I grew up on the Jersey shore in the 1950s, an era of fairly blatant racism. Neighborhoods were either white or black, not yet mixed, and very few of my black friends were "tracked" into my academically advanced high school classes. I rarely encountered black families in the local diners, department stores or movie theaters.

That kind of racial bias is largely gone from the world my kids are growing up in. So that's a good thing. But many social critics believe strongly that racism never really went away, that bias against blacks has simply been driven underground in our era of political correctness. In this view, even the most progressive of thinkers may harbor dark, discriminatory impulses that can surface when least expected or desired.

And it's not just race. A large and growing number of psychologists now argue that a welter of prejudices are simmering just below the surface of society: prejudices against many ethnic groups, against women, gays, the elderly, and outsiders like the homeless and drug addicts. The big question is whether these unconscious animosities are potent enough to actually shape our actions, to make us do things we ourselves find shameful. A new study suggests that, unhappily, the answer is yes.

But let me back up just a bit. About a decade ago psychologists developed an instrument that they claim can actually tap into unconscious attitudes about minority groups of all sorts. The so-called Implicit Association Test, or IAT, has spawned a cottage industry of psychological research and has now been administered to hundreds of thousands of Americans. Basically, what the test asks you to do is to very rapidly sort various words—words like glorious and happy to the left and words like disaster and awful to the right. Then it randomly intersperses the words with photographs—of blacks and whites, for instance, or youthful and elderly people—and asks that you sort them simultaneously. For example, black images and positive words to the left, and white images and negative words to the right. Then they switch it all around. The idea is that if you harbor a subtle bias against any group, your unconscious mind will take milliseconds longer to process links between images of that group and words like wonderful and joyous. (The IAT is hosted on Harvard University's Web site, for anyone who wants to peek inside his own unconscious.)

The test has sparked a heated controversy among both psychologists and legal scholars, some of whom are arguing for a radical rethinking of antidiscrimination law to accommodate such hidden prejudice. Stereotypes are as robust as ever, they say, and more insidious because they are not overt. Critics respond that what the test measures is not prejudice at all but simply a lack of familiarity with blacks or whites or lesbians or heroin addicts. They argue further that even if the test is tapping into unconscious fears or animosities, it does not mean that people will actually act on those impulses.

It's this disputed link between unconscious attitudes and behavior that the new study set out to explore, and it did so in an unusual way. Since it's very hard to get anyone to admit to prejudicial behavior involving race or age or sexuality, William von Hippel of Australia's University of Queensland conducted a very focused study designed to sidestep that issue. He and his colleagues looked at hidden biases against IV drug users. But they didn't study these attitudes in the general population. Instead, they deliberately studied people one would expect to have the most empathy and understanding for drug addicts, and who indeed claimed such sympathy: nurses working in the substance abuse field.