Jennifer Richmond, the book was Blink.

“But the truth is that much prejudice — perhaps most of it — flourishes below the level of conscious thought. Which means, alarmingly, that it’s entirely possible to hold strong beliefs that point in one direction while demonstrating behaviour that points in the other. The classic (if controversial) demonstration of this is Harvard’s Project Implicit, made famous in Malcolm Gladwell’s book Blink. You can take the test here: whatever your race, there’s a strong chance you’ll take a split second longer to associate positive concepts with black faces than white ones.”