Peter Wason

Psychological pioneer of human reasoning who rescued his subject from the dark days of behaviourism
  • The Guardian,
Most psychologists design experiments to test hypotheses. Peter Wason, who has died aged 78, sought always to invent tasks to reveal that thinking was not what it seemed. A pioneer of the study of human reasoning, he rescued the subject from the dark days of behaviourism.

Born in Bath, Wason was educated at Stowe school and, after war service as a liaison officer in an armoured brigade, read English at New College, Oxford. It was here that he made his first psychological observation - detecting when he had irritated his tutor, Lord David Cecil. Cecil liked to draw curtains: if they were open, he would close them; if they were closed, he would open them. A good essay, Wason noted, might shift the curtains just once.

After a stint teaching English at Aberdeen University, in 1950 Wason read psychology at University College London, and stayed there until his retirement more than 30 years later.

Nothing in his studies or background - except, perhaps, that his great uncle Cathcart Wason, a Liberal MP, passed the time in the Commons knitting - presaged Wason's unique genius for research. His most famous task came from his reflections on the logic of "if". He maintained that psychologists should never know quite why they were doing an experiment - a precept that worked brilliantly for him, but was hard for his students to emulate.

The task was deceptively simple: the experimenter laid down four cards, bearing on their uppermost faces A, B, 2 and 3 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 an A on one side, then it has a 2 on its other side."

Most people selected the A card and, perhaps, the 2 card. Surprisingly, they failed to select the 3 card. Yet, given an A on the other side of the 3, the assertion is false.

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.

Another task devised by Wason examined inductive inferences, and was provoked by Sir Karl Popper's emphasis on falsifiability in science. It appeared to show that individuals try to confirm their conjectures rather than disconfirm them, though the interpretation of this result also remains controversial.

Yet another early experiment pioneered psychological investigations of language. This showed that individuals are faster to evaluate affirmative assertions than negative assertions. Strikingly, they evaluate affirmatives as true faster than as false, but evaluate negatives as false faster than as true. This study, published in the late 1950s, showed that not all psycholinguistic phenomena could be explained in terms of syntax.

Wason had several other brilliant ideas for experimental tasks, though not all of them worked; the conjecture that participants who had to write in invisible ink would be free to write more fluently was not successful. The editor of a journal was happy to publish it, Wason reported, but only in invisible ink. Yet, no one in Wason's generation, or since, has created so many experimental paradigms that continue to inspire investigation.

He certainly excited students about his research. In his postgraduate seminar, they talked about experiments before carrying them out - and, somehow, found themselves working on problems that also interested Wason. Afterwards, pipe in hand, he would explain where you had gone wrong; his aquiline demeanour fitted your picture of Sherlock Holmes, for whom you had just given a passable imitation of Dr Watson.

Wason published many scientific papers and four books, including one on his passion, chess (in collaboration with WT Hartstone). When he began his research, there was a mere handful of published studies on the psychology of reasoning; his work, and that of his students and critics, has led to a literature too vast for any one reader to master.

Wason and his wife Marjorie met at Oxford, and were married for more than three decades. She predeceased him in 1984; he is survived by two daughters.

· Peter Cathcart Wason, psychologist, born April 22 1924; died April 17 2003

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