Women with a twin brother earn less than those with twin sisters

NHH By Ingrid Aarseth Johannessen

19 June 2019 09:47

Women with a twin brother earn less than those with twin sisters

Women with a twin brother drop out of upper secondary and higher education more often than those with a twin sister, a new study shows. ‘Parents need to be aware of these consequences,’ says NHH Professor.

The study is based on data from over 13,000 twin births in Norway over 11 years, and has followed up the twins from birth until their mid-thirties.

Girls with a twin brother are also less likely to be married and have fewer children and worse pay. Whether twins have a twin brother or twin sister therefore plays a major role in how they get on later in life.

‘We believe this is because the women have been exposed to testosterone by sharing the womb with their twin brother,’ says Professor at NHH Aline Bütikofer.

The results support previous research and the hypothesis that women who have twin brothers are exposed in utero to testosterone in the amniotic fluid or the mother’s blood supply.

‘Makes no difference to men’

‘What should parents and teachers do?’

‘Parents need to be aware of these consequences. The research on this is pronounced and we see clear differences. Parents with a twin girl and also teachers should consider providing extra support through their course of education,’ says Bütikofer.

The researchers also looked at the lives of women whose twin brother died during birth or his first year of life. The results indicate the same tendencies, which Bütikofer believes supports the hypothesis that the findings are not caused by social or cultural factors.

‘Women who lost their twin brother shortly after birth experienced the same effects of sharing the womb with their brother. We therefore believe that the long-term effects we have identified are not due to the fact that they have grown up with a male co-twin,’ the NHH researcher says.

The study shows no effect on boys who have a twin sister and their life prospects. The same applies to male twins.

‘It makes no difference to men whether they have shared the womb with a boy or girl. We have not found any significant differences here. Having a twin is neither good nor bad for men,’ says Bütikofer.

READ THE RESEARCH ARTICLE

Must repeat the study

The study is based on a dataset concerning twins born at the end of the 1960s and in the 1970s, and Bütikofer believes that the results do not necessarily apply to twins born after this period and up to the present day.

 ‘Women with twin brothers are obviously still exposed in utero to testosterone, but we won't find out whether they face the same challenges until 30 years from now, and when we repeat the study,’ says the NHH professor. 

She points out the fact that Norway has undergone major socio-economic changes, including in education and gender equality.