Cognitive dissonance is the psychological theory that people feel mental discomfort when they hold two clashing beliefs, or when their actions conflict with their values, and that they are driven to reduce this discomfort, often by changing their beliefs to fit what they have done. Proposed in the 1950s, it is one of the most influential ideas in social psychology, supported by a long line of experiments and visible throughout everyday life.

The theory was developed by the American psychologist Leon Festinger. In a famous early study, he and his colleagues secretly joined a small group that believed the world would end on a particular date. When the prophecy failed and the world did not end, the most committed members, rather than abandoning their belief, became even more convinced and began recruiting others. Festinger argued they were resolving the unbearable clash between their deep commitment and the plain facts.

In Aesop's fable the fox, unable to reach the grapes, decides they must be sour, a classic case of easing dissonance.
In Aesop's fable the fox, unable to reach the grapes, decides they must be sour, a classic case of easing dissonance.

At its core the theory says that holding inconsistent thoughts creates a genuine, unpleasant tension, which people are motivated to relieve. They may do so by changing one of the beliefs, by adding new justifications, or by playing down the importance of the conflict. Crucially, when people cannot undo an action, they often adjust their attitudes to match it, coming to genuinely believe that what they did was right.

In the best known study, people were paid to tell a lie, describing a deathly dull task as enjoyable. Some were paid a large sum and others only a tiny one. Surprisingly, those paid very little ended up actually believing the task had been fun, while those paid well did not. With no large reward to justify the lie, the poorly paid had to resolve the dissonance by changing their own opinion, a result reproduced many times since.

Brain imaging studies have found measurable neural signatures associated with cognitive dissonance.
Brain imaging studies have found measurable neural signatures associated with cognitive dissonance.

The theory illuminates much of ordinary life. It explains buyer's remorse and the way people talk themselves into being happy with a purchase, the tendency to dismiss as sour what we cannot have, just like Aesop's fox and the grapes, and the way smokers and others downplay risks they choose to run. It is a powerful lens on how people maintain a consistent and favourable picture of themselves.

More than half a century after it was proposed, cognitive dissonance remains one of the most widely cited and tested theories in psychology, refined but never overturned. Its central insight, that people are driven not only to be right but to feel that they have been right, has reshaped the understanding of belief, persuasion, and human self justification.