http://en.wikipedia.org/wiki/Cognitive_dissonance

In psychology, cognitive dissonance is the mental stress or discomfort experienced by an individual who holds two or more contradictory beliefs, ideas, or values at the same time, or is confronted by new information that conflicts with existing beliefs, ideas, or values.[1][2]

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"The Fox and the Grapes" by Aesop. When the fox fails to reach the grapes, he decides he does not want them after all. Rationalization (making excuses) is often involved in reducing anxiety about conflicting cognitions, according to cognitive dissonance theory.