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.

Virpi Lummaa of the University of Sheffield, in Britain, and her colleagues made the claim after studying detailed data from several generations of church records from many parishes in Finland. To ensure their findings were not skewed by modern health care, they confined their investigation to the years before Finns gained access both to contraception and assisted conception.

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. And finally—to rule out any influence of sharing a house as well as a womb—Dr Lummaa checked the results were the same for women whose twin brothers died before they were three months old. They were. The researchers reported their findings in this week's Proceedings of the National Academy of Sciences.

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.

Dr Lummaa's results also suggest that, if a woman wishes to maximise the chances of passing on her genes, she would do better to avoid producing pairs of twins consisting of one boy and one girl and go for a single-sex combination instead. Mothers included in the study who produced opposite-sex twins had 19% fewer grandchildren than did mothers who gave birth to same-sex twins.

Evolutionary theory thus predicts that there should be fewer pairs of girl-and-boy non-identical twins than single-sex pairs of non-identical twins. Whether that is so requires another set of figures. Finnish church records, helpful as they are, do not distinguish non-identical same-sex twins from identical ones. In the eyes of God, unlike those of natural selection, twin girls are created equal.