Women With a Twin Brother Are More Likely to Face Penalties at School and Work

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Women With a Twin Brother Are More Likely to Face Penalties at School and Work

Pranjal Mehta, an experimental psychologist at University College London who studies the effects of hormones on behavior, cautioned against drawing simplistic conclusions from the results. “Testosterone might not have direct effects on behaviors,” he said. “It depends on context, but it also depends on other characteristics of the person.”

Men and women have varying levels of testosterone, and it affects people in different ways; everyone sits somewhere along a spectrum, scientists say. Much depends on culture and environment, and on individual factors like stress levels and self-perception. Also, it’s a responsive hormone, meaning its levels change based on what happens to a person. Testosterone levels rise when men watch their sports team win, for instance, and decrease when they become fathers.

But the new study suggests that, on average, there are consequences when females are exposed before birth to higher-than-usual levels of testosterone.

“The evidence here is there likely are biological effects of prenatal testosterone, but how they actually manifest is a product of a particular society,” said Chris Kuzawa, a co-author of the paper and a professor of anthropology at Northwestern. “What behaviors are considered problematic or encouraged is a cultural phenomenon.”

Scientists do not measure prenatal levels of testosterone or manipulate the levels for the purpose of an experiment, so they cannot prove direct effects. But other research has suggested similar effects when females are exposed to male sex hormones in utero. Studies have found that the girls behaved more like typical boys. This has also been observed in mice and other animals born in litters that are predominantly male.

Other studies of opposite-sex twins have also found effects of testosterone on females, in behaviors like aggression and rule breaking. But it had been impossible to show that they were for biological reasons, as opposed to a result of being raised with a brother. By also studying females whose male twins died early in life, the Norway research was able to show that the differences were mainly because of testosterone exposure.

Socialization also plays a role. A new study published this month shows that in the United States, women with a younger brother earned around 7 percent less in adulthood, in part because their parents had lower academic expectations for them and they took on more traditional gender roles.

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