Cognitive Dissonance Theory
Proposed by Festinger, the cognitive dissonance theory asserts that people often have two conflicting or inconsistent cognitions which produce a state of tension or discomfort (also known as "dissonance"). People are then motivated to reduce the dissonance, often in the easiest manner possible.
For example, if you are a pacifist, but punched someone, there is inconsistency -- you think you should be passive, but you became angry enough to punch someone -- which would likely produce tension (you would feel discomfort from this - "how could I do this" ..." I don't believe in violence" ...etc.). You may reduce this tension by claiming that you don't believe in violence, EXCEPT in certain circumstances, like this one! In Festinger's classic study of dissonance, people who had engaged in a boring task for along period of time had to tell the next participant who was going to engage in the same task that it was actually a lot of fun (dissonance = telling a lie, but most people do not view themselves as liars).
Participants were either paid $1 or $20 for engaging in the boring task. It turned out that people who were paid $1 told the biggest lies - they said the task was great, so much fun, etc...while the people paid $20 said it wasn't so great. Why? How can someone who just did a boring task for along time, and got paid so little for doing it, tell someone else how much fun it was?
They change their attitude to actually believe that they DID enjoy the task. The people who received $20 didn't have to justify anything - the task was boring, but you get paid $20, so who cares. There is little or no dissonance in the $20 situation.
Check our in-depth Cognitive Dissonance section for more about this theory, key people, factors, how to reduce dissonance, opposition to cognitive dissonance theory, and more.