What is cognitive dissonance and how do people reduce it?

Cognitive dissonance, proposed by Leon Festinger, is the uncomfortable tension that arises when a person holds two contradictory beliefs or when their behavior conflicts with their attitudes. For example, a smoker who knows smoking causes cancer experiences dissonance between the behavior and the knowledge. People are motivated to reduce dissonance by changing their behavior to match beliefs, changing beliefs to match behavior, or adding new beliefs to justify the inconsistency.