The case of Lee Boyd (John) Malvo, arrested in the region's sniper slayings, ought to raise a question for all of us about whether the execution of minors should be permitted in this country. News reports on the relationship between the 17-year-old and his father figure, John Allen Muhammad, portray the older man as a hot-tempered, authoritarian bully who controlled every detail of Malvo's existence. Even if Malvo did fire the gun in one or more of the killings, the odds are great that he did so under the influence of an older and more powerful person.

Whether a 17-year-old has the psychological wherewithal to resist the pressure of an older accomplice to commit a crime is one of many questions my colleagues and I are studying under the auspices of the MacArthur Foundation Research Network on Adolescent Development and Juvenile Justice, a national initiative that is examining juvenile justice policy in light of what we know about psychological development during adolescence. Most reasonable people agree that there is some age below which people are not mature enough to be held to adult standards of criminal responsibility, but there is no such agreement about where this line should be drawn. Suppose Malvo were 15, or 12, or 10, rather than 17? Would we feel differently about his facing the death penalty at any of these ages? If so, where, and on what basis, should we draw the line between those whose lack of judgment mitigates their culpability and those whom we expect to be fully responsible for their behavior?

The Supreme Court, in Atkins v. Virginia, held that mentally retarded people are inherently less culpable than those of normal intelligence. Justice John Paul Stevens, writing for the majority, noted that "mentally retarded persons frequently know the difference between right and wrong . . . but, by definition, they have diminished capacities to understand and process information, to communicate, to abstract from mistakes and learn from experience, to engage in logical reasoning, to control impulses, and to understand others' reactions. Their deficiencies do not warrant an exemption from criminal sanctions, but diminish their personal culpability."

Like many mentally retarded people, adolescents generally know the difference between right and wrong, but they often suffer from diminished capacities to reason logically, control their impulses, think through the future consequences of their actions and resist the coercive influences of others. For this reason, adolescents who commit crimes should not be exempt from criminal sanctions, but the sanctions they face should not be as severe as those faced by adults found guilty of the same offenses.

The Supreme Court has held that the death penalty is unconstitutional for people who are under 16 at the time of their offenses. Currently, only 22 states permit the execution of juveniles -- and in practice, only seven have executed juveniles since the death penalty was reinstated in 1976. Half the states that permit capital punishment allow for the execution of juveniles, and 40 percent of states that permit capital punishment allow the execution of juveniles as young as 16. To those who say that 16 is old enough to know better, it is worth pointing out that in virtually every other area of the law, we treat 16-year-olds as if they are inherently less mature than adults.

Malvo is not the best poster boy for repeal of the juvenile death penalty. But ongoing research on the links between brain maturation and psychological development in adolescence is beginning to explain why adolescents are not as capable of planning, or as thoughtful and self-controlled, as adults and, more important, why these deficiencies may be inherent to being an adolescent. It is true that the science is a long way from providing a definitive answer to the question of where we should set the age boundary for adult criminal responsibility. But executing juveniles puts the United States in rare company: The only other country that is still committed to the execution of juveniles is Iran. Until the data are in, we should join the rest of the world and prohibit the execution of people under the age of 18.

The writer is Distinguished University Professor of Psychology at Temple University. He will discuss this piece in a Live Online discussion at 1 p.m. tomorrow at www.washingtonpost.com.