A problem of concept formation in which people are given the ordered triple of numbers (2, 4, 6) and are invited to try to generate further examples of triples conforming to an unspecified rule that the example obeys, trying to home in on the correct rule on the basis of simple right/wrong feedback after every guess. The actual rule is any ascending sequence, but the example invites people to form more specific hypotheses, such as ascending even numbers or numbers ascending by equal intervals. It was introduced in 1960 by the English psychologist Peter C(athcart) Wason (1924–2003), who found that people tend to try examples consistent with such more specific hypotheses, such as (10, 20, 30) and seldom try examples that would refute them such as (10, 11, 30), thus manifesting confirmation bias and failing to find the right answer but often becoming increasingly convinced of the rightness of their incorrect hypotheses. Also called Wason's 2-4-6 problem. See also problem solving.