"Black people aren't allowed to make mistakes," musician Prince declared last year, in a statement that spoke for many. Screw up once, the thinking goes, and you're out, having confirmed the view of many whites that you didn't deserve a shot in the first place. New research provides evidence of that destructive dynamic on one of America's largest stages: the National Football League. A new study concludes that, after a sub-par performance, black NFL quarterbacks are roughly twice as likely to be benched the following week than their white colleagues. After taking a series of variables into account (including age,...