The unconscious is playing a political role this year, for the evidence is overwhelming that most Americans have unconscious biases both against blacks and against women in executive roles.

At first glance, it may seem that Barack Obama would face a stronger impediment than Hillary Clinton. Experiments have shown that the brain categorizes people by race in less than 100 milliseconds (one-tenth of a second), about 50 milliseconds before determining sex. And evolutionary psychologists believe we’re hard-wired to be suspicious of people outside our own group, to save our ancestors from blithely greeting enemy tribes of cave men. In contrast, there’s no hard-wired hostility toward women, though men may have a hard-wired desire to control and impregnate them.

Yet racism may also be easier to override than sexism. For example, one experiment found it easy for whites to admire African-American doctors; they just mentally categorized them as “doctors” rather than as “blacks.” Meanwhile, whites categorize black doctors whom they dislike as “blacks.”

In another experiment, researchers put blacks and whites in sports jerseys as if they belonged to two basketball teams. People looking at the photos logged the players in their memories more by team than by race, recalling a player’s jersey color but not necessarily his or her race. But only very rarely did people forget whether a player was male or female.

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

“We can make categorization by race go away, but we could never make gender categorization go away,” said John Tooby, a scholar at the University of California, Santa Barbara, who ran the experiment. Looking at the challenges that black and female candidates face in overcoming unconscious bias, he added, “Based on the underlying psychology and anthropology, I think it’s more difficult for a woman, though not impossible.”