Do people who undergo training usually shed their biases? Researchers have been examining that question since before World War II, in nearly a thousand studies. It turns out that while people are easily taught to respond correctly to a questionnaire about bias, they soon forget the right answers. The positive effects of diversity training rarely last beyond a day or two, and a number of studies suggest that it can activate bias or spark a backlash. Nonetheless, nearly half of midsize companies use it, as do nearly all the Fortune 500.