NBC Miami: When it comes to preventing risky teen sex, there may be no better deterrent than a doting dad.

Teenagers whose fathers are more involved in their lives are less likely to engage in risky sexual activities such as unprotected intercourse, according to a new study.

The more attentive the dad -- and the more he knows about his teenage child's friends -- the bigger the impact on the teen's sexual behavior, the researchers found. While an involved mother can also help stave off a teen's sexual activity, dads have twice the influence.

"Maybe there's something different about the way fathers and adolescents interact," said the study's lead author Rebekah Levine Coley, an associate professor at Boston College. "It could be because it's less expected for fathers to be so involved, so it packs more punch when they are."

Understanding a father's influence in teen sexual behavior is important, experts say. One in four American adolescents under the age of 15 has had sexual intercourse and, by age 18, two-thirds have had sex, according to research. The concern is, many sexually active young people aren't using protection, a contributing factor in rising teen birth rates. Approximately 750,000 teenagers become pregnant each year and about 3 in 10 teenage girls become pregnant at least once before age 20, according to government statistics.

For the new study, which was published in the journal Child Development, Coley and her colleagues surveyed 3,206 teens, ages 13 to 18, once a year for four years. The teens, who all came from two-parent homes, were asked about their sexual behaviors and about their relationships with their parents.

Researchers posed a series of questions about both mothers and fathers, such as "how much does s/he know about whom you are with when you are not at home?" The teens were also asked how often they interacted with their parents in activities such as eating dinner, playing games or attending religious activities.

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