Genetics may explain why children who live in homes without fathers have sex at a younger age than others, according to a report published today.
The study, published in the American journal Child Development, found a genetic theory to challenge "environmental" theories which previously explained the link.
Researchers looked at more than 1,000 cousins aged 14 and older, testing for genetic influences as well as factors such as poverty, education opportunities and religion.
It compared children who were related in different ways to each other, and who differed in whether they had lived with their fathers.
The more genes the children shared, the more similar their ages of first intercourse - regardless of whether or not the children had an absent father.
This finding, the researchers say, suggests environmental theories do not fully explain the link. Instead, genetic influence can help understand the tie between fathers' absence and early sex.
"Our study found that the association between fathers' absence and children's sexuality is best explained by genetic influences, rather than by environmental theories alone," according to Jane Mendle, assistant professor of psychology at the University of Oregon, which led the study.
Among prior environmental explanations of why children who live in homes without fathers have sex earlier are that early childhood stress accelerates children's physical development; that children who see their parents dating may start dating earlier, and that it's harder for a single parent to monitor and supervise children's activities and peers.
Prof Mendle added: "While there's clearly no such thing as a 'father absence gene', there are genetic contributions to traits in both mothers and fathers that increase the likelihood of earlier sexual behaviour in their children.
"These include impulsivity, substance use and abuse, argumentativeness, and sensation seeking.
"These traits get passed down from parents to children, resulting in a situation known as 'passive gene-environment correlation,' because the same genetic factors that influence when children first have intercourse also affect the likelihood of their growing up in a home without a father."
The study was conducted by researchers at the University of Oregon, the University of Virginia, the University of Chicago, the University of Indiana, Columbia University and the University of Oklahoma.