The confirmation bias is the most insidious of all cognitive errors. It stems from 'cognitive dissonance', that is, the inability for the mind to hold two opposing ideas simultaneously. Secondly, it stems from a lack of basic reasoning skills. Confirming evidence is not as important as disconfirming evidence. The confirmation bias can create 'echo chambers' which can lead to some really dark ideas. This point cannot be overstated.
Now, why is confirmation so bad?
We can confirm almost anything we like.
For example, consider the evidence for Santa Clause's existence. We find presents under the christmas tree; figurines of him in stores; songs written about him; he's in movies; and so on. This is all evidence for establishing the existence of Santa Clause.
Yet, we all know this is silly. Thus, the important factor in reasoning is the disconfirming evidence. The disconfirming evidence in this case involves staying up all and night watching your parents put the presents under the christmas tree, not Santa. Or more simply, your parents tell you they the presents there.
So, what is the confirmation bias?
Confirmation bias refers to the tendency to recall information that confirms or reinforces our own beliefs rather than contradicting them, thereby making our judgement or decision more positive than it should be. 
People tend to ignore disconfirming evidence and are 'disinclined to change his or her belief once he or she arrives at a conclusion.' 
...when people are forced to look at disconfirming evidence, they will find a way to criticize, distort, or dismiss it so that they can maintain or even strengthen their existing belief. 
Terrible, I know.
A Simple Test:
A way to test yourself and other people for the confirmation bias is to ask the following question:
'What evidence runs against my/your position?'
If you/they cannot answer this question, there's a strong chance you're/they're trapped in the horrors of confirmation. What are the chances there is absolutely zero evidence in favour of the opposition? Very slim. If only life were that simple...
Now, some fascinating examples of the confirmation bias in action...
1. The Error of Confirmation Taken to the Extreme:
Assume that I told you that I had evidence that the football player O.J. Simpson (who was accused of killing his wife in the 1990s) was not a criminal. Look, the other day I had breakfast with him and he didn’t kill anybody. I am serious. I did not see him kill a single person. Wouldn’t that confirm his innocence?
I took a nap the other day on the railroad track in New Rochelle, New York, and was not killed. Hey, look at me, I am alive, I would say, and that is evidence that lying on the train is risk-free. 
These absurd examples are a good illustration of the ludicrousy of confirmation.
2. Political Infatuation
After the first televised presidential debate between Nixon and Kennedy, the comedian Lenny Bruce wrote:
I would be with a bunch of Kennedy fans watching the debate and their comment would be, “He’s really slaughtering Nixon.” Then we would all go to another apartment, and the Nixon fans would say, “How do you like the shellacking he gave Kennedy?”
And then I realized that each group loved their candidate so that a guy would have to be this blatant— he would have to look into the camera and say: “I am a thief, a crook, do you hear me, I am the worst choice you could ever make for the Presidency!” And even then his following would say, “Now there’s an honest man for you. It takes a big guy to admit that. There’s the kind of guy we need for President.” 
3. Capital Punishment Study
In one experiment, researchers selected people who either favored or opposed capital punishment and asked them to read two scholarly, well-documented articles on the emotionally charged issue of whether the death penalty deters violent crimes.
One article concluded that it did; the other that it didn’t. If the readers were processing information rationally, they would at least realize that the issue is more complex than they had previously believed and would therefore move a bit closer to each other in their beliefs about capital punishment as a deterrence.
But dissonance theory predicts that the readers would find a way to distort the two articles.
They would find reasons to clasp the confirming article to their bosoms, hailing it as a highly competent piece of work. And they would be supercritical of the disconfirming article, finding minor flaws and magnifying them into major reasons why they need not be influenced by it. This is precisely what happened. Not only did each side discredit the other’s arguments; each side became even more committed to its own. 
4. In Advertising
From a marketing perspective, confirmation bias presents a real problem when consumers search internally for only positive information about the competition.
One way marketers attack this problem is to draw attention to negative aspects of competitive brands through comparative advertising.
Apple has done this with ads comparing its easy-to-use computer operating systems with those made by Microsoft for PC‘s. By presenting comparative information in a convincing and credible way, marketers may be able to overcome confirmation bias. 
5. Belief in Fate
For instance, a person may believe that fate is unfair to him, as proven by a series of observations showing that whenever he waits for a bus the bus is always late.
He will then tend to register as "evidence” only those cases when the bus comes late but not those when it arrives on time.
Confirmation bias makes it hard to refute a hypothesis and therefore may be responsible for the frequently discussed unfounded perseverance and conservatism of beliefs. 
6. People Only Read Books They Agree With
During the 2008 U.S. presidential election, researcher Valdis Krebs at orgnet.com analyzed purchasing trends on Amazon.
People who already supported Obama were the same people buying books that painted him in a positive light. People who already disliked Obama were the ones buying books painting him in a negative light. Just as with pundits, people weren’t buying books for the information, they were buying them for the confirmation.
Krebs has researched purchasing trends on Amazon and the clustering habits of people on social networks for years, and his research shows what psychological research into confirmation bias predicts: you want to be right about how you see the world, so you seek out information that confirms your beliefs and avoid contradictory evidence and opinions. 
7. The Job Evaluation Study
In a 1979 University of Minnesota study by Mark Snyder and Nancy Cantor, people read about a week in the life of an imaginary woman named Jane. Throughout the week, Jane did things that showcased she could be extroverted in some situations and introverted in others.
A few days passed. The subjects were asked to return. Researchers divided the people into groups and asked them to help decide if Jane would be suited for a particular job.
One group was asked if she would be a good librarian; the other group was asked if she would be a good real estate agent. In the librarian group, people remembered Jane as an introvert. In the real estate group, they remembered her being an extrovert.
After this, when each group was asked if she would be good at the other profession, people stuck with their original assessment, saying she wasn’t suited for the other job.
The study suggests even in your memories you fall prey to confirmation bias, recalling those things that support even recently-arrived-at beliefs and forgetting those things that contradict them. 
* * *
 Mistake Were Made (But Not By Me): How We Justify Foolish Beliefs, Bad Decisions and Hurtful Acts by Carol Tavris & Elliot Aronson
 How to Talk Dirty and Influence People by Lenny Bruce
 The Black Swan: The Impact of the Highly Improbable by Nassim Nicholas Taleb
 Consumer Behaviour, 5th Ed by Hoyer & Maclinnis
 Wayne A. Wallace, The Effect of Confirmation Bias in Criminal Investigative Decision Making - paper
 Personal Control in Action: Cognitive and Motivational Mechanisms edited by Miroslaw Kofta, Gifford Weary, Grzegorz Sedek
 You Are Not So Smart: Why You Have Too Many Friends on Facebook, Why Your Memory Is Mostly Fiction, and 46 Other Ways You're Deluding Yourself by David McRaney
Categorisation: psychology → human misjudgement → cognitive bias
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