IF you’re a politician, there may be something worse than being called a liar, a cheater, a jerk or a wimp. In America, you don’t want to be known as a flip-flopper. Jeb Bush and Hillary Rodham Clinton have both been criticized for changing their positions from yes to no on invading Iraq. A decade earlier, flip-flopping was one of the biggest knocks against John Kerry. In a JibJab video gone viral, an animated George W. Bush mocked Mr. Kerry: “You have more waffles than a house of pancakes.”

The English, too, have an intense distaste for inconsistency. “U-turn if you want to,” Margaret Thatcher pronounced. “The lady’s not for turning.”

We don’t just loathe inconsistencies in others; we hate them in ourselves, too. But why? What makes contradictions so revolting — and should they be?

Leon Festinger, one of the great social psychologists in history, coined the term cognitive dissonance to describe the discomfort you feel if you say or do something that is inconsistent with one of your beliefs. In a series of classic experiments, he and his colleagues demonstrated that people will go to great lengths to avoid this discomfort. If you’re against gun control and are paid $100 to give a speech in favor of gun control, your beliefs won’t change; you can just say, I was paid so much, I did it for the money. But if you’re paid only $1 to give that same speech, you’ll actually convince yourself that gun control is a decent idea. If it wasn’t, why would you have supported it for such a paltry sum? You lack external justification, so you have to convince yourself that you believe what you said.