The teenage brain is like a car with a good accelerator but a weak brake.

NEW YORK - The teenage brain, Laurence Steinberg says, is like a car with a good accelerator but a weak brake. With powerful impulses under poor control, the likely result is a crash.

And, perhaps, a crime.

Steinberg, a Temple University psychology professor, helped draft an American Psychological Association brief for a 2005 case in which the U.S. Supreme Court outlawed the death penalty for crimes committed before age 18.

That ruling relies on the most recent research on the adolescent brain, which indicates the juvenile brain is still maturing in the teen years and reasoning and judgment are developing well into the early to mid 20s. It is often cited as state lawmakers consider scaling back punitive juvenile justice laws passed during the 1990s.

"As any parent knows," wrote Justice Anthony Kennedy for the 5-4 majority, youths are more likely to show "a lack of maturity and an underdeveloped sense of responsibility" than adults. "These qualities often result in impetuous and ill-considered actions and decisions."

He also noted that "juveniles are more vulnerable or susceptible to negative influences and outside pressures, including peer pressure," causing them to have less control over their environment.

Some child advocates have pointed to the Supreme Court decision and the research as evidence that teens -- even those accused of serious crimes -- should not be regarded in the same way as adults in the criminal justice system.

Dr. David Fassler, a psychiatry professor at the University of Vermont College of Medicine who has testified before legislative committees on brain development, says the research doesn't absolve teens but offers some explanation for their behavior.

"It doesn't mean adolescents can't make a rational decision or appreciate the difference between right and wrong," he said. "It does mean, particularly when confronted with stressful or emotional decisions, they are more likely to act impulsively, on instinct, without fully understanding or analyzing the consequences of their actions."

Experts say that even at ages 16 and 17, when compared to adults, juveniles on average are more:

* impulsive.

* aggressive.

* emotionally volatile.

* likely to take risks.

* reactive to stress.

* vulnerable to peer pressure.

* prone to focus on and overestimate short-term payoffs and underplay longer-term consequences of what they do.

* likely to overlook alternative courses of action.

Violence toward others also tends to peak in adolescent years, says psychiatrist Dr. Peter Ash of Emory University. It's mostly likely to start around age 16, and people who haven't committed a violent crime by age 19 only rarely start doing it later, he said.

The good news here, he said, is that a violent adolescent doesn't necessarily become a violent adult. Some two-thirds to three-quarters of violent youth grow out of it, he said. "They get more self-controlled."

MALCOLM RITTER