Classic experiment.
In another thread when we were basically arguing nature vs. nurture when it comes to racial differences (when it comes to things like sports, academics, crime rates, etc.) and I argued that nurture was a far bigger factor, people swore I was wrong (like when I argued that the fact that black people excel in American athletics probably doesn't have much to do with genetics). Truth be told though, that really is what most of this stuff comes down to. Even if it's not explicit, if a group of people internalize stereotypes/expectations then it tends to become a self-fulfilling prophecy, good or bad.