Belief Bias

What is belief bias?

Belief bias occurs when the believability of a conclusion influences a judgment about whether an argument is logically valid. A valid argument is one whose conclusion follows from its premises. That is a different question from whether the conclusion sounds true.

Belief bias example

An invalid argument with a plausible conclusion may be accepted, while a valid argument with an implausible conclusion may be rejected. In a syllogism, the task is to evaluate the relationship between the premises and conclusion.

For a concrete illustration: “All dogs are mammals. All poodles are mammals. Therefore all poodles are dogs.” The conclusion is believable, but it does not follow from those premises alone. Sharing a category does not establish that one group belongs to the other. Accepting the argument because you know poodles are dogs would confuse truth with validity.

What the research found

Evans, Barston, and Pollard found effects of both belief and logic in three experiments. The result is not that people ignore logic entirely. Their judgments can be influenced by both the argument’s structure and what they already believe.

A useful check

Ask two questions separately: “Does this follow from the premises?” and “Do I believe it?” Prior knowledge is useful in many decisions. In a validity task, it becomes a problem when it replaces the required logical test.

Source: On the conflict between logic and belief in syllogistic reasoning.