Oh, Brother! Female Twins Less Likely to Have Kids

Female twins with twin brothers are less likely to marry and have kids than those with twin sisters, a new study shows, and those who do reproduce have fewer children. Elevated exposure to testosterone in utero appears to be the culprit, say British scientists who reached back over two centuries to find records unaffected by modern health care.

Researchers measured the impact of the hormone by analyzing Finnish records dating from 1734 to 1888, the BBC reports, and turned up 754 twins. They attributed the decreased fertility of the females from opposite-sex twins—which persisted even if the male twin died at birth—to greater likelihood of masculine characteristics and elevated risk of diseases such as reproductive cancers. (More estrogen stories.)