Nobody ever said having a brother was easy, but for girls who start life by sharing a womb with a male twin, the consequences can be drastically life-altering, scientists say.
According to the largest study of its kind, women who share their mother's womb with a twin brother end up being less likely to graduate high school or university than female twins who share with a sister – but it doesn't end there.
In addition to being 15.2 percent less likely to complete high school and 3.9 percent less likely to finish college, they're also 11.7 percent less likely to get married.
Similarly, their earning power on average is less (by 8.6 percent), as is their rate of entering the workforce (3.2 percent less), and their fertility rates are lower, producing 5.8 percent fewer children on average than girl-girl twins.
At least, that's what the figures suggest based on data on twin births in Norway between 1967 and 1978, comprising some 728,842 births in total, including 13,800 twins – and after accounting for factors like month and year of birth, maternal education and age at the time of birth, and the child's birth weight.
While the experience of twin girls born in different countries and at different times may be different in itself, the findings nonetheless are strongly indicative of the hypothesis that sharing the womb with a brother can have lasting and detrimental effects on girls.
"Nobody has been able to study how male twins impact their twin sisters at such a large scale," says economist Krzysztof Karbownik from Northwestern University.
"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."
To isolate whether these changes in the girl's lives were due to nature (exposure to foetal testosterone from the twin brother) or nurture effects (postnatal socialisation in the experience of growing up with a twin brother), the team filtered the data.
When they isolated the cohort – looking only at girls whose twin brother died shortly after birth, or in the first year of life – the same results presented, virtually unchanged.
In other words, even when girls were brought up not as one of a pair of twins, but effectively as single children, their life prospects in terms of success and family life were still negatively impacted.
In the researchers' assessment, that means the hypothesis of in utero testosterone transfer between twins is real, and likely affects a small but growing subset of females worldwide, owing to greater number of twins being born now due to increased IVF procedures.
Essentially, by being exposed to testosterone from their twin brother in utero via amniotic fluid or through the mother's bloodstream, the idea is that girls end up being changed in measurable ways.
"We are not showing that exposed females are necessarily more 'male-like'," says one of the team, economist David Figlio.
"But our findings are consistent with the idea that passive exposure to prenatal testosterone changes women's education, labour market and fertility outcomes."
It's a remarkable and concerning trend; also, it's a one-way effect. Boys, it seems, don't experience any kind of lasting impact by sharing the womb with a girl twin.
"We found that males exposed in utero to a female co-twin had similar long-term outcomes as males exposed in utero to a male co-twin," the authors write in their paper.
"Females have much lower baseline testosterone levels than males, and thus passive exposure to testosterone from a male co-twin represents a relatively larger increase in exposure to the hormone."
There's a lot to unpack here, and we're far from having all the answers. But the issue, to the extent the researchers have detected it, is only growing more pressing.
In Norway at least, since the time the cohort in this study was being born, about twice as many girls are starting their lives after sharing the womb alongside their twin brother for nine months.
What goes on inside that place remains something of a mystery, and nobody's saying socialisation effects don't occur too: a study published less than a fortnight ago found having a younger brother in the US reduced women's adult earning potential by seven percent.
The findings are reported in PNAS.