In 1999 one of the scores of death-row appeals that land at the Supreme Court each year caught the eye of Justice Stephen Breyer. Thomas Knight, who had been sentenced to death by the State of Florida in 1975 for killing a married couple, argued in the appeal that he had been living in anguished anticipation of execution for so long that his sentence had become a form of cruel and unusual punishment. Knight's case was joined with that of Carey Moore, who'd been on Nebraska's death row for nineteen years for killing two cabdrivers.

No court in the United States had ever lifted a death sentence for this reason. But Breyer had a hunch that courts in other countries might have done so, and he asked his clerks to investigate. He had issued a similar directive earlier that term, when he wanted to know more about other countries' approaches to campaign-finance law. "I remember his exact words," says one of Breyer's clerks from that year. "He said, 'We're not the only court in the world. See what they have to say.'" Breyer has come to refer to proponents of this approach—namely, judges who use international legal precedents for context as they interpret the U.S. Constitution—as "comparativists."

In their death-penalty research Breyer's clerks found several cases in which courts from around the world, including the European Court of Human Rights and the Supreme Court of India, had expressed serious reservations about whether executions conducted after lengthy delays were humane. When the Court declined to review Knight and Moore's case, Breyer took the unusual step of commenting on it anyway. In a dissent to the Court's denial of review he wrote, "A growing number of courts outside the United States—courts that accept or assume the lawfulness of the death penalty—have held that lengthy delay in administering a lawful death penalty renders ultimate execution inhuman, degrading, or unusually cruel."

None of Breyer's colleagues signed on to the dissent. And in a preview of the resounding rejection of comparativism soon to come from other hard-line conservatives, Clarence Thomas wrote an opinion ridiculing Breyer's approach. "Were there any support in our own jurisprudence," he argued, "it would be unnecessary for proponents of the claim to rely on the European Court of Human Rights, the Supreme Court of Zimbabwe, the Supreme Court of India, or the Privy Council."

Twenty years ago the exchange of law among nations consisted almost entirely of a "vigorous overseas trade in the Bill of Rights," as one British barrister has put it. Canada, for example, followed the United States' lead on freedom of religion; India did so in balancing the values of freedom of expression and privacy in libel law. But times have changed, and many countries' highest courts have now come into their own. The courts of South Africa and India, for example, have upheld rights to education, housing, and health or medical care that American courts have not recognized. In Europe, too, courts are becoming bolder, despite a tradition of parliamentary supremacy. Breyer feels strongly that the United States can learn from these developments abroad. "Really, it isn't true that England is the moon, nor is India," he said in a debate with Justice Antonin Scalia last January at American University, in Washington, D.C. Judges in those countries "have problems … similar to our own."

Breyer is no longer a lonely crusader: comparativism is startlingly in the ascendant in the U.S. Supreme Court, among both the relatively liberal justices and the centrist conservatives. Foreign precedents were at the heart of the Court's recent decisions to strike down sodomy laws (Lawrence v. Texas, 2003) and to ban the execution of juveniles (Roper v. Simmons, 2005). Both opinions were written by Justice Anthony Kennedy, who often votes with the Court's conservative bloc. They did not sit well with observers on the right. Some conservatives have called for Kennedy's impeachment—and have introduced bills in the House and Senate that would ban American courts from employing any foreign judgment other than centuries-old English common law when interpreting the Constitution.

Kennedy and the other justices who have sympathized with comparativism (Breyer, Ruth Bader Ginsburg, David Souter, and John Paul Stevens, along with Sandra Day O'Connor, who announced her resignation last summer) have trod lightly in view of the conservative uproar. Breyer argues that comparativism has a modest aim—to "cast an empirical light" that will reveal new solutions to shared legal problems. He and his allies have pointed out that looking abroad for precedents is not new for the Court: in cases from the 1950s through the 1980s (and also during the 1890s) majorities took account of foreign practices in deciding whether a punishment was permissible. This attracted little notice until Scalia decreed, in the 1989 case Stanford v. Kentucky (which allowed juvenile execution), that in weighing whether a punishment was cruel and unusual the Court would consider only "American conceptions of decency" (his italics).

In overturning Stanford this year the Roper majority stressed that American courts are not bound to follow rulings from abroad. But of course foreign judgments don't need to be formally binding to be persuasive. Kennedy observed in his opinion that the United States was the only country in the world that still executed minors.

