The duty of every American prosecutor is to see that justice is done, not to convict criminal defendants by any means. For a generation, the district attorney’s office in New Orleans brazenly flouted this standard.

The office chronically failed to hand over evidence to criminal defendants when there was “a reasonable probability” that such evidence would change the outcome in a case. This constitutional duty of prosecutors, established in the 1963 Supreme Court case Brady v. Maryland, is central to fundamental fairness in criminal cases.

Last week, the Supreme Court heard arguments in Smith v. Cain, a case involving another egregious episode in which New Orleans prosecutors withheld evidence from the defendant.

The current Brady rule requires prosecutors to make two judgments: Is evidence favorable to the defendant? If so, is it likely to affect a decision about guilt or punishment? Too often, prosecutors avoid disclosing evidence by answering no to the second question.