In a wonderful book “The Undercover Philosopher”, the author, Michael Philips, writes on the confirmation bias.

“When information doesn't conflict with our preexisting beliefs, we tend to be surprisingly charitable. If what we read or hear is clear, we tend to look for instances that confirm it. If what we read or hear is vague, we tend to interpret it in a way that makes it plausible or true. This tendency to find the truth in what we read or hear is part of what keeps so many of us believing in psychics and astrologers.

The experimental psychologist Ray Hyman did a lot of interesting work on our understanding of vague sentences. He became interested in this subject after reading palms as a teenager. He did not believe in palm reading when he started, but the more he did it, the more he heard responses like "That's amazing! How could you possibly know that about me!" For a while, these responses got him to think that maybe there was something to palmistry after all. Eventually, though, he realized how it works. The terms we use to describe character traits are vague. Almost everyone is courageous and fearful, sensitive and thick-skinned, sociable and shy in some ways and in some contexts. When we are told by a palmist, astrologer, or psychic reader that we are foolish with money or shy with our friends, most of us can think of contexts in which that is true. And that's exactly what we tend to do. We look for confirming, instances in these cases and we almost always are able to find them. We ignore the rest.

Hyman's hypothesis has been tested many times by psychologists who employ what are called" stock spiels." A stock spiel is a very general personality description that is often presented as a profile of a particular individual (for example, as the result of a personality test). Here's an example:

Some of your aspirations tend to be pretty unrealistic. At times you are extroverted, affable, sociable while at other times you are introverted, wary and reserved. You have found it unwise to be too frank in revealing yourself to others. You pride yourself on being an independent thinker and do not accept others' opinions without satisfactory proof. You prefer a certain amount of change and variety, and become dissatisfied when hemmed in by restrictions and limitations. At times, you have serious doubts as to whether you have made the right decision or done the right thing. Disciplined and controlled on the outside, you tend to be worrisome and insecure on the inside.

This stock spiel was first used by Bertram Forer in a class-room demonstration. He asked his students how accurately the description fit them. On a scale of one to five, his students gave the test an average accuracy rating of 4.26. Sixteen of his thirty-nine students rated it a five (a perfect description of themselves). In another test, the psychologists Synder and Shenkel presented a very similar spiel to two groups of students. One group was told that the sketch is generally true for all people and was asked to say how well it fit them. The other group was told that the description was made specifically for them. The former rated the test between average and good. The latter rated the test midway between good and excellent.

When we are dealing with statements that don't conflict with our established beliefs, we interpret them generously and look for ways in which they are true. Interestingly, Hyman believes that we can learn by treating statements in this manner. If an astrologer tells a generally timid person that they are courageous, that person may think of a way in which they are courageous which may not have occurred to them before. I think this point can be generalized. Since people often make rather general statements with something more particular in mind, our tendency to look for confirming instances helps us to see the truth in what they are saying; or, anyway, it helps us see what they had in mind. If instead we immediately look for disconfirming instances, we might miss their point and lose their insight. Of course, not everyone looks for confirming instances first. Lawyers, philosophers, scientists, and other intellectuals are trained to look for disconfirming instances (and some people are contentious in this way without the benefit of formal training).

In the Hyman cases, the sentences we try to confirm are vague and open to a variety of interpretations. In at least some contexts we also look for confirming rather than disconfirming instances when sentences are clear. Psychologists call this "confirmatory bias" and "biased recruitment of evidence" and have shown experimentally that we do it in a number of contexts. Like so many experimental results in psychology and social psychology, it is hard to know to what extent these results extend beyond the contexts in which they have been tested. There is, however, one famous (though also controversial) experiment that suggests that our tendency to look for confirming instances is quite general. In this experiment - the Wason selection test - subjects are dealt four cards. There are letters on one side of each card and numbers on the other. Two cards are dealt with the number side up and two cards are dealt with the letter side up. In the original version of the experiment the visible letters are "K" and "A" and the visible numbers are "2" and "7." The subjects are asked which cards they need to turn over to tell whether the following sentence is true: "If there is a vowel on one side of the card, there is an even number on the other side." Typically, about four percent of the people given this task get it right. Everyone recognizes that you need to turn over the "A" card to see whether there is an even number on the other side. That is, they look immediately for the case that obviously confirms or disproves the statement. But typically about eighty percent of the people also think you need to turn over the "2." You don't. What's on the other side of that card is irrelevant. The statement would be true whether the latter were a vowel or a consonant. Still, a vowel is a confirming instance and that is why people are drawn to it. The amazing thing is that only ten percent of the subjects looked for (or, anyway, found) a way to disconfirm the hypothesis. You do that by turning over the "7." If there is a vowel on the other side, we know that the target sentence is false. The fact that so few people think of this suggests that our tendency to look for confirming instances is much stronger than our tendency to look for disconfirming instances (for these four cards, the target sentence is true if and only if there is an even number on the other side of the "A" card and a consonant on the other side of the “7”.

