The Discomfort of Contradiction
People have a deep drive to feel that their beliefs, attitudes, and actions all hang together. When they clash, the result is an uncomfortable mental tension that Leon Festinger named cognitive dissonance. To relieve it, we adjust something — and often it is the belief, not the behavior, that gives way.
Consider Festinger and Carlsmith's classic study. Participants did a crushingly boring task, then were paid either 1 dollar or 20 dollars to tell the next person it was fun. Who ended up actually believing the task was enjoyable? Counterintuitively, the ones paid 1 dollar. Those paid 20 dollars had a solid external justification for lying ("I did it for the money"), so they felt little dissonance. Those paid only 1 dollar had no such excuse — the only way to resolve "I said it was fun but it wasn't" was to change their attitude: "well, it actually was kind of fun."
This reveals the engine of dissonance reduction. When behavior cannot be undone and external justification is weak, the cheapest fix is to change the attitude to match the behavior. Dissonance also drives effort justification (we value things more when we suffered to get them) and post-decision rationalization (we talk up the option we chose after choosing it).
Common pitfall: predicting that the 20-dollar group would believe the lie more, on the logic that "more reward changes attitudes more." Dissonance theory predicts the reverse: the larger reward provides ample external justification, so it produces less attitude change. The small reward, lacking justification, forces the belief to shift.