A simple but tricky logical puzzle introduced in 1966 by the English psychologist Peter C(athcart) Wason (1924–2003) and widely used to study problem solving. Four cards are laid on a table with their uppermost faces showing the following letters and numbers: E, K, 4, 7. The respondent is told that each card has a number on one side and a letter on the other. The following rule may or may not be true: If a card has a vowel on one side, then it has an even number on the other side. Which cards would have to be turned over to determine whether or not the rule is true? The vast majority of respondents choose only the card showing E, or the card showing E and the card showing 4. The correct answer is E and 7. If the card showing E turned out to have an odd number on the other side, or if the card showing 7 turned out to have a vowel on the other side, that would disprove the rule. But the card showing 4 is irrelevant, because the rule does not state that a card with an even number on one side must have a vowel on the other, and therefore turning over this card could neither prove nor disprove the rule. The failure of most respondents to select the card showing 7, which could disprove the rule, and their tendency to select the irrelevant card showing 4, were interpreted by Wason as evidence of a confirmation bias. According to some researchers, errors in this task may also be due partly to a matching bias—a tendency to focus attention on evidence that contains the letters and numbers mentioned explicitly in the rule that is being tested. The problem can be framed in realistic rather than abstract terms. In one influential Italian experiment, participants were shown a sealed and an unsealed envelope face down, and an envelope with a 50-lire stamp and an envelope with a 40-lire stamp face up, and were asked which envelopes would have to be turned over to prove or disprove the rule: If a letter is sealed, then it has a 50-lire stamp on it. This is the identical problem; but when it is presented in this way, most people solve it easily—a classical framing effect. Also called Wason's four-card selection task or simply the four-card problem.