Twin brothers can leave quite an impression. The mere presence of a boy in the same womb as his sister causes her to develop bigger teeth than she otherwise would. Girls with twin brothers perform better on spatial-ability tests. They have better ball skills than most females; squarer, more masculine jaws and are more likely to be short-sighted. Now it seems that sharing the womb also has a deleterious effect on the sexual reproduction of women with a twin brother.Prenatal levels of testosterone strike again. No word on how these women play the ultimatum game when they grow up.
They report that women with a twin brother were 15% less likely to get married than were women with a twin sister. Those with a male twin also had a 25% lower chance of giving birth even though they lived just as long as those with a female twin. When the researchers considered only married women, those with a twin brother on average had two fewer children during their lifetimes than did women with a twin sister.
As with the teeth and the jaw lines, the purported cause of atypical female biology is early exposure to testosterone. This hormone is made by a male fetus's developing testes from about seven weeks after conception and is thought to diffuse through the amniotic fluid, influencing his sister's growth. But the exact mechanism by which a twin brother lowers his sister's chances of reproductive success is unclear.
Lesbianism is one possibility. (To what extent is impossible to tell, because the Lutheran ministers charged with collecting exhaustive demographic details did not probe quite that far.) But physiology could also play a part. Some cancers of the reproductive system, and a condition called polycystic ovary syndrome, which reduces fertility, are more common in women with relatively high early exposure to male hormones.
via The Economist