Proof-Positive Cards
In one experiment participants were shown 4 cards, each of which had a letter or number facing up: A, B, 2, and 3. They were told that the cards had a letter on one side and a number on the other.
Participants were then asked, by judiciously turning over the minimum number of cards, to determine whether “all cards with a vowel on one side have an even number on the other.”
The common response was to turn over the A and 2 cards. The 2 card was uninformative: a vowel on the flip side would confirm, but a consonant would be irrelevant.
The 3 card was rarely turned over, even as it could be dispositive: a vowel on the other side would disprove the hypothesis.
Besides illustrating bias toward confirmation, this experiment showed that the tendency to seek information consistent with a hypothesis does not stem from an interest in its truth. Instead, we blithely accentuate the positive.
