Females who share a womb with a male may be less likely to graduate from university or get married than those born with a female counterpart, according to a new Norwegian study.
'Evidence that prenatal testosterone transfer from male twins reduces the fertility and socioeconomic success of their female co-twins' was published by Illinois' Northwestern University in the Proceedings of the National Academy of Sciences journal.
The study, which analysed data on 13,800 twin births between 1967 and 1978 in Norway, found females exposed to a male in utero were less likely to graduate from high school and university or get married than those born with another female.
They also had lower fertility rates and made less money over their lives.
"Nobody has been able to study how male twins impact their twin sisters at such a large scale," said co-author Krzysztof Karbownik.
"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."
The research suggests the effects are caused by the female being exposed to more testosterone.
"We are not showing that exposed females are necessarily more 'male-like', but our findings are consistent with the idea that passive exposure to prenatal testosterone changes women's education, labour market, and fertility outcomes," said co-author David Figlio.
The hormone increases aggression, competition and risk-taking within the females.
To prove the effects of the testosterone, the researchers also analysed the lives of female twins whose twin sibling died shortly after birth and found similar results, suggesting the effects came from prenatal conditions.
The researchers did say that some positive effects of testosterone exposure may exist and the effects may be affected by societal standards.
"It is important to emphasise that our findings apply to Norwegian society during the timeframe of the study, but may not apply equally across other societies or cultural settings," said co-author Christopher Kuzawa.
"Basically we find that there are some very interesting long-term biological effects of being a sister to a twin brother.
"But whether we view those effects as 'positive' or 'negative' may be culturally dependent."
There was little evidence of oestrogen from females affecting the lives of the males.
Newshub.
