Wason's Four-card Task
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[edit] Definition
The Four-Card Task, as postulated by Peter Wason and Philip N. Johnson-Laird in 1972, is a seemingly simple logic question involving four cards. The cards have a letter on one side and a number on the other, and are presented thus:
The other side of the cards is not shown.
A claim is then made: "If a card has a vowel on one side, then it has an even number on the other side." The question is, which of the cards would one need to turn over to see if the statement is true or false?
Wason and Johnson-Laird gave this test to 128 University-educated adults. The most common answer given was "A and 4," by 59 students (46%), with "A" being a close second (42, or 33%). Only 5% gave the proper answer.
[edit] The Correct Answer
Most people easily hit upon "A" as being one of the necessary cards to turn over. If there is an even number on the other side, then the claim is at least partially confirmed. On the other hand, if there is an odd number on the other side, then the claim is falsified.
The next tendency is to turn over the "4" and see if there is a vowel on the other side. But this really doesn't tell you anything; the claim is, "If a card has a vowel on one side, then it has an even number on the other side." But finding a card with a consonant on one side and an even number on the other doesn't falsify the claim, as it doesn't say that only vowels have even numbers on the other side. Likewise, turning over the "D" tells you nothing, even if you do find an even number on the other side.
The other necessary card to turn over is the 7. In order for the claim to be true, there must be a consonant on the other side. Finding a vowel would falsify the claim. So, the correct answer is "A and 7."
Most people are fooled by this task because of confirmation bias. They attempt to confirm the claim, while forgetting that it is also important to try and falsify it. Finding a pairing of vowel and even number does nothing to support the claim; it's the failure to find a vowel with an odd number that confirms it.
At The Amazing Meeting 1, 26 people out of 156 answered a version of the Four Card problem correctly, showing that even skeptics are susceptible to this effect.
The original problem was modified by Wason to the Two Card problem. The original premises were stated the same as the Four Card problem, but the question was worded, "Which one of these cards would you turn over?" The choices were "1" and "2". Wason said that this resulted in a majority of people getting it right, but others have observed that a significant number of college students still get it wrong.
[edit] References
Psychology of Reasoning: Structure and Content by P.C. Wason & P.N. Johnson-Laird.
