We've long known that the death penalty disproportionally kills people of color. David Baldus, a University of Iowa law professor, and his colleagues studied more than 2,000 homicides in Georgia in the 1970s and 1980s for evidence of bias. Their landmark research, known popularly as the Baldus study, found vast racial disparities in Georgia's capital-punishment system.

A black inmate named Warren McCleskey, who had received a death sentence for murdering a white Atlanta police officer, challenged it before the Supreme Court in 1987 using the Baldus study, arguing that Georgia's racially discriminatory system violated his Eighth and Fourteenth Amendment rights. Justice William Brennan, who opposed the death penalty in all circumstances, explained in his dissent why the statistical evidence mattered:

At some point in this case, Warren McCleskey doubtless asked his lawyer whether a jury was likely to sentence him to die. A candid reply to this question would have been disturbing. First, counsel would have to tell McCleskey that few of the details of the crime or of McCleskey's past criminal conduct were more important than the fact that his victim was white. Furthermore, counsel would feel bound to tell McCleskey that defendants charged with killing white victims in Georgia are 4.3 times as likely to be sentenced to death as defendants charged with killing blacks. In addition, frankness would compel the disclosure that it was more likely than not that the race of McCleskey's victim would determine whether he received a death sentence: 6 of every 11 defendants convicted of killing a white person would not have received the death penalty if their victims had been black, while, among defendants with aggravating and mitigating factors comparable to McCleskey's, 20 of every 34 would not have been sentenced to die if their victims had been black. Finally, the assessment would not be complete without the information that cases involving black defendants and white victims are more likely to result in a death sentence than cases featuring any other racial combination of defendant and victim. The story could be told in a variety of ways, but McCleskey could not fail to grasp its essential narrative line: there was a significant chance that race would play a prominent role in determining if he lived or died.

Five justices disagreed with this assessment. The majority decision in McCleskey v. Kemp ruled that aggregate empirical evidence of racial discrimination could not invalidate an individual defendant's death sentence. "At most, the Baldus study indicates a discrepancy that appears to correlate with race," Justice Louis Powell wrote, "but this discrepancy does not constitute a major systemic defect."

Unchecked by the judiciary, the death penalty's racial discrepancy survived and thrived. Eleven years after McCleskey, Baldus studied 667 homicides in Philadelphia between 1983 and 1993 and found that black defendants there were nearly four times likelier than white defendants to receive a death sentence for the same crimes. Racial disparities in crime rates aren't a factor in this because homicide, the predominant capital offense, is an overwhelmingly intra-racial crime. Federal statistics show that 84 percent of white victims and 93 percent of black victims between 1980 and 2008 were murdered by someone of the same race. But death-row statistics don't reflect those rates: Although roughly half of all U.S. homicide victims are black, more than three-quarters of victims of death-row defendants executed since 1976 were white.