As I said, this experiment is controversial. Some argue that people may ignore the disconfirming card in this highly abstract context because they simply don't understand the meaning of "if. . . then" sentences in highly abstract contexts. Some experiments in fact suggest that in less abstract cases - where we are not dealing with numbers and letters but sentences that have plausible connections to one another - people make this mistake less often. However that may be, it is clear that confirmation bias exists in a significant range of cases. If we read that a stitch in time saves nine, most of us will think about cases where prompt action is called for. If we are told that haste makes waste, we will think about cases where patience is wise. Again, though, this tendency is limited to cases in which what we read or hear is consistent with our preexisting beliefs. It is also limited to cases in which we do not feel under attack. When we are fighting with our loved ones we don't respond to statements that begin "You always. . ." and “You never. . ." by looking for confirming instances.

The advantage of confirmation bias is that it helps us to see the truth in what someone else says. The disadvantage is that it leads us to ignore what is false. That is dangerous. Almost every generalization, however inaccurate, has some confirming instances. If we think only of those instances, we may find ourselves agreeing with a lot of false and even dangerous propositions. For example, we may find ourselves nodding in agreement to racist stereotypes ("Most Italians have Mafia connections;" "Most young black men are felons;" "Most Muslims support terrorism;" "Most Jews are money hungry;" etc.). If our tendency is to look for confirming instances, we will almost always be able to find some. This tendency is exploited by demagogues, public relations people, televangelists, advertisers, ordinary politicians, and everyone else schooled in the arts of persuasion. The trick is to support a generality with a vivid confirming instance so that when people hear the generality they think of that confirming instance. “You ask me what these homosexuals are like and why they are destroying the American family. Let me tell you about Horace Green. . .”

Anyone who thinks about it for a moment realizes that this is bad reasoning. It invites us to move from a single case or a few cases to a generality (to start with "a few" or "some" and conclude "most" or "all"). Nonetheless, this is a very powerful technique. We may recognize the general point that it is a logical fallacy and still fall for it in a particular case. As noted earlier, iconic representations - vivid confirming instances - can become deeply seated in our imagination and are hard to escape. To immunize ourselves against them, we need to do more than recognize their danger in the abstract. We need to train ourselves to look for disconfirming instances. We need to complement our normal, more charitable ways of understanding each other with the style of thinking characteristic of philosophers and scientists (on the job).

As we've seen, we also learn by interpreting vague and ambiguous sentences in ways that make them true. But this is also dangerous. For example, it can lead us to believe that our personalities and the opportunities and hazards we face each day depend on the alignment of the stars. It can also be exploited in the cause of knavery. Consider biblical interpretation. By assigning specific contemporary references to vague biblical passages, some fringe Christian sects insist that black people are subhuman ("mud people") and Jews are demonic ("spawn of Satan"). By assigning specific contemporary references to passages from the Book of Revelation, a much larger number expect that the Apocalypse is just around the corner. One can only guess how this affects attitudes toward global warming, looming water shortages, and other ecological issues. Since fundamentalist Christians are currently an influential political force in America, their style of biblical prophecy may have worldwide consequences. Somehow it does not occur to them that there were equally plausible "references" for their end-of-the-world passages at almost every period in European history. The Last Days are long overdue.”