Women with a boy as twin are less likely to MARRY or have kids

While having a twin might sound fun, a new study suggests that it could come with some worrying side effects.

The study, by researchers from Northwestern Academy of Sciences, has revealed that women with a twin brother are less likely to marry, have children, or go to university. And they will probably end up earning less money on average than either their brother or women who have a female twin, the study claims.

Exposure to high levels of the male s-x hormone testosterone is believed to explain the phenomenon.

The implications for society are especially important, say scientists. The number of twins being born has nearly doubled in many countries since 1980.

Women are conceiving later in life as they put off starting a family for their career - leading to increasing reliance on IVF (in-vitro fertilisation).

It means a rising number of females worldwide are exposed to pre-natal testosterone from their male twin.

The study of 13,800 twin births in Norway over a period of 12 years - the biggest of its kind - add to previous evidence these girls are at a disadvantage.

Corresponding author Dr Krzysztof Karbownik, an economist at Northwestern University in Illinois, said:

Nobody has been able to study how male twins impact their twin sisters on such a large scale.

This is the first study to track people for more than 30 years - from birth through schooling and adulthood - to show being exposed in utero to a male twin influences important outcomes in their twin sister, including school graduation, wages and fertility rates.

It showed that compared to women whose twin was also female, these girls were about 15 and four percent less likely to graduate from high school or complete college respectively.

They were nearly 12 percent less likely to get married. Fertility rates were almost six percent lower and life-cycle earnings 8.6 percent less.