Leon Festinger  developed Cognitive Dissonance Theory in 1957 to explain how a person's beliefs can change when they are in conflict. According to Festinger , cognitive dissonance is the noxious mental state that results from beliefs being in conflict with each other. Because cognitive dissonance is unpleasant, we are motivated to find some way to resolve the conflict that is causing it. As cognitive dissonance increases, we become more motivated to modify our behavior to resolve the conflict.
The strength of cognitive dissonance is a direct function of two factors: the number of beliefs in conflict and the importance of those beliefs. We experience the most cognitive dissonance when many important beliefs are in conflict. A prime example is the abortion issue. A person might believe that everyone deserves equal rights, but a pro-life stance assumes that the child's rights are more important than the mother's while a pro-choice stance assumes that the mother's rights are more important than the child's. The issue becomes even more complex when the pregnancy puts the mother's life in danger or the pregnancy was the result of sexual assault. The issue can be emotionally charged if it directly affects the person or a close family member or friend.
Cognitive dissonance can be reduced in two ways: a) adding new beliefs or b) changing existing ones. Adding new beliefs can reduce dissonance if the new beliefs add weight to one side of the conflict or if they reduce the importance of the dissonant beliefs. Likewise, changing existing beliefs reduces dissonance if their new content makes them less contradictory with others or their importance is reduced.
The resolution of cognitive dissonance is generally a subconscious process. We often change our beliefs without even realizing we have done so. Consider an early experiment of cognitive dissonance performed by Festinger . Subjects were asked to perform a tedious task that consisted of putting knobs on pegs, turning them a quarter turn, and then taking them off again, for an hour. After this boring task was finally completed, subjects in the control group rated how interesting the experiment was. For the experiment groups, the experimenter told the subject that his assistant had not shown up yet so he needed the subject to help him by telling the next subject that the experiment task was fun and interesting. Subjects in one experiment group were given $1 to help while subjects in the other experiment group were given $20 to help. After convincing the next subject (who was really the experimenter's assistant) that the task was fun, each subject was asked to rate how much he/she really enjoyed the experiment.
Both experiment groups rated the task as being more enjoyable than the control group. The group that was paid $20 rated it only slightly higher than the control group, whereas the group that was paid only $1 rated it much higher than the control group.
Cognitive Dissonance Theory explains these results in terms of "insufficient justification." It is assumed that subjects come into the experiment with the belief "I do not lie without a good reason." The subjects in the experiment groups then go on to tell a lie. The subjects in the $20 group had a good reason to lie (being paid $20), so the decision and commitment to lie in this case was consistent with their preexisting belief. The subjects in the $1 group had insufficient justification for lying, so the fact that they lied without good reason was inconsistent with the belief that they do not lie without a good reason. To reduce the cognitive dissonance created by these inconsistent beliefs, the subjects had to change one of them. "I do not lie without good reason" is an important belief in most people's self-perception, so it would be hard to change that belief. It is easier to just say "I did not lie." The subjects can not deny that they said that the experiment was fun, so they subconsciously change their belief that the experiment was boring and end up believing that the experiment was fun.
The application of Cognitive Dissonance Theory to truth maintenance requires the definitions of several terms to be operationalized. The importance of a belief is the number of beliefs that rely on that belief directly for justification. The importance of a belief will be measured by the number of valid children that are derived directly from that belief. In a reasoning system, the only type of conflict that can exist is the direct logical contradiction of two beliefs. The number of beliefs in conflict can only be measured by the number of beliefs that directly support each of the conflicting beliefs. Therefore, the evidence of the beliefs in conflict will be measured by the number of valid justifications for each belief.
In the TMS-CD, resolving cognitive dissonance is framed as a decision process. The TMS-CD must decide which of the conflicting beliefs to keep and which to reject. Therefore, measuring the strength of the dissonance does not help to decide between the beliefs. Instead, the TMS-CD measures the strength of each conflicting belief as a linear combination of evidence and importance. The strengths of the beliefs can then be compared to make the decision. With these operational definitions, we can apply Cognitive Dissonance Theory to truth maintenance.