Bias 4: Reasoning Fallacy


Reasoning Fallacy

Answer 1: The answer is 47 ( the patch was twice as small on the 47th day – half the size of the lake). Most people answer 24.

Answer 2 : Most people, even in famous American universities such as MIT or Harvard, answer 10 ct for the first question (the right answer is 5 ct) and ‘’right’’ for the second question but it is the wrong reasoning.

Reasoning FallacyWhy? In the first question, people tend to jump from the uncommon question ”costs £1.00 more than” to the common question ”costs £1.00”.

Answer 3: 5 minutes. But most people tend to say automatically 100 minutes as our mind is used to symmetrical reasoning.

What do these small games show? That it is not always a good idea to use heuristics or intuition to solve problems. Numerous mathematical reasonings are counter-intuitive and in such cases it is better to use our analytical systems.


Frederick, S. (2005). Cognitive reflection and decision making. The Journal of Economic Perspectives, 19(4), 25-42.

Tiphaine Saltini


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