The proof is in the killing, and a newly released study of death sentences in North Carolina proves that our year-old Racial Justice Act was not only well-motivated, but urgently needed.

This doesn't mean that every death sentence is a miscarriage of justice. It does mean that we need the brake on human passions that the act provides.

The study, covering death sentences from 1980 through 2007, was conducted by researchers from the University of Colorado in Boulder and the Northeastern University school of criminology and criminal justice in Boston. They examined more than 15,000 homicides.

Unlike earlier findings that blacks convicted of capital crimes are more likely to be executed than whites convicted of capital crimes, this one focused on victims. And it found that people accused of killing whites are three times as likely to be sentenced to die as people who are accused of killing blacks.

That's bound to feel like a punch in the gut to prosecutors, because the decision to try a homicide case as a capital case rests with the prosecutor, not the jury. And the vast majority of prosecutors think of themselves as scrupulously fair.

"Prosecutors do not look at skin color," said Seth Edwards, president of the N.C. Conference of District Attorneys. "We consider lots of things, but race is not one of them."

That certainly reflects the ideal. In fact, it reflects perfection, which means it is to some degree at odds with reality because there are no perfect humans. You don't have to "look at" race in any formal or even conscious way for it to have an influence.

If Edwards can point to scientific error in the researchers' methodology, so be it. But "three times as likely" is no statistical anomaly. The researchers considered variables, too. And while researchers are human and therefore imperfect, these findings are not inconsistent with related research.

What has been proved here - guilt, or innocence? Neither. What has been proved is that there's reasonable doubt that such decisions are free of racist taint. And that is one of the things the Racial Justice Act was crafted to address.

Under the law, death row inmates can have their sentences set aside if they make a convincing case that race was a significant factor in the decision to seek or the decision to impose the death penalty. (Such arguments are subject to rebuttal.) Inmates who succeed are resentenced to life without parole. Those who fail return to death row. Nobody "walks."

The Racial Justice Act is not an adversary of justice. It is its handmaiden.