Cognitive dissonance is a state of tension that occurs whenever a person holds two cognitions (ideas, attitudes, beliefs, opinions) that are psychologically inconsistent. 
An example of ‘psychologically inconsistent’ is “smoking is a dumb thing to do because it could kill me” and “I smoke two packs per day”.
We reduce cognitive dissonance by using self justification. Self justification logically follows cognitive dissonance, thus grouping the two together.
The Break Down:
(a) Cognitive dissonance is '…mental discomfort, ranging from minor pangs to deep anguish' 
(b) This is produced by holding 'two cognitions (ideas, attitudes, beliefs, opinions) that are psychologically inconsistent' 
(c) The person seeks to reduce the discomfort through ‘Self justification’ 
(d) Self justification are self-serving thoughts that tend to ignore personal criticism.
1. The Fox and the Grapes Fable
In the story, a fox sees some high-hanging grapes and wishes to eat them.
When the fox is unable to think of a way to reach them, he decides that the grapes are probably not worth eating, with the justification that the grapes probably are not ripe or that they are sour.
The moral that accompanies the story is "Any fool can despise what he cannot get". The pattern in this example is 'one desires something, finds it unattainable, and reduces one's dissonance by criticizing it.' 
This is an extremely common pattern of thinking. Observe this exact pattern in the people around you when they refer to the 'rich', or the 'attractive', or the 'successful'.
2. The Pleasure of Murder
A bizarre and extreme case of cognitive dissonance reduction occurs in men sentenced to life imprisonment without the possibility of parole for a crime—let us say a spousal murder, using a knife repeatedly.
Surprisingly few will admit that the initial act was a mistake. Quite the contrary: they may be aggressive in its defense. “I would do it again in a second; she deserved everything she got.”
It is difficult for them to resist reliving the crime, fantasizing again about the victim’s terror, pain, unanswered screams for help, and so on. They are justifying something with horribly negative consequences (for themselves as well now) that they cannot change.
Their fate is instead to relive the pleasures of the original mistake, over and over again. 
...the most direct way for a smoker to reduce [cognitive] dissonance is by quitting.
But if she has tried to quit and failed, now she must reduce [cognitive] dissonance by convincing herself that smoking isn’t really so harmful, or that smoking is worth the risk because it helps her to relax or prevents her from gaining weight (and after all, obesity is a health risk, too), and so on.
Most smokers manage to reduce dissonance in many such ingenious, if self-deluding, ways. 
4. The Purchase of Expensive Items
An example of cognitive dissonance is a person's unwillingness to believe that a very expensive car or one that is considered a status symbol could have anything wrong with it or could be defective in any way. 
Take a boy who goes along with a group of his fellow seventh graders who are taunting and bullying a weaker kid who did them no harm.
The boy likes being part of the gang but his heart really isn’t in the bullying. Later, he feels some dissonance about what he did. “How can a decent kid like me,” he wonders, “have done such a cruel thing to a nice, innocent little kid like him?”
To reduce dissonance, he will try to convince himself that the victim is neither nice nor innocent: “He is such a nerd and crybaby. Besides, he would have done the same to me if he had the chance.”
6. Acceptance Into Fraternities
The greater the difficulty to be accepted into a club, the more they value the club.
Subjects were split into two groups, one comprising people who would endure a painful or embarrassing test to join a group and the other people who would pay a modest fee.
Then each was asked to evaluate the group based on a tape of a group discussion arranged to be as dull and near—incoherent as possible.
Those who suffered the higher cost evaluated the group more positively than did those who paid the small entry fee. And the effect is strong. The low—cost people rated the discussion as dull and worthless and the people as unappealing and boring. This is roughly how the tape was designed to appear.
By contrast, those who paid the high cost (reading sexually explicit material aloud in an embarrassing situation) claimed to find the discussion interesting and exciting and the people attractive and sharp. 
7. Public Speeches
...in one study, Sakai found that being asked to make a speech contrary to one’s attitudes produced more attitude change when it was made publicly than when it was made privately.” “...they resolved their dissonance by changing their private attitudes to the topic of their speech. 
People will change their views to the side they're forced to support.
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 Mistake Were Made (But Not By Me): How We Justify Foolish Beliefs, Bad Decisions and Hurtful Acts by Carol Tavris & Elliot Aronson
 Fables: Babrius and Phaedrus (Loeb Classical Library No. 436) edited by Ben Edwin Perry, pg 31 & 303.
However, this quote is from Wikipedia.
 Kaplan and Sadock's Synopsis of Psychiatry: Behavioral Sciences/Clinical Psychiatry, 10th Ed by Benjamin J. Sadock , Virginia Alcott Sadock, Pedro Ruiz
 The Folly of Fools: The Logic of Deceit and Self-Deception in Human Life by Robert Trivers
 Cognitive Dissonance: 50 Years of a Classic Theory by Joel Cooper
Founder: Leon Festinger, American social psychologist
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