Cognitive dissonance is such an elegantly simple idea that it's hard to forget: anybody who took Psych 101 can remember it. In fact, as Carol Tavris and Elliot Aronson remind us in Mistakes Were Made (But Not By Me), the term is now even a mainstay of pop culture.
As you may remember, cognitive dissonance is "a state of tension that occurs whenever a person holds two distinct cognitions (ideas, attitudes, beliefs, opinions) that are psychologically inconsistent," write the authors. Obviously, it isn't easy coping with two distinct and contradictory ideas, such as "smoking is a dumb thing to do because it could kill me" and "I smoke two packs a day," at the same time. So cognitive dissonance produces "mental discomfort," ranging from "minor pangs to deep anguish," and people don't rest easy until they've found a way to reduce it.
The resolution for cognitive dissonance is universal: you make up your mind about something you've struggled over, and then you find good reasons to support your decision. After you've made up your mind, you engage in a determined campaign of self-justification to reduce the unpleasant recollection of the dissonance.
This theory has a marvelous creation story. In the 1950s, a young social scientist, Leon Festinger, and two colleagues infiltrated a group that believed the world would end on December 31. The scientists wanted to know what would happen when their prophecy failed. What Festinger and company discovered was that when the world didn't come to an end, the group's disappointment unexpectedly turned into exhilaration: they reasoned that the world had been miraculously spared because of their faith. Their cognitive dissonance—belief that the world would end and the reality that it hadn't—was reduced by their new belief that they'd witnessed a miracle!