For example, when the black job candidate is highly qualified, there is no discrimination. Yet in a more muddled gray area where reasonable people could disagree, unconscious discrimination plays a major role.

Image Nicholas D. Kristof Credit... Fred R. Conrad/The New York Times

White participants recommend hiring a white applicant with borderline qualifications 76 percent of the time, while recommending an identically qualified black applicant only 45 percent of the time.

John Dovidio, a psychologist at Yale University who has conducted this study over many years, noted that conscious prejudice as measured in surveys has declined over time. But unconscious discrimination  what psychologists call aversive racism  has stayed fairly constant.

“In the U.S., there’s a small percentage of people who in nationwide surveys say they won’t vote for a qualified black presidential candidate,” Professor Dovidio said. “But a bigger factor is the aversive racists, those who don’t think that they’re racist.”

Faced with a complex decision, he said, aversive racists feel doubts about a black person that they don’t feel about an identical white. “These doubts tend to be attributed not to the person’s race  because that would be racism  but deflected to other areas that can be talked about, such as lack of experience,” he added.

Of course, there are perfectly legitimate reasons to be against a particular black candidate, Mr. Obama included. Opposition to Mr. Obama is no more evidence of racism than opposition to Mr. McCain is evidence of discrimination against the elderly or against war veterans. And at times, Mr. Obama’s race helps him: it underscores his message of change, it appeals to some whites as a demonstration of their open-mindedness, and it wins him overwhelming black votes and turnout.

Still, a huge array of research suggests that 50 percent or more of whites have unconscious biases that sometimes lead to racial discrimination. (Blacks have their own unconscious biases, surprisingly often against blacks as well.)