Cognitive dissonance - distress caused by inconsistency between a person's two beliefs, or a belief and an action.  This distress becomes motivation to change the BELIEF.

Selective Exposure - the principle that individuals only pay attention to ideas they ALREADY believe because discrepant information would lead to mental distress.

Post Decision Dissonance - distress as a result of doubts about a decision AFTER the decision has been made.  The result is a need for reassurance.  Generally the higher the importance of the decision, the greater the need for reassurance.

Minimal Justification Hypothesis - The idea that the best way to achieve attitude change is to offer just enough reward or punishment to elicit compliance.

Now, the classic Stanford Experiment.