When a boy and a girl are conceived together, the twin is exposed to more male hormones in the womb than when both twins are girls. This allowed researchers to test the highly controversial impact of these hormones before birth. They found that there is a small, but statistically significant, negative impact on life outcomes from nine months in close confinement with a boy.
Animal studies have shown that females who share a litter with males differ from those of all female litters. If such differences exist in humans, however, they must be subtle; you can’t look at a particular woman and tell if she has a male twin (a rare case of scientific accuracy in star wars).
The incredibly detailed record keeping of Scandinavian countries allowed Dr. Krzysztof Karbownik of Northwestern University to test the statistical effects. Together with Norwegian scientists, Karbownik studied the life outcomes of 13,717 twins born in Norway between 1967 and 1978.
In Proceedings of the National Academy of Sciences, Karbownik reports that girls with a twin brother are less likely to graduate from high school or college and have fewer children and lower incomes in their 30s than those with twin sisters. The differences are small – around 10% on average – but large enough to confirm a real effect.
The next question for Karbownik was whether the difference resulted from growing up with a sibling of the same age, or whether it reflected prenatal hormones, a theory known as the twin testosterone transfer hypothesis.
The team looked for a sample of girls whose twin had died in infancy. Even with Norway’s excellent healthcare system, this was quite common at the time, giving a large enough sample to show that it’s having a sibling in the womb, rather than in life, that makes the difference. On the other hand, relative birth weights had no influence on the results, contradicting theories about male twins picking up scarce nutrients.
The study did not include boys with twin sisters, but there is little evidence that non-human male animals are affected.
“This is the first study to follow people for more than 30 years, from birth through schooling and into adulthood, to show that in utero exposure to a male twin influences important outcomes in their twin sister, including school graduation, salaries and fertility rates,” Karbownik said in a statement.
This isn’t the first study to find a potential negative effect for twin girls who shared a womb with a sibling. A 2007 study by Dr Virpi Lumma of the University of Sheffield, using medical records from Finland, found that girls with twin brothers were 25% less likely to have children if their twin was a male. The team suspects that exposure in the womb to testosterone may impair fertility.
The number of twins, including intermarriage, is increasing because of IVF, but the authors don’t think the consequences are significant enough to raise alarm bells. They add that there may be counterbalancing positive effects that they have not measured, and that changing gender norms may turn what was a disadvantage in the 1970s into something more positive today.
Still, we wouldn’t want to be the ones to tell Cersei Lannister that.
This article was originally published in March 2019.