In the field of psychology, cognitive dissonance is described as a mental phenomenon in which people unknowingly or subconsciously hold fundamentally conflicting cognitions. Being confronted by situations that create this dissonance or highlight these inconsistencies motivates change in their cognitions or actions to reduce this dissonance, maybe by changing a belief or maybe by explaining something away.

Relevant items of cognition include peoples' actions, feelings, ideas, beliefs, values, and things in the environment. Cognitive dissonance exists without outward sign, but surfaces through psychological stress when psychological discomfort is created due to persons participating in an action that creates conflicting beliefs, attitudes, or behaviors, or when new information challenges existing beliefs.

According to this theory, when an action or idea is psychologically inconsistent with the other, people automatically try to resolve the conflict, usually by reframing a side to make the combination congruent. Discomfort is triggered by beliefs clashing with new information or by having to conceptually resolve a matter that involves conflicting sides, whereby the individual tries to find a way to reconcile contradictions to reduce their discomfort.

In When Prophecy Fails (1956) and A Theory of Cognitive Dissonance (1957), Leon Festinger proposed that human beings strive for internal psychological consistency to function mentally in the real world. Persons who experience internal inconsistency tend to become psychologically uncomfortable and are motivated to reduce the cognitive dissonance. They tend to make changes to justify the stressful behavior, by either adding new parts to the cognition causing the psychological dissonance (rationalization), believing that "people get what they deserve" (just-world fallacy), taking in specific pieces of information while rejecting or ignoring others (selective perception), or avoiding circumstances and contradictory information likely to increase the magnitude of the cognitive dissonance (confirmation bias). Festinger explains avoiding cognitive dissonance as "Tell him you disagree and he turns away. Show him facts or figures and he questions your sources. Appeal to logic and he fails to see your point."

Leon Festinger, born in 1919 in New York City, was an American social psychologist whose contributions to psychology include cognitive dissonance theory, social comparison theory, and the proximity effect.

Festinger graduated from the City College of New York in 1939; he then received his PhD in Child Psychology from the University of Iowa. He was initially inspired to enter the field of psychology by Kurt Lewin, known as the "father of modern social psychology", and his work in Gestalt psychology. Studying under Kurt Lewin for most of his academic career, Festinger returned to collaborate with Lewin at the Research Center for Group Dynamics at the Massachusetts Institute of Technology. In a 2002 American Psychological Association article, Festinger is cited as the fifth most eminent psychologist of the 20th century, just after B.F. Skinner, Jean Piaget, Sigmund Freud, and Albert Bandura, respectively. Festinger's cognitive dissonance theory is still one of the most influential social theories in modern social psychology.

Throughout this research, Festinger noticed that people often like to stick to consistent habits and routines to maintain order within their lives. These habits may include everyday activities like preferring a specific seat during their daily commute or eating meals at consistent times. Any disturbance to this order can lead to mental unease, which may manifest in altered thought processes or beliefs. Festinger concluded that the sole means of alleviating this discomfort is by adjusting either their actions or beliefs to restore consistency.

Since his publication of A Theory of Cognitive Dissonance in 1957, Festinger's findings have helped to understand peoples' personal biases, how people reframe situations in their heads to maintain a positive self-image, and why one may pursue certain behaviors that misalign with their judgments as they seek out or reject certain information.