The idea of putting a person to death for a murder committed at age 17 or younger strikes many of us as grotesque. So it may seem fitting that five Supreme Court justices held on March 1 that juvenile executions violate "the evolving standards of decency that mark the progress of a maturing society" — the touchstone since 1958 for determining whether punishments are unconstitutionally "cruel and unusual."

Justice Anthony Kennedy’s opinion for the majority gives six cogent-sounding reasons for this judgment:

1) The trend in state legislatures has been toward ending juvenile executions.

2) Only six states have executed someone convicted as a juvenile since 1989.

3) Juveniles are less calculating than adults and thus less likely to be deterred by fear of death.

4) Juveniles’ crimes tend to be less "morally reprehensible" and less indicative of "irretrievably depraved character" because they are less mature and have a less developed sense of moral responsibility.

5) Emphasizing the Court’s assertion three years ago that "in the end, our own judgment will be brought to bear on the question of the acceptability of the death penalty under the Eighth Amendment," Kennedy declares his personal view, joined by four others, that juvenile executions should be banned.

6) "The United States now stands alone in a world that has turned its face against the juvenile death penalty," with all other nations having officially ended the practice.

All of this is good enough to convince me that we should end juvenile executions. So why did four justices dissent in the case, Roper v. Simmons? Because they were right to oppose ending juvenile executions by judicial fiat. The dissenters shred each of the majority’s six arguments:

1) A trend that 20 of the 38 death-penalty states have declined to join is far from being the national "consensus" — the traditional measure of "evolving standards of decency" — that Kennedy claims it is. Indeed, just 16 years ago, the Court upheld the death penalty for 16- and 17-year-old murderers, in Stanford v. Kentucky. While four more state legislatures have ended juvenile executions since then, for a total of 18, that’s not even a majority of the 38, let alone a "consensus."

Kennedy pads his bogus "consensus" by adding to these 18 states the 12 others that have entirely abolished the death penalty. But none of the 12 suggested that juvenile killers should be ineligible for the maximum penalty faced by adult killers.

2) The number of juvenile executions has held steady or even gone up since Stanford. And their infrequency reflects only the facts that most murderers are adults and that capital juries are instructed to consider youth as a mitigating factor.

3) The defendant in this very case, Christopher Simmons, showed how calculating a juvenile killer can be. He told friends he wanted to murder someone; planned to break into a house, tie up his victim, and throw her off a bridge; and he assured accomplices that they "could get away with it" because they were juveniles — a prediction now partially validated by the Court. On entering his victim’s bedroom and recognizing her, Simmons bound and gagged her with duct tape, took her to a railroad bridge, tied her hands and feet with electrical wire, and threw her into the Meramec River.

4) As further evidence of his moral depravity, Simmons bragged to friends that he had killed "because the bitch seen my face." The defense lawyer, stressing that, under state law, Simmons was too young to drink, serve on a jury, or see certain movies, argued that he did not deserve death. The jury disagreed.

Some of the mental-health experts who successfully urged the Court to find that juveniles lack the moral-reasoning ability to be held responsible for murder have made inconsistent arguments in past cases. When the issue was whether minors should have access to an abortion without parental involvement, the American Psychological Association asserted that girls as young as 14 "develop abilities similar to adults in reasoning about moral dilemmas."

5) It is presumptuous and anti-democratic for five life-tenured lawyers to appoint themselves the nation’s moral conscience and to look inward — rather than to elected representatives, voters, juries, or the Constitution itself — to discern the nation’s "evolving standards of decency."

Especially when the justices’ own moral consciences are so malleable. The same Kennedy who authored Simmons had joined Scalia’s 1989 ruling that nothing in the Constitution forbids juvenile executions. As Scalia stresses, Kennedy’s explanation for this reversal is "not, mind you, that this Court’s decision 15 years ago was wrong, but that the Constitution has changed." This brand of "interpretation" mocks Alexander Hamilton’s injunction in Federalist 78 that the judiciary — the "least dangerous" branch — should be "bound down by strict rules and precedents, which serve to define and point out their duty in every particular case that comes before them."

6) One key to understanding the Court’s reliance on international and foreign law, laid out in friend-of-the-court briefs by the European Union and several foreign countries, may be the justices’ summer sojourns to glittering international conferences. They may be embarrassed by their foreign friends’ concern that America seems so indifferent to world opinion — so barbaric, even.

I might be embarrassed too. But should the meaning of our Constitution be determined — and should democratic governance be set aside — by what Scalia calls "the subjective views of five members of this Court and like-minded foreigners"?

And if international standards are to be our guide, what of the facts that — by decree of the Supreme Court — the United States alone broadly bars prosecutors from using illegally seized evidence; is one of only six countries to allow abortion on demand until the fetus is viable; and is quite exceptional in requiring strict separation of church and state?

What of the fact that the United Nations Convention on the Rights of the Child, which the Court cites approvingly for its ban on executing juveniles, also bans sentencing them to life without parole — a penalty that all, or almost all, 50 states authorize and that Kennedy cited with approval. By the way, the United States has refused to ratify that convention, except to the extent that the Court has now implicitly arrogated the treaty-ratification power to itself.

The subjectivity of the justices’ "independent judgment" is also underscored by Kennedy’s side debate with Justice Sandra Day O’Connor’s separate dissent. She agrees with the majority’s interpretative method, its internationalist bent, and its 2002 precedent in Atkins v. Virginia (which she joined) banning execution of mentally retarded murderers.

But 16- and 17-year-old murderers should not enjoy the same constitutional protection, O’Connor asserts in Simmons, while suggesting that she would welcome a statutory ban on juvenile executions. Why? Because there is "continuing public support" for juvenile executions and because "at least some 17-year-olds" may deserve death. Kennedy, on the other hand, stresses that the impropriety of the juvenile death penalty "gained wide recognition earlier than the impropriety of executing the mentally retarded."

Scalia’s own "purely originalist approach" (as he describes it) has its problems, however. Scalia would uphold any punishment deemed constitutional at the time of the Framers, leaving it to elected officials to discern "evolving standards of decency."

That would make the Eighth Amendment a dead letter. When it was adopted, children as young as 7 could be executed, among other punishments now universally deemed barbaric.

So the Court must draw a line somewhere to designate how young is too young for the death penalty. In 1988, it drew a more defensible line, over Scalia’s dissent, in Thompson v. Oklahoma, holding that killers 15 years old and younger should not be executedBut in its impatience with 20 states’ current unwillingness to draw their own legal lines where it (or I) would like, the current Court majority has assumed the power to act essentially as a continuing constitutional convention. And not only in the death-penalty context.

"It seems inevitable," editorialized The New York Times, "that, one day, Americans will look back on this latest narrowing of the categories of people eligible for execution as another intermediate step toward the Court’s entire rejection of the death penalty."

Oh, good. I don’t like the death penalty, either. And if the voters in the 38 death-penalty states remain too benighted to do the right thing themselves, what standing do they have to second-guess the "evolving standards of decency" decreed by five moral guardians and the world’s greatest newspaper?