It's one thing to say teens don't control their impulses as well as adults, but another to show that they can't, he said. As for peer pressure, the new brain research "gives credence to the idea that this isn't a choice that kids are making to give in to their friends, that biologically, they're more vulnerable to that," he said.
Consider the lobes at the front of the brain. The nerve circuitry here ties together inputs from other parts of the brain, said Dr. Jay Giedd of the National Institute of Mental Health.
This circuitry weighs how much priority to give incoming messages like "Do this now" versus "Wait! What about the consequences?" In short, the frontal lobes are key for making good decisions and controlling impulses.
Brain scans show that the frontal lobes don't mature until age 25, and their connections to other parts of the brain continue to improve to at least that age, Giedd said.
The inexplicable behavior and poor judgments teens are known for almost always happen when teens are feeling high emotion or intense peer pressure, conditions that overwhelm the still-maturing circuitry in the front part of brain, Giedd said.
As Steinberg sees it, a teenager's brain has a well-developed accelerator but only a partly developed brake.
By around 15 or 16, the parts of the brain that arouse a teen emotionally and make him pay attention to peer pressure and the rewards of action — the gas pedal — are probably all set. But the parts related to controlling impulses, long-term thinking, resistance to peer pressure and planning — the brake, mostly in the frontal lobes — are still developing.
"It's not like we go from becoming all accelerator to all brake," Steinberg said. "It's that we go from being heavy-foot-on-the-accelerator to being better able to manage the whole car."
Giedd emphasized that scientists can't yet scan an individual's brain and draw conclusions about how mature he is, or his degree of responsibility for his actions.
Brain scans do show group differences between adult and teen brains, he said, "but whether or not that should matter (in the courtroom) is the part that needs to be decided more by the judicial system than the neuroscientist."
Steinberg, who frequently testifies on juvenile justice policy and consults with state legislators on the topic, said it's not clear to him how much the research on teen brains affects lawmakers. They seem more swayed by pragmatic issues like the cost of treating teens as adults, he said. But he noted that he has been asked to testify more in the past few years than before.
In any case, experts say, there's nothing particularly magic about the age 18 as a standard dividing line between juveniles and adults in the courtroom.
Different mental capabilities mature at different rates, Steinberg notes. Teens as young as 15 or 16 can generally balance short-term rewards and possible costs as well as adults, but their ability to consider what might happen later on is still developing, he said.
A dividing line of age 18 is better than 15 and not necessarily superior to 19 or 17, but it appears good enough to be justified scientifically, he said.