Donald Trump has predicted there will be riots in the streets if he is not crowned victor at the Republican National Convention in Cleveland this summer. Given his exhortations to his supporters to "knock the crap out of" anyone disrupting his rallies and his promise to "pay for the legal fees," I'll take him at his word. Trump's comments are what sociologist Robert K. Merton called a self-fulfilling prophecy: "a false definition of the situation evoking a new behavior which makes the originally false conception come true." Trump likes to play the tough guy, and his rallies have become a...