Studies have shown that racially diverse juries deliberate longer, consider a wider variety of perspectives and make fewer factual errors than all-white juries, and that predominantly black juries are less likely to impose the death penalty.

Excluding jurors based on race has been illegal since 1875, but after Reconstruction, all-white juries remained the norm in the South.

“It really made lynching and the Ku Klux Klan possible,” said Christopher Waldrep, a historian at San Francisco State University and the author of a forthcoming book about a lawyer who was able, in a rare case, to prove jury discrimination in Mississippi in 1906. “If you’d had a lot of black grand jurors investigating crimes, it would have made lynching impossible.”

Back then, judges and prosecutors often argued that blacks lacked the intelligence or education to serve. That such claims persist is evidence, said Bryan A. Stevenson, the executive director of the Equal Justice Initiative, that jury selection remains largely unscrutinized.

“There’s just this tolerance, there’s indifference to excluding people on the basis of race, and prosecutors are doing it with impunity,” Mr. Stevenson said. “Unless you’re in the courtroom, unless you’re a lawyer working on these issues, you’re not going to know whether your local prosecutor consistently bars people of color.”

In jury selection, potential jurors are first dismissed for cause  reasons like scheduling conflicts or opposition to the death penalty. Then, both sides can ask questions and take turns dismissing jurors using what are called peremptory strikes (the number of strikes varies by state, but it is often enough for one side to eliminate all qualified minorities).

In a 1986 case, Batson v. Kentucky, the Supreme Court ruled that if a pattern of discrimination emerged during peremptory strikes, lawyers must provide nonracial reasons for their strikes. The reason does not have to be “persuasive, or even plausible,” the Supreme Court ruled in a later case in which a prosecutor said he dismissed one black juror because he had long hair, and another because he had a goatee, saying, “I don’t like the way they looked.” It is up to the judge to decide if there was deliberate discrimination.