Harold Hongju Koh, the dean of the Yale Law School, is one of the leading academic proponents of comparativism. Koh is a veteran litigator who led a partly successful and attention-getting battle in the mid-1990s to force the federal courts to grant a broad array of rights to Haitian émigrés, at Guantánamo Bay and on their way to American shores. He also served as President Bill Clinton's assistant secretary of state for human rights from 1998 to 2001. Diplomatic work runs in Koh's family: during the 1950s his father was South Korea's minister to the United States. (The family chose to stay in this country after a 1961 coup.)

Harold Koh loved his work as a Clinton-era diplomat but came to hate some of the American practices he was asked to defend—especially capital punishment. He felt that the death penalty alienated U.S. allies in Europe and Latin America and gave countries in the Middle East and Asia an excuse to ignore American entreaties to improve their human-rights records. Since his term in the State Department ended, Koh has attacked the death penalty as antithetical to American foreign-policy interests, most recently in an amicus curiae brief in Roper. If the United States isn't keeping pace with the rest of the civilized world, Koh argues, then for its own good it needs to change.

One of the leading opponents of comparativism is John Yoo—a former student of Koh's who is now a law professor at the University of California at Berkeley. Like Koh, Yoo is of Korean descent; he immigrated to the United States as a child. He took three of Koh's classes at Yale Law School from 1989 to 1992, when Koh was a young professor; he also worked for Koh as a teaching assistant and co-authored a paper about international economic power with him. "I wouldn't be where I am without Harold, without his guidance and support," Yoo told me. Yet Yoo never shared Koh's embrace of international law. His early major academic articles attacked Koh's theories, and he has continued to write skeptically about the delegation of authority to international institutions. In 2001 Yoo went to work for George W. Bush, becoming, along with Koh, one of only three Korean-Americans ever to win a high-level appointment in a U.S. administration. As a deputy assistant attorney general in John Ashcroft's Justice Department, Yoo wrote the memorandum that stripped foreign detainees at Guantánamo Bay and in Afghanistan of the protections of the Geneva Convention, and he co-authored the notorious "torture memo" that justified the administration's authorization of extreme interrogation tactics.

Yoo says he was simply giving the government, as his client, "a good sense of the lines that the law draws." But Koh found Yoo's actions unforgivable. "If a client asks a lawyer how to break the law and escape liability," he said before the Senate Judiciary Committee during the confirmation hearings for Attorney General Alberto Gonzales, "the lawyer's ethical duty is to say no." Backed by more than 200 law professors and lawyers who expressed similar sentiments in a public statement, Koh called the torture memo "perhaps the most clearly erroneous legal opinion I have ever read."

Yoo says he and Koh have not spoken for some time. In print, however, they continue to clash over the questions raised by Roper and Lawrence. Because foreign judges "are not responsible to the American political system," Yoo argues in an essay that will be published in the Hawaii Law Review, relying on their decisions is at odds with the Constitution. Viewed through this lens, invoking what Kennedy referred to in Roper as "the overwhelming weight of international opinion" is an end run around American democracy. Richard Posner, a federal-appeals-court judge and a law professor at the University of Chicago, agrees. "Such nose-counting is like subjecting legislation enacted by Congress to review by the United Nations General Assembly," he wrote last year in the magazine Legal Affairs.

To Koh, on the other hand, it is the job of the courts to bring Congress and the states to heel. "It's very basic in the Bill of Rights," he says. "We hold these truths to be self-evident, whether the majority agrees with them or not. Rights are in some sense anti-democratic; to trumpet democracy as the all-important value is to denigrate them."

The strongest argument against comparativism is a matter of practice, not principle. Those who draw on foreign laws and precedents tend to use them selectively, when it suits their purposes. This makes their opponents cry foul. "We are one of only six countries in the world that allows abortion on demand at any time prior to viability," Scalia said to Breyer in their American University debate. "Should we change that because other countries feel differently?"

If comparativism continues to gather momentum, as seems likely, Scalia's question may be taken up in earnest. "The first opposition strategy is to argue that these materials are illegitimate," says Mark Tushnet, a professor of law at Georgetown University. "But if you lose that battle, then you start citing them yourself." A victory for comparativism could have conservative judges citing international precedents that support their own arguments—laws, say, that restrict abortion in Germany and Ireland, or limit free speech in Canada. That would be the foreign-law version of "If you can't beat 'em, join 'em."