Common Reasons for False Beliefs
Misleading information from other people
The "others" that mislead us may be authorities or people promoting a product or friends or parents or just about anybody or everybody. They may believe what they say or they may be intentionally deceptive or they may be blatantly lying. Nevertheless, a great deal of what we believe, true or false, we believe because we have heard it from others. We are forced to depend on others for much of our knowledge, so we need to learn as much as we can about how to separate the reliable from the unreliable.
This is a special case of believing others: when virtually everyone in the culture shares the belief. This can be particularly tricky since we are likely to just assume the belief is true. We may never realize there could be a question about it. In many cultures in the past, it was assumed that women were not suited for leadership roles. Everyone simply "knew" it. It was not something that needed to be justified or explained.
It is common to find people who have an unrealistic belief in something because that is what they wish were true. They may overestimate their chances of becoming a professional athlete or writing a best selling novel. They may think their child who is slow in school is really very bright but just never given a chance by the teachers. They may believe that some unrealistic claim for a diet or exercise program or arthritis cure is genuine because they want it to work.
Jumping to conclusions
We may draw a conclusion based on too little evidence or evidence that is invalid. If Joe tripped coming up the steps the first time you saw him, you might assume he is clumsy (but this may be the first time he has tripped in five years). If we read about a couple of airliner crashes, we may assume airliners are unsafe. In this case the evidence is misleading because the newspapers report mainly the crashes - rarely the much more common safe landings.
Quantities too big or small to be intuitive
When a senator talks about a program that wastes five billion dollars and another program that wastes five hundred thousand dollars, both these numbers are outside the bounds of our normal intuition. The first waste is ten thousand times more costly than the second. Small probabilities are a similar problem. How great is the risk of being killed by a terrorist while on a European vacation as opposed to being killed on a ten mile bicycle excursion? We can easily form very unrealistic priorities based on quantities that we can't evaluate intuitively.
Polarization: Emotional attachment to one side of an issue
A common, but very serious situation occurs when we become emotionally attached to one side of an issue. Often the problem starts because someone we respect persuades us that a particular "cause" is vitally important but threatened by sinister and corrupt opponents. We start to think of our side as good and the other side as evil. We trust the evidence for our side without questioning whether it might be faulty, and we hastily reject arguments against our position as deception by our evil opponents. It is difficult for us to evaluate new evidence fairly since we are so afraid of siding with the enemy and we hate to disagree with our allies. We might even become so emotionally involved that we feel violence or other unethical practices are justified. Many bitter political, religious, ethnic, and even family conflicts involve polarization.