Everyone knows teenagers are impulsive, take bad risks and do stupid things.

Now scientists understand why, and the reasons may have far-reaching implications for how teenage murderers are punished.

In the past five years, new brain-imaging research has shown that the brains of teenagers are different from those of adults. Teens' brains are still growing and changing. In fact, a key part of the brain that affects judgment may not be in place until men and women reach their early 20s.This revelation played a pivotal role in persuading the U.S. Supreme Court in March to bar the death penalty for anyone younger than 18.

"It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime," the court said.

"As any parent knows and as the scientific and sociological studies . . . tend to confirm, a lack of maturity and underdeveloped sense of responsibility are found in youth more often than in adults and are more understandable among the young. These qualities often result in impetuous and ill-considered actions and decisions."

Now, advocates of juvenile justice reform hope the studies will trigger changes in state laws such as Colorado's that sentence teenagers to life without parole when they are charged as adults and convicted of first-degree murder.

In Colorado, a convicted murderer must be 18 to receive the death penalty.

The research suggests an anatomical basis for teens' risky behavior and poor judgment.

Scientists previously believed the brain was 90 percent of its adult size by the age of 6 and finished developing by 12.

But they were wrong.

Teams of researchers at the National Institute of Mental Health and the University of California have learned through magnetic resonance imaging that the brain continues to mature during the teen years - beginning a final push around 16 or 17.

"We found that the frontal lobes were the last to develop," said UCLA brain researcher Paul Thompson. "These brain regions control inhibition, rash actions, rage and anger." They also control decision-making, risk perception and impulse control.

"While a 14- or 17-year-old knows the difference between right and wrong, they don't have the same abilities to control their behaviors and assess risks the way adults do," said Steven Drizin, a Northwestern University law professor who helped author one of the briefs the Supreme Court considered in its ruling on juvenile death penalty.

Relying on gut reactions

Just before puberty, a growth spurt takes place in children's brains, the researchers found. Then new connections between neurons are formed to make nerve transmissions faster, more efficient and accurate.

Until their brains mature, teens rely heavily on a more primitive part of the brain, the amygdala, which is responsible for "gut" reactions including "fight or flight" responses and aggressive behaviors, said David Fassler, a Vermont psychiatrist who has testified frequently in support of efforts to overturn laws that allow juveniles to be executed.

At the same time, teens are besieged by raging hormones. In boys, levels of testosterone, which is linked to aggressive behavior, skyrocket.

"What we're finding is that adolescents actually use a different part of their brain than adults do, in particular when responding to things with emotional content," Fassler said. "They are much more likely to use the more primitive parts of their brain."

This accounts for much of the risky behavior associated with the teenage years, he said.

"Almost all of us do things that are impulsive, irresponsible and out of character (as teenagers). That's really tied to how (young people's) brains work," Fassler said. "Fortunately, most people don't commit heinous crimes. But almost everyone can look back on things they did as adolescents and say to themselves, 'What was I thinking?' "

A study by Harvard neuropsychologist Deborah Yurgelun-Todd showed that adolescents frequently misinterpret the emotions of others. When asked to assess emotions depicted in photographs, they often misidentified expressions of fear as anger or hostility.So what implications does this research have for sentencing juveniles who commit murder?

Organizations such as the American Bar Association, American Medical Association, American Psychiatric Association and American Psychological Association joined the battle to overturn the death penalty for juveniles.

None has yet taken a formal position on life sentences for juveniles.

The brain researchers themselves are reluctant to join activists who are clamoring for change."Yes, the brain of a 16-year-old is different than the brain of a 25-year-old," Jay Giedd, a brain imaging researcher at the National Institute of Mental Health, said in an interview on PBS NewsHour last October. "But what should that mean for the judicial system or other systems? I think that it's just too great of a leap at this point."

UCLA researcher Thompson said many of his colleagues signed petitions to overturn the death penalty for juveniles, but he was not among them.

"I think it's too early for brain development research to be used in legal cases," he said. "Some experts say it is highly relevant to assessing behavioral immaturity. But I don't think it should be used as a potential excuse for criminal behavior. The science just isn't there."

Trying to assess the blame

Nevertheless, juvenile justice experts who oppose life sentences for teens contend that the brain research bolsters earlier behavioral studies on teenagers.

They say the legal concept of proportionality supports prohibiting life without parole for teens. Proportionality holds that fair punishment should reflect the harm caused as well as the defendant's blameworthiness.

For a host of reasons, the blameworthiness of teenagers is less than adults who commit the same crimes, said Laurence Steinberg, a Temple University professor of psychology whose studies were cited in the Supreme Court's death penalty ruling.

"Somebody who can't foresee the consequences of what he is doing, can't resist pressure from others to do something he knows is wrong and can't control his impulses, is not as responsible as someone who can do all those things," he said. "The brain research gave the psychosocial research more credence. Arguing that someone doesn't think ahead or control their impulses is less persuasive reasoning than arguing that someone can't control their impulses."Still, while all youths undergo the same stages of brain development, only a tiny fraction commit the gruesome crimes that land them in prison for life.

"Immature brain development is not an excuse for juvenile criminal activity," Drizin acknowledged. "(But) it is an explanation which mitigates their culpability. It explains why they should be held accountable and in some cases punished severely, but never as severely as an adult who commits the same crime."

But prosecutors disagree.

"The taking of a human life is the ultimate crime. Irreversible. It does not matter to the victim of a murder whether the person holding a gun is 16 or 60," said Bob Grant, executive director of the Colorado District Attorney's Council.

Attorneys can use brain development arguments in a teen's defense at trial, Grant said, but he opposes any change to Colorado's law.

Teens need hope, expert says

Those who support laws that allow for the possibility of parole argue that teens can be rehabilitated because their personality and character are not yet formed.

"Life without parole for kids presumes that they can't be changed," Steinberg said.

Psychiatrists and psychologists do not diagnose antisocial personality disorder in anyone younger than 18 precisely because teen personalities are works in progress, Fassler said. "Traits in childhood will often not persist in adulthood," he said.

Drizin argues teens need hope to live up to their potential."The overwhelming majority of juveniles are involved in impulsive or risky, even delinquent, behaviors during the teenage years. The majority . . . go on to become productive citizens who don't commit crimes," he said.

"When you impose life without parole on someone, you are basically crushing any hope for them to develop into that person," he said. "It doesn't matter what you do in prison to improve yourself, overcome your problems. You are what you did. Not only is that a tremendous waste of state resources, it's a naive and incomplete view of who adolescents are."

The issue

One in eight lifers in Colorado - 46 out of 360 - were sentenced for murders committed when they were younger than 18.

In the wake of a U.S. Supreme Court ruling that ends the death penalty for juveniles, a national debate is emerging over laws such as Colorado's that require life without parole for teens convicted as adults of first-degree murder.

• Those who want to change Colorado's law say:

New research shows juveniles' brains are not fully developed so their faulty judgment makes them less responsible for crimes than adults. Juveniles could be rehabilitated and should get a chance at parole.

• Those who oppose changes to the law say:

Inmates serving life without parole took others' lives and have forfeited their right to freedom.

The young killers, if released, would pose too great a risk to society.

lindsays@RockyMountainNews.com or 303-892-5181

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