Women with twin brothers do worse in school and make less money than those with twin sisters, a large new study has found. In their 30s, the women wound up earning 9 percent less. They were also less likely to graduate from school, marry and have children.
The researchers said the effects were because the women were naturally exposed to their brothers’ testosterone in the womb. The study, which was published Monday, included all births in Norway for 11 years.
The findings might also help explain a paradox — overall, girls are doing better than boys in school, but men are doing better than women in the workforce. Researchers have found many potential explanations. Girls seem to be encouraged to be competent, while boys are encouraged to be confident, and school today requires a lot of self-control, which most boys develop later. Once people start working, women face sexism and a host of other inequalities (many related to motherhood).
Testosterone, which all females are exposed to in utero, might be another contributor. The hormone is associated with certain behaviors — including aggression, competition and risk taking — that might contribute to boys’ underperformance in school, but that are often rewarded in the workplace. Females exposed to an elevated level oftestosterone might act more like boys when they’re young, but then face sexism at work when they’re older. Women are penalized, research shows, when they show many of the same behaviors that benefit men in the workplace.
The new study was focused on studying twins and did not analyze these other issues. Also, the findings are specific to people born in Norway several decades ago. But it suggests broader effects, said David Figlio, an economist and dean of the School of Education and Social Policy at Northwestern, who wrote the paper with colleagues at Northwestern and the Norwegian School of Economics.
“Women exposed to testosterone have some of the educational challenges more frequently associated with men,” Figlio said. “However, to the extent to which labor market discrimination exists in society, they don’t have the discriminatory benefits that men enjoy.”
The study, published in PNAS, was of 728,842 people, including 13,800 twins — everyone born in Norway from 1967 to 1978 — in addition to records about their family, education and work.
Women with a male twin were 15 percent less likely to graduate from high school than women with a female twin, and those who went to college were 4 percent less likely to finish it.They had a 12 percent lower probability of being married, and a 6 percent lower probability ofhaving children.
Notably, the research did not show an effect on the careers people pursued. Among those who graduated from college, women with male twins were no more likely to pursue degrees in traditionally male-dominated fields like science, engineering, math and economics. The study did not find that men had long-term effects from having a female twin.
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.
In the Norway study, the effects on girls’ educational achievement — perhaps because of more disruptive behavior in the classroom — probably drove the long-term economic effects, the researchers said. Roughly half of their decreased earnings could be explained by their lower rates of graduation from high school and college.
One potential explanation for the women’s lower rates of marriage is that testosterone could indirectly influence their behavior in relationships. Their decreased fertility was not because of marrying less often — it was true among women who married and those who didn’t. Exposure to excess testosterone in utero has been found to decrease fertility in the females of multiple species, including sheep and mice.
In the last four decades, the rate of twins has nearly doubled in many countries, largely because women are having babies later and using in vitro fertilization more often, both of which increase the chance of twins. In the United States, 1.1 percent of newborn girls have male twins, up from 0.6 percent in 1971, the researchers estimated.
The findings could inform parents, doctors and teachers about ways to support these children, the scientists said. They also noted that the findings were based on national averages, and do not predict the result of any individual fertility decision. Not all girls with twin brothers are affected, or affected in the same way, and the researchers did not study other parts of life in which females with male twins might excel.
Also, a person’s biology alone does not determine a life course. Family, environment and personality shape it, too.
“There are always interactions between biology and culture and the environment,” said Bernard Crespi, an evolutionary biologist at Simon Fraser University in Canada who peer-reviewed the paper. “You can always change the cultural and social environment. Humans are plastic.”
This article originally appeared in The New York Times.