A small U.S. study appears to confirm adult fears that teens who spend more than the average amount of unsupervised time "hanging out" with peers have higher odds of smoking cigarettes and marijuana and drinking alcohol.

Researchers also found that teens more heavily involved in sports were less likely to smoke tobacco or pot, but more likely to drink, whereas kids who worked part-time jobs were more likely to smoke and drink but less likely to use pot.

Unfortunately, greater than average involvement in structured school and after-school activities did not seem to offer a protective effect.

The study team expected structured activities to have the strongest negative relationship to kids' use of forbidden substances, and therefore the greatest predictive power, said lead author Kenneth Lee, a doctoral student in education at the University of California, Irvine. "But we're seeing it's unsupervised time with peers that's being the most predictive of substance abuse," Lee told Reuters Health by email.

Lee and co-author Deborah Lowe Vandell, an education researcher at U.C. Irvine, point out in the Journal of Adolescent Health that other studies have shown unsupervised time with peers and lack of structure can increase the risk of delinquency and illegal acts.

Most research of this kind, however, tends to look at single contexts, like sports participation, and also often focuses on ways to stop kids from using illicit substances, rather than preventing them from ever starting, Lee said.

"We thought it would be interesting to identify when, where and with whom adolescents are partaking of substance use. We thought the best way to do this would be to look at various contexts - especially in the out-of-school time environment," Lee said.

The study used data from a National Institute of Child Health and Human Development study on 766 kids from 10 cities who were enrolled as infants in 1991. Researchers focused on the teens' activities at age 15 and then again at the end of high school.

They found that teens who spent the most unsupervised time with peers relative to the average amount for the entire group were 39 percent more likely to smoke cigarettes, 47 percent more likely to drink alcohol and 71 percent more likely to smoke marijuana than average.

Teens who spent the most time in sports were 19 percent more likely than average to drink alcohol but less likely to use marijuana. And those with the most paid employment were 46 percent more likely than average to use tobacco and 28 percent more likely to drink.

For all three substances, having used them already by age 15 raised the odds three- to four-fold of use during the study.

Organized time, such as arts classes at school, religious activities outside school or community volunteer work had a very modest protective effect. Kids with the most time in these activities showed a 7 percent to 18 percent lower than average risk of drinking or smoking.

Lee noted that the study cannot prove cause and effect and hoped the results could be repeated and tested across different populations.

"First and foremost, I think it was an interesting way to examine a topic that we've looked at for decades in a slightly new way," said Jenn Matheson, a marriage and family therapist in private practice in Longmont, Colorado, who was not involved in the study.

But Matheson, also an adjunct professor at the University of Colorado, Denver, said parental influence on teen substance abuse would also be important to consider.

"Seeing parents or other adults in home smoking, using marijuana or drinking is often a major predictor of whether the kids themselves will," said Matheson, adding that parents need to explain the risks of substance use to their children.

"Parents need to set rules around alcohol, marijuana and tobacco use among teenagers and there have to be consequences," she said. "Kids know a rule without consequences is nothing."