WASHINGTON, D.C. — The Supreme Court declared Monday that the Constitution requires that juries come to an unanimous verdict to convict Americans of serious crimes, overturning a state law in Louisiana that critics say was used to find black defendants guilty even when one or two black jurors disagreed.

The court’s opinion by Justice Neil M. Gorsuch provides a striking example of how “originalism” —a doctrine favored by conservatives — can sometimes yield rulings that produce liberal results.

Gorsuch said that when Sixth Amendment and the right to a jury trial was added to the Constitution in 1791, it was understood to mean that jurors had to reach a unanimous jury verdict. Yet the Supreme Court until now had refused to enforce that requirement on the states. While juries in federal cases must be unanimous, a divided court in 1972 did not impose the rule on the states.

Gorsuch said this was a mistake. “When the American people chose to enshrine that right in the Constitution, they weren’t suggesting fruitful topics for future cost-benefit analyses. They were seeking to ensure that their children’s children would enjoy the same hard-won liberty they enjoyed,” he wrote in Ramos v. Louisiana.

“As judges, it is not our role to reassess whether the right to a unanimous jury is ‘important enough’ to retain,” Gorsuch continued, quoting from the court’s 1972 decision, which is now overturned. “With humility, we must accept that this right may serve purposes evading our current notice. We are entrusted to preserve and protect that liberty, not balance it away aided by no more than social statistics.”

The court’s decision overturns the conviction and life prison term of Evangelisto Ramos, who was convicted by a 10-2 jury of murdering a prostitute in New Orleans. As Gorsuch noted, Louisiana had adopted the rule of non-unanimous juries in 1898 at a state convention established to maintain “the supremacy of the white race.” In the Reconstruction era, Southern states were told they may not exclude all blacks from juries, but the Louisiana rule allowed for convicting black defendants even if one or two blacks refused to go along.

Gorsuch’s opinion was joined by Justices Ruth Bader Ginsburg, Stephen G. Breyer and Sonia Sotomayor. Justices Clarence Thomas and Brett M. Kavanaugh concurred in the result, but for different reasons.

Justice Samuel Alito said in dissent that the court should not overturn the 1972 precedent that allowed for non-unanimous juries, and Chief Justice John G. Roberts and Justice Elena Kagan agreed.