Peter Cathcart Wason (1924 - 2003)

Peter Cathcart Wason was an English cognitive psychologist, who worked on the psychology of reason. He made great progress in explaining why people make certain consistent mistakes in logical reasoning. He designed logical problems and tests to demonstrate these processes, for example the Wason selection task, the THOG problem and the 2-4-6 problem.

The task was deceptively simple: the experimenter laid down four cards, bearing on their uppermost faces A, S, 4 and 9 respectively. The participants knew that each card had a letter on one side and a number on the other side. They had to select just those cards that they needed to turn over to find out whether the following assertion was true or false: "If a card has a vowel on one side, then it has an even number on the other side."

This selection task, which Wason tried out during a year at the Harvard centre for cognitive studies in 1963, was not published until three years later. It has launched more investigations than any other cognitive puzzle. To this day - and to Wason's delight - its explanation remains controversial. Its continued popularity among researchers is borne out by its current ban from a major psychological journal.

Here's the little test: Pictured to the right are four cards. Each card contains a letter on one side, and a number on the other. Which cards must you turn over to prove the following statement false? "If a card has a vowel on one side, then it has an even number on the other side."

Researchers Peter Wason and Philip Johnson-Laird gave a similar test to 128 college-educated subjects in 1972. The most frequently given answer was "A and 4," (46 percent), with "only A" the second most popular (33 percent). Only 5 percent gave the correct answer, which is "A and 9."

It's fairly obvious that you must turn over the A-card: if there is an odd number on the other side of the card, you have proven the statement false. The popular tendency is to also turn over the 4-card to see if there is a vowel on the other side. However, the statement does not say an even-numbered card cannot have a consonant. For the same reason, turning over the S-card proves nothing, since the statement makes no claims about cards with consonants. On the other hand, turning over the 9-card and finding a vowel proves the statement false.

Why does this test fool so many people? The answer is a common act of reasoning called "Confirmation Bias". Research shows that most people prefer confirming something rather than proving something wrong. Therefore, we gravitate toward confirming our beliefs, even when our task is to disprove something. (By turning over the 4-card we're trying to find further confirmation of the statement.) In the process, we make some flawed assumptions...