“If it corresponds to my knowledge and beliefs, it must be true.”
Belief bias refers to our tendency to rely on our pre-existing beliefs to evaluate a conclusion. This leads us to overestimate the validity of a credible conclusion, independently of its true logical validity . This is one of the most studied and demonstrated phenomena in the field of reasoning. The tendency is accentuated when we are obliged to reason quickly .
Experts in logic differentiate between the logical validity of a given reasoning (or argument), and the veracity of the premises and the conclusion. Logical validity depends on the logical relationship that links the premises and the conclusion within the argument. The veracityof the premises or the conclusion refers to whether the information presented in them is true in the real world. It is important to understand that an argument may be logically valid, but nonetheless contain premises or conclusions that are false, just as it may be logically invalid but arrive at a conclusion that is true.
When we examine the two arguments below, we tend to see argument A as logically valid and to reject argument B, simply because we know that the conclusion in A is true in the real world. However, both of these arguments are logically invalid because their conclusions do not follow from the stated premises.
Premise 1: All flowers have petals
Premise 2: Roses have petals
Conclusion: Therefore roses are flowers
Premise 1: All automobiles have motors
Premise 2: Airplanes have motors
Conclusion: Therefore airplanes are automobiles
According to many theoretical models, the belief bias is the result of heuristic processes that speed up the processing of information. Such processes may be used instead of slow, deliberate reasoning that would need more effort. From this point of view, reasoning that depends on the credibility of the conclusion rather than logical analysis allows one to obtain rapid and often useful results based on pre-existing knowledge about the world, rather than on the logical validity of a proposition.
When belief in a conclusion is in harmony with the logical validity of the inference, belief bias can facilitate reasoning. However, when there is a conflict between them, belief bias may interfere with the reasoning. This can have impacts on all areas of information processing. For example, we rarely search for proof for or against an argument we consider credible, because we overestimate its validity. On the other hand, when presented with a conclusion relating to something about which we have no prior knowledge, we evaluate the arguments in a more neutral manner. When evaluating arguments that are opposed to our beliefs, we judge even more carefully.
Thoughts on how to act in light of this bias
Be aware of our tendency to confuse logical validity with credibility.
Question credible conclusions.
Concentrate on the validity of the proposition.
How is this bias measured?
Tools developed to measure belief bias often ask subjects to evaluate arguments with and without both logical validity and credible conclusions. For instance, in a sequence of 12 propositions, half will be logically valid and half invalid, and half the conclusions will be credible and half not credible. This will result in 6 deductions with conflicts (3 valid but not believable, and 3 invalid but believable), and 6 deductions without conflict (3 valid and believable, and 3 invalid and not believable). A measure of belief bias will be obtained by calculating the difference between correct evaluations of conflictual deductions versus non-conflictual deductions.
This bias is discussed in the scientific literature:
This bias has social or individual repercussions:
This bias is empirically demonstrated:
 Evans, Jonathan. S. B., Julie L. Barston & Paul Pollard (1983). On the conflict between logic and belief in syllogistic reasoning. Memory & Cognition, 11(3), 295-306.
 Evans, Jonathan. S. B. & Jodie Curtis-Holmes (2005). Rapid responding increases belief bias: Evidence for the dual-process theory of reasoning. Thinking & Reasoning, 11(4), 382-389.
Klauer, Karl Christoph, Jochen Musch & Birgit Naumer (2000). On belief bias in syllogistic reasoning. Psychological review, 107(4), 852-884.
Markovits, Henry & Guilaine Nantel (1989). The belief-bias effect in the production and evaluation of logical conclusions. Memory & cognition, 17(1), 11-17.
Oakhill, Jane, Philip N. Johnson-Laird & Alan Garnham (1989). Believability and syllogistic reasoning. Cognition, 31(2), 117-140.
Individual level, Availability heuristic, Need for cognitive closure, Need for cognitive consonance
Émilie Gagnon-St-Pierre is a PhD candidate in social and cognitive psychology at the Université du Québec à Montréal. She is affiliated to the Laboratoire des processus de raisonnement and the Laboratoire Culture, Identité et Langue. She is also co-founder of Shortcuts.
Translated from French to English by Susan D. Renaud.