CHICAGO — A surprising number of teenagers — nearly 15 percent — think they're going to die young, leading many to drug use, suicide attempts and other unsafe behavior, new research suggests.

The study, based on a survey of more than 20,000 kids, challenges conventional wisdom that says teens engage in risky behavior because they think they're invulnerable to harm. Instead, a sizable number of teens may take chances "because they feel hopeless and figure that not much is at stake," said study author Dr. Iris Borowsky, a researcher at the University of Minnesota.

That behavior threatens to turn their fatalism into a self-fulfilling prophecy. Over seven years, kids who thought they would die early were seven times more likely than optimistic kids to be subsequently diagnosed with AIDS. They also were more likely to attempt suicide and get in fights resulting in serious injuries.

Borowsky said the magnitude of kids with a negative outlook was eye-opening.

Adolescence is "a time of great opportunity and for such a large minority of youth to feel like they don't have a long life ahead of them was surprising," she said.

The study suggests a new way doctors could detect kids likely to engage in unsafe behavior and potentially help prevent it, said Dr. Jonathan Klein, a University of Rochester adolescent-health expert who was not involved in the research.

"Asking about this sense of fatalism is probably a pretty important component of one of the ways we can figure out who those kids at greater risk are," he said.

The study appears in the July issue of Pediatrics, released Monday.

Scientists once widely believed that teenagers take risks because they underestimate bad consequences and figure "it can't happen to me," the study authors say. The new research bolsters evidence refuting that thinking.

Cornell University professor Valerie Reyna said the new study presents "an even stronger case against the invulnerability idea."

"It's extremely important to talk about how perception of risk influences risk-taking behavior," said Reyna, who has done similar research.

Fatalistic kids weren't more likely than others to die during the seven-year study; there were relatively few deaths, 94 out of more than 20,000 teens.

The researchers analyzed data from a nationally representative survey of kids in grades 7 to 12 who were interviewed three times between 1995 and 2002. Of 20,594 teens interviewed in the first round, 14.7 percent said they thought they had a good chance of dying before age 35. Subsequent interviews found these fatalistic kids engaged in more risky behavior than more optimistic kids.

The study suggests some kids overestimate their risks for harm; however, it also provides evidence that some kids may have good reason for being fatalistic.

American Indians, blacks and low-income teens — kids who are disproportionately exposed to violence and hardship — were much more likely than whites to believe they'd die young.