A child's sexual behaviour could be influenced by their genes just like environmental factors, says a new study.
The research, led by Jane Mendle, assistant professor of psychology at the University of Oregon, looked at the age that more than 1,000 cousins first had sexual intercourse.
It found that there was more of a pattern with genetically similar children, than with those whose living situation was alike.
Ms Mendle pointed out that there is no specific gene to look out for, but that personality traits such as impulsivity and argumentativeness can be passed down to children and will probably lead to them having sex earlier.
She said: "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."
A recent study at the University of Michigan indicated that working long hours in adult environments can lead to young people having older sexual partners.