Sept. 18 (HealthDay News) -- Girls who live in poor neighborhoods are more likely to have sexual intercourse in early adolescence and to have their first sexual experiences with older boys, a Canadian study says.

The study included 2,596 Canadian teens who were followed from ages 12 to 15.

"Young girls who live in disadvantaged neighborhoods are more likely to initiate sex at an early age, especially those young women with conduct problems," lead author Veronique Dupere said in a University of Montreal news release. "The results suggest that neighborhoods shape peer groups, which in turn influence when girls become sexually active."

The researchers also found that teen girls from poor neighborhoods who have a history of conduct problems are more likely to associate with deviant peers and to have their first sexual experiences with males who are three or more years older.

"Deviant peers are thought to provide a pool of willing partners and cultivate a sense that early sexual activity is desirable," Dupere said.

About 13 percent of the adolescents in the study were considered to have conduct problems, defined as engaging in three at-risk behaviors -- such as bullying, fighting, vandalism, stealing, running away, and staying out all night -- over one year.

Among boys, living in a poor neighborhood wasn't directly associated with the start of sexual activity in early adolescence, the study found.

"Contrary to girls for whom peers were of primary importance, family and individual risk appeared more influential in boys' timing their first sexual experience," Dupere said.

The study was published in the journal Child Development.

From a public health perspective, it's important to identify when and why young girls become sexually active, Dupere said.

"Other studies show that early initiators are more likely to contract sexually transmitted diseases, undergo unwanted teen pregnancy and to report involuntary sexual experiences," she said.

"By identifying young adolescents who are particularly at risk, this study provides valuable insights for future intervention efforts. To maximize effectiveness, prevention programs need to take the larger social context into account and make special efforts to enroll vulnerable young adolescents."

SOURCE: University of Montreal, news release, Sept. 16, 2008