If our beliefs are firmly established on solid evidence and valid confirmatory experiments, the tendency to give more attention and weight to data that fit with our beliefs should not lead us astray as a rule.
Of course, if we become blinded to evidence truly refuting a favored hypothesis, we have crossed the line from reasonableness to closed-mindedness.] One need dig no deeper than confirmation bias when looking for an explanation as to why so many intelligent people believe that the positions of stars, planets, the sun, and the moon affect or determine such things as personality traits or personal fates.
Phrenology has been thoroughly discredited and has been recognized as having no scientific merit, yet it was advocated by many scientists and medical professionals in the 19th century in Europe and America.
The believers in phrenology had no trouble finding cases that fit their beliefs but nobody, it seems, attempted to test the belief by trying to falsify it, as good science requires.
Astrology is just one example of a belief system easy to confirm with data.
The problem is that this and similarly grounded belief systems are not tested by trying to such claims.
They compound the problem by proceeding in ways that avoid dealing with data that would contradict their hypotheses.
See the entry on negativity bias.) Confirmation bias refers to a type of selective thinking whereby one tends to notice and to look for what confirms one's beliefs, and to ignore, not look for, or undervalue the relevance of what contradicts one's beliefs.
For example, if you believe that during a full moon there is an increase in admissions to the emergency room where you work, you will take notice of admissions during a full moon, but be inattentive to the moon when admissions occur during other nights of the month.
For more on the popularity and application of eugenics in the United States before it became part of Hitler's attempt to eliminate inferior people (as defined by him) and promote the absurd idea of the Aryan thoroughbred see the chapter on eugenics in Siddhartha Mukherjee's , p. Authors such as Malcolm Gladwell have turned confirmation bias into a successful formula for writing best sellers.
The trick is to make a claim that something is a necessary condition for something else (A is necessary for B to occur) and then back it up with dozens of entertaining and colorful anecdotes where A happened and then B happened.
Search for validating feelings:
It is much easier to see how a piece of data supports a position than it is to see how it might count against the position.