Cognitive dissonance is a theory of social psychology that describes the discomfort we feel when we hold two conflicting ideas at the same time. The theory is important in explaining why we have a hard time changing our beliefs, even when we know that they are wrong.

The theory was first proposed by Leon Festinger in 1957. He suggested that people have a need for consistency in their beliefs. When we encounter information that is inconsistent with our beliefs, we experience a state of mental discomfort, or dissonance. To reduce this dissonance, we either change our beliefs to match the new information, or we find a way to rationalize the inconsistency.

For example, imagine that you believe that smoking is harmful to your health. You may have read about the dangers of smoking, or seen statistics that show how many people die each year from smoking-related illnesses. However, you may also know someone who smokes and seems to be in good health. This inconsistency between your beliefs and your observations creates cognitive dissonance.

To reduce this dissonance, you may decide to stop believing that smoking is harmful. Or, you may rationalize the inconsistency by telling yourself that the person you know who smokes must be an exception to the rule.