transfer Effects

The data from the study come from the Norwegian Registry Data, which is a compact database that covers Norway's entire population. For the research, the team looks into all the twin pairs born from 1967 to 1978. All the entries have at least one twin who did not manifest any birth defect, survived the first year of life, and at least one twin is observed at 32 years of old, when long-term outcomes are evaluated. They exclude those with observations for education information only (absence of income data), because of reasons such as childhood death or emigration. With these restrictions, the team was able to gather a total of 13,717 samples, with 36.2 percent female to female pairs, 37 percent male to male pairs, and 26.7 percent female to male pairs.

The researchers then study the effects of testosterone transfer. The theory designates that such transfer occurs in both female to female, and female to male twin pairs.

Effect Of Males On Females

Results strengthen the theory that females in female to male twin pairs are subject to more testosterone while inside the womb through the amniotic fluid or mother's bloodstream that the siblings share.

The numbers show negative results, literally. About -15.2 percent females are able to finish high school, -3.9 percent are able to complete college, -11.7 percent get married, -8.6 percent present life earnings, and -5.8 percent show good fertility rates.

In-utero Effect Or Postnatal Socialization Result?

The long-term effects of having a male twin are partially explained by changes in behavior over time. In the past, girls with boy twins are observed to have such changes. Boys, on the other hand, are not known to have long-term effects from having exposure to female twins while inside the womb.

When babies are inside the womb, ovaries and testes generate steroids such as testosterone, which create distinct differences between males and females. Some small studies show that exposure to hormones of the opposite sex may result in behavior and trait changes. Others link these changes to interaction after being born, or females being raised alongside their male twin.

To rule out the effects of post-birth socialization to the results of the current study, the researchers focus on female babies, whose twins, either male or female, died shortly after birth. Such babies are considered singletons. The findings turn out unchanged, proving that the long-term effects of having a male twin is due to what happens inside the womb, not on what occurs outside after birth.

To Each His Own

While the study is considered big for its kind, the researchers emphasize that the results particularly apply to Norwegian society. However, they do not completely limit the possibility of having the same effects on other cultures.

"Basically we find that there are some very interesting long-term biological effects of being a sister to a twin brother," says Christopher Kuzawa, coauthor and anthropology professor. He adds that the choice as to whether people see the findings as negative or positive depends on the cultural norms.

The study is published in the Proceedings of the National Academy of Sciences of the United States of America.

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