Myeshia Price and Dr. Janet Hyde from the University of Wisconsin say adolescents who engage in sexual acts before the age of 15 are likely to do so without adequate protection, putting them at higher risk of sexually transmitted infections and, for girls, unwanted pregnancies.

Price and Hyde studied 273 adolescents ages 13 to 15 -- 146 girls and 127 boys -- who took part in the Wisconsin Study of Families and Work.

Boys who had engaged in early sexual activity were further into their puberty, spent more time watching television, had lower self-esteem, showed signs of attention-deficit hyperactivity disorder and oppositional defiant disorder and had poor relationships with their parents.

The study, published in the Journal of Youth and Adolescence, found one factor had a particularly large effect on early pregnancy -- the amount of time spent watching television.

However, a parent's positive influence goes a long way to reduce risky sexual behavior during adolescence, the researchers said.