Teenagers are more likely to engage in underage sex if their parents smoke or don't wear seatbelts in cars, a study suggests.

Researchers in the United States have found a strong link between what they describe as risky behaviour by adults and teenage sex.

They also believe there is a link between parents who smoke and drink heavily and children who get involved with drugs or crime.

The principle of young people having sex early is a difficult one to entangle

Spokeswoman, Family Planning Association

The researchers suggested that teenage sex could be curbed if public health campaigns tried to get parents to give up smoking.

Researchers at the Southwest Texas State University and The City University of New York based their findings on an analysis of the US National Longitudinal Study of Adolescent Health.

Sexual behaviour

This study includes information on the sexual behaviour of 19,000 adolescents between the ages of 12 and 18.

It also includes data on one of the children's parents, namely whether they smoked, drank heavily or used a car seatbelt.

The researchers found that teenagers were more likely to have had sex before the age of 16 if their parents smoked.

Teenagers were most likely to have sex early in their lives if their parents drank heavily as well.

Boys were also likely to have sex before they left school if their parents drove without wearing a seatbelt. The same did not apply for girls.

The researchers found no evidence to suggest that risky behaviour by parents increased the chances of their children practising unsafe sex.

But they suggested that it did increase the likelihood of children smoking and drinking heavily themselves.

'Sexually active'

There was also a link between parents who smoked and drank and children becoming involved with drugs and the police.

The study found that children were less likely to have sex as teenagers if their parents kept a close eye on them.

There was a similar pattern if parents were at home in the morning and evening, before and after school and again at bed-time.

"Adolescents whose parents engage in risky behaviour, especially smoking, are especially likely to be sexually active," the researchers said.

"They are also more likely to smoke, drink, associate with substance-abusing peers and participate in delinquent activity."

They added: "Because parents serve as important role models for their children, it stands to reason that parents who exhibit unsafe behaviours are especially likely to have children with similar tendencies."

'Complex reasons'

The researchers said the findings highlighted the need to encourage parents to stop smoking and to drink less.

"Public health campaigns that urge parents to act responsibly by engaging in health-conscious behaviours are likely to help reduce precocious and unsafe sexual activity among teens."

A spokeswoman for the UK's Family Planning Association said the reasons why teenagers have sex at an early age were complex.

"I think the principle of young people having sex early is a difficult one to entangle."

But she added: "If young people are brought up in a household without open communication or positive role models then they may be more likely to have sex early."