IT HAS long been a puzzle that girls who grow up without their fathers at home reach sexual maturity earlier than girls whose fathers live with them. For years, absent fathers have taken the blame for this, because growing up quickly has negative consequences for girls. For example, early-bloomers are more likely to suffer depression, hate their bodies, engage in risky sex and get pregnant in their teen years.

It could be a simple matter of not having as many eyes, particularly suspicious fatherly ones, watching over daughters. Or it could be a complicated physiological response to stress, in which girls adapt their reproductive strategy to their circumstances. If life is harsh, the theory goes, maybe they need to get their babies into the world as quickly as possible.

The animal world suggests that the effect is not restricted to humans. Young mice, pigs, goats and even a few primates get signals from their kin which inhibit sexual development; a strange male in their midst, by contrast, really speeds things up. Research in humans has shown that girls growing up with stepfathers mature even more quickly than fatherless girls and that stepbrothers have a measurable effect too. However, Jane Mendle of the University of Oregon and her colleagues have suggested another putative cause: genes. Specifically, the same genes that might make a dad more likely to leave his family could be behind early sexual development as well.

The researchers came to their conclusion after analysing data collected through the American National Longitudinal Survey of Youth. Dr Mendle looked at 1,382 boys and girls, each of whom was related to at least one other subject through their mother. Most of the mothers were pairs of sisters, but some were identical twins or first cousins raised as sisters.

The NLSY surveys asked the mothers about many things, including whether the father of their children lived with them. They were surveyed every year from 1979 to 1994 and then every second year. From age 14 their children were given annual questionnaires, and asked if they had engaged in sexual intercourse yet.

What the researchers wanted to know was whether the age at which a young person first had sex was something that ran in the family—regardless of whether the father had been around or not. To find out, they compared young people who had grown up without their dads with cousins whose dads remained at home. If the environmental effect of a father's absence was causing children to mature faster, they reasoned, that would show up.

It didn't. In fact, the more closely related the cousins were—by having mothers who were identical twins, for instance, versus cousins—the closer their age at first sexual experience, says Dr Mendle. The researchers found it was as true for boys as it was for girls. They published their work in the current issue of Child Development.

Dr Mendle now suspects that the same genetic factors that influence when a child first has intercourse also affect the likelihood that they would grow up in a home without their dads. What kind of genes could cause both? They could be ones that predispose a person to impulsivity, for instance, or sensation seeking. Or they could simply be genes that cause early puberty, suggests Dr Mendle—leading to early sexual experimentation, unintended pregnancy and a partner you never really chose and do not want to spend your life with.