No matter how egalitarian, unbiased and unprejudiced we claim to be and believe to be, underneath it all many of us are quite different.
If you ask people whether men and women should be paid the same for doing the same work, everyone says yes. But if you ask volunteers how much a storekeeper who runs a hardware store ought to earn and how much a storekeeper who sells antique china ought to earn, you will see that the work of the storekeeper whom volunteers unconsciously believe to be a man is valued more highly than the work of the storekeeper whom volunteers unconsciously assume is a woman. If you ask physicians whether all patients should be treated equally regardless of race, everyone says yes. But if you ask doctors how they will treat patients with chest pains who are named Michael Smith and Tyrone Smith, the doctors tend to be less aggressive in treating the patient with the black-sounding name. Such disparities in treatment are not predicted by the conscious attitudes that doctors profess, but by their unconscious attitudes—their hidden brains. (source)
And even if most of our actions are guided by our conscious beliefs, some will be caused by unconscious prejudice, in which case we’ll have identified a cause of discrimination, a cause that will be very hard to correct.