LOS ANGELES - Gang-prevention officer Jeff Norat drives a bunch of sullen teens through the gang-riddled streets of a Los Angeles neighborhood, not because they're in trouble with the law, but so they'll stay out of trouble.

"These kids are all at risk of joining gangs - look where they live," said Norat, motoring through Boyle Heights, where some gangs are in their third generation. "But some kids don't."

What prompts some children to join gangs and others not to is a question that has long baffled experts. City officials, who have made little headway denting the ranks of street gangs, now say they might find the answer through a multiple-choice test.

"If you could identify who those at-risk kids were, then you could microtarget them with resources," said Jeff Carr, director of the mayor's office on gang reduction and youth development.

That premise marks a new strategy in the city's fight against gangs, which claim roughly 40,000 members in Los Angeles, making it the nation's gang capital.

The city spends about $20 million a year on gang prevention and intervention. Until now, much of that funding has gone to what the anti-gang czar calls a "shotgun approach" to prevention - flooding gang-infested neighborhoods with social programs.

But Carr points to research showing only about 15 percent of children in a given neighborhood join gangs, according to University of Southern California social psychologist Malcolm Klein and others. Klein found 10 factors that channel children into gangs, including poor parenting, justifying delinquent behavior, and traumatic events.

Researchers at USC's Center for Research into Crime used those findings to develop the 74-question survey called the Youth Services Eligibility Test. A child with at least five factors is deemed "at risk" and offered programs such as counseling, anger management, and tutoring.

Many struggling parents embrace the test as a means to get their children much-needed help.

After a gang-prevention program at the Hollenbeck Police Activities League, Norat takes the children home. One lives a few doors from a White Fence gang house. Two blocks further on, in Evergreen turf, he drops off two brothers whose gangbanger father had recently been killed.

The test found several of those children at risk of joining a gang, but not others. Some critics say that is one problem with the test. In such a high-poverty, gang-saturated milieu, it excludes a lot of less-troubled children who still need help.

No one knows if the test works. The city has contracted for a study in three years to see if children the test failed to identify as gang-joiners actually did join a gang.

"In some ways it's an experiment," Carr admitted. "But the gang problem has been endemic in L.A. for 25, 30 years. We have to innovate our way out of it."