When a boy and girl are conceived together, the female twin is exposed to more male hormones in the womb than when both twins are girls. This has allowed researchers to test the much-debated impact of these hormones prior to birth. They have 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 females that share a litter with males differ from those from all female-litters. If such differences exist in humans, however, they must be subtle; you can't look at an individual 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 for 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 girls with a twin brother are less likely to graduate from high school or university and have fewer children and lower earnings into their 30s than those with twin sisters. The differences are limited – averaging around 10 percent – but large enough to confirm a real effect.
The next question for Karbownik was whether the difference arose from growing up with a brother of the same age, or if it reflects pre-natal 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 health system, this was disturbingly common at the time, giving a sample large enough to show it is having a brother in the womb, rather than in life, that makes the difference. On the other hand, relative weights at birth had no influence on outcomes, contradicting theories about male twins capturing scarce nutrients.
The study didn't look at boys with twin sisters, but there is little evidence non-human male animals are affected.
"This is the first study to track people for more than 30 years, from birth through schooling and adulthood, to show that being exposed in utero to a male twin influences important outcomes in their twin sister, including school graduation, wages, and fertility rates,” Karbownik said in a statement.
It's not the first study to find a potential negative effect for twin girls that shared a womb with a brother. A 2007 study carried out by Dr Virpi Lumma from the University of Sheffield, using the health records of Finland, found that girls with twin brothers were 25 percent less likely to have children if their twin was male. The team suspected exposure in the womb to testosterone may damage fertility.
Twin numbers, including mixed-sex pairs, are rising because of IVF, but the authors don't think the consequences are large enough to raise alarm. They add there may be counterbalancing positive effects they have not measured, and changing gender norms may make what was a disadvantage in the '70s into something more positive today.
Still, we wouldn't want to be the ones to tell Cersei Lannister though.