Teenage pregnancy could be "contagious" and caught by girls influenced by their elder sisters who have children while young, new research suggests.
A team studied the records of thousands of women over decades and found that those with elder sisters who had children in their teenage years were twice as likely to do the same as those without.
The research showed that although there is evidence that better education of women leads to lower teenage pregnancy rates, in families with teenage mothers the chances of a younger girl having a child in her teens doubled from one in five to two in five.
The scientists believe their research may show that new policies may be needed to tackle teenage pregnancies, which take into account this "sibling effect".
Professor Carol Propper, who co-authored the British and Norwegian study, said: "These findings reveal the positive sibling effect (on conception rates) still dwarfs the negative effect of education. These findings provide strong evidence that the contagious effect of teen motherhood in siblings is larger than the general effect of being better educated.
"This suggests that more policies aimed directly at decreasing teenage pregnancy may be needed in order to reduce teen births."
The study of 42,000 women chose to look at sister-to-sister relationships because sisters generally spend more time together than with schoolmates or other friends.