Parents who don't want their teens to engage in risky sexual behaviour should make family time a priority, a new study suggests.

Adolescents who take part in family activities more often have sex less frequently, less unprotected sex and fewer sex partners, Dr Rebekah Levine Coley of Boston College and her colleagues have found.

Most research on parenting and teen sexual behaviour simply look at whether or not a teen has had sex and not the degree of sexual risk taken, Dr Coley said.

However, given that two out of three American teens have sex before they turn 19, more specific data will provide a better understanding of the risks involved, Dr Coley and her team point out in the Journal Of Adolescent Health.

To investigate, as well as to better define whether parental qualities influence a child's sexual behaviour rather than vice versa, Dr Coley and her team analysed the survey results of 4,950 American teens, 1,058 of whom were siblings.

The adolescents - 12 to 16 years old when the study began ? did the survey every year for three years.

By comparing parenting quality and sexual behaviour for siblings raised in the same household, Dr Coley noted that it is possible to tease out potential cause-and-effect relationships.

The more times a week that an adolescent reports having dinner with his family, "doing something religious" as a family, or having fun with his family, the less likely he is to engage in risky sexual behaviour, the researchers find.

However, having a parent with "negative and psychologically controlling" behaviour increases the likelihood that a teen will have risky sex. This includes "criticising the ideas of the adolescents, controlling and directing what they think and how they feel", Dr Coley explained.

On the other hand, family activities are important supports for children, providing opportunities for emotional warmth, communication and transmission of values and beliefs.