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Cognitive Biases
Common thinking errors and how they shape decisions
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Cognitive biases are predictable patterns of thinking that can lead people away from accurate judgment. They matter because students, scientists, doctors, voters, and consumers all make decisions using limited time and incomplete information. Biases are not signs of low intelligence, but shortcuts the brain uses to reduce mental effort. Learning about them helps people reason more carefully and evaluate evidence more fairly.
Many biases come from heuristics, which are fast mental rules that often work well but sometimes fail. For example, a vivid recent event can feel more common than it really is, and evidence that agrees with a belief can seem stronger than evidence that challenges it. In science and daily life, reducing bias often means slowing down, comparing alternatives, checking base rates, and asking what evidence would change a conclusion. Good thinking is not bias-free thinking, but thinking that includes safeguards against predictable errors.
Key Facts
- Cognitive bias = a systematic error in thinking that affects judgment or decision-making.
- Confirmation bias occurs when people favor evidence that supports what they already believe.
- Availability heuristic: judged likelihood increases when examples are easy to remember.
- Base rate thinking uses prior probability: P(hypothesis | evidence) depends on P(hypothesis) and P(evidence | hypothesis).
- Bayes' rule: P(A | B) = P(B | A)P(A) / P(B).
- Expected value can improve decisions: EV = probability x payoff.
Vocabulary
- Cognitive bias
- A cognitive bias is a predictable error in thinking that influences how a person judges information or makes decisions.
- Heuristic
- A heuristic is a quick mental shortcut that helps people make decisions with limited time or information.
- Confirmation bias
- Confirmation bias is the tendency to notice, seek, or trust information that supports an existing belief.
- Availability heuristic
- The availability heuristic is judging how likely something is by how easily examples come to mind.
- Anchoring
- Anchoring is relying too heavily on the first number, idea, or piece of information encountered.
Common Mistakes to Avoid
- Thinking bias only affects other people. This is wrong because cognitive biases are normal features of human thinking and can influence experts as well as beginners.
- Treating a vivid example as strong statistical evidence. This is wrong because memorable stories can be rare, while less dramatic data may better represent the real pattern.
- Ignoring base rates when new evidence appears. This is wrong because the starting probability of an event strongly affects the final probability after evidence is considered.
- Assuming confidence equals accuracy. This is wrong because people can feel very certain when their reasoning is affected by anchoring, confirmation bias, or incomplete information.
Practice Questions
- 1 A student estimates that a test score will be 90 because the first practice problem looked easy. The actual average score for the class on similar tests is 72. If the student revises by averaging the anchor and the base rate, what estimate do they get?
- 2 A disease affects 2 percent of a population. A test is 95 percent accurate for people with the disease and has a 10 percent false positive rate for people without it. Out of 10,000 people, about how many true positives and false positives would you expect?
- 3 A person reads three online reviews of a product and ignores a large survey of 2,000 buyers because the reviews feel more personal. Identify the likely bias and explain what better reasoning would include.