For many people, the word prejudice evokes the sort of bigotry that fuels the most bald-faced acts of intolerance, from name-calling and deliberate discrimination to hate crimes and genocide. Although that kind of overt prejudice has by no means vanished, its force in American society is much diminished, as the election of the nation's first black president attests.

But even the most egalitarian-minded among us harbor biases that reside outside our conscious control, and even sometimes without our awareness. Such implicit biases, as researchers call them, are often subtle, but they can cloud our perceptions and affect how we treat others.

Because people can't reliably describe attitudes and beliefs that operate automatically, scientists measure implicit bias indirectly, by assessing how strongly concepts such as female and career or African American and criminal are linked in a person's memory. (Measure your own bias using the Implicit Association Test, a prominent method for measuring automatic biases.)

Using such methods, psychologists have learned that stereotypes about social groups—black and white, gay and straight, fat and thin, old and young—can be automatically activated and can influence judgment and behavior subconsciously.

For example, researchers found in a 2007 experiment that physicians who showed stronger implicit race bias were less likely to give lifesaving "clot-buster" drugs to a hypothetical black patient complaining of chest pain. "Most doctors just can't quite believe that race would have anything to do with the level of treatment that they provide," says Harvard University's Alexander Green, M.D., the study's lead author. "They're trained to be objective and to base their judgments on medical evidence. But doctors are just like everyone else, and race skews their decisions."

In another study done in the months before the election, San Diego State University psychologist Thierry Devos, Ph.D., and University of Chicago colleague Debbie Ma, Ph.D., found that when participants used race to categorize photos of Barack Obama and other politicians, they were slower to associate symbols of American identity such as the American flag and the U.S. Capitol building with Obama than with white politicians like Hillary Clinton, John McCain, or even former British Prime Minister Tony Blair—who is not, of course, American. And the less that people implicitly viewed Obama as American, the less willing  they were to support him—regardless of their explicit beliefs.

Bias in the brain

Almost from birth, our industrious brain begins to automatically categorize objects in the environment and help us observe associations between those objects. If we couldn't do so, life would be nightmarish: Imagine having to learn every day that an icy sidewalk is slippery, or that a steaming cup of coffee will burn you … again.

But this same ability to form associations effortlessly also makes us susceptible to bias. Consider a child whose mother grips her hand a little tighter when a black man passes by on the street. The mother may not have intended to convey an indiscriminate fear of blacks, and may not even realize she has done so. But that's no matter to the child's brain: the neural architecture connecting "black person" and "danger" has been reinforced.

Further stacking the deck, the brain also devotes more resources to cataloging information about members of one's own social group. Brain areas responsible for face recognition are more active when we see faces of our own race, and people are quicker to recognize same-race faces than other-race faces. "These basic differences in how we perceive members of one group versus another pave the way for differences in how we feel about and judge members of those groups," says Ohio State University psychologist Wil Cunningham, Ph.D.

Overcoming implicit prejudice

The encouraging news is that implicit attitudes and stereotypes are learned, and scientists are discovering that with effort, they can be unlearned—or at least overruled. "It's easy to get discouraged because the problems are huge," says Yale University psychologist John Dovidio, Ph.D. "But in the long run, implicit attitudes are habits of mind. If I have habits of mind that are prejudiced, I can practice habits of mind that are egalitarian."

Studies have shown that one way to weaken stereotypical associations is to change the context in which people make social judgments. In one study, participants showed less race bias if black faces were superimposed on images of a church, rather than, for example, a dilapidated street corner.

Other studies indicate that people can deliberately weaken stereotypical associations by consciously practicing non-prejudiced behavior. The first step to consciously changing implicit bias, though, is to acknowledge it. "If I'm willing to become aware that I have unconscious biases and if I really care about social justice," Devos says, "then maybe I'm going to do something when I realize that I have unconscious, automatic biases."

How much change is coming?

Will the election of Barack Obama as President of the United States usher in the "post-racial" era that some pundits have predicted, erasing racial wounds that have festered for centuries?

"I think it's simplistic to say that," says Devos. "There are still very pervasive racial biases all over the place .… The lesson from a huge body of research is that changing stereotypes is challenging. There's no doubt that it's going to be a powerful thing to see a black representing this country, to see the Obama family living in the White House. But one risk is that people will see Obama as a likable, counter-stereotypical exemplar, but an exception. If so, then he may do little to change our attitudes toward African Americans as a whole."

But, he adds, "I hope that having an African American president will give us an opening to have more discussion about race in our society."

For more information and to test your own biases visit Project Implicit's website.

Comment: Will Barack Obama's presidency help reduce racial bias?

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Siri Carpenter is a science writer in Madison, Wisconsin. She is a social psychologist by training and covers psychology, neuroscience, health, and medicine. She is the first author of the textbook Visualizing Psychology (John Wiley & Sons, 2007).