May 18 (HealthDay News) -- American teens' attitudes toward sex and other issues are strongly influenced by their peers, but they also tend to pick friends that think like they do, a new study finds.

Researchers at the University of Illinois, Chicago, compared sexual attitudes and behaviors among 1,350 male and female students, aged 15 to 18, from across the United States.

The study found that teens whose friends had intercourse without a condom were more likely to have intercourse without a condom over the following year. Teens with friends who believe that sex can have undesirable consequences were less likely to have intercourse without a condom over the following year.

The effect of friends' attitudes on sexual behavior was stronger in girls than in boys.

The study also found that teens tend to select new friends with sexual attitudes that are similar to their own.

"This study has two implications for prevention. First, it supports the use of adolescent leaders for preventive interventions. Second, it suggests that interventions that use attitude change to change behavior may be more effective among females than among males," lead author David B. Henry, an associate professor of psychiatry, said in a prepared statement.

The findings are published in the May/June issue of Child Development.