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Liar, Liar No More
A mathematical program draws out honest opinions
By Tracy Staedter

Do you like Joan Miró’s painting Bleu II? Even if you think it looks like something your two-year-old could have created, you might—because you want to appear cultured—answer yes. MIT psychologist Dražen Prelec has developed a mathematical “truth serum” that promotes honesty in people’s answers and opinions.

Prelec asks individuals in a group of 10 or more to answer a question and to predict how many others in their group will give the same answer. The sessions are run like a game, with Prelec awarding higher scores to people whose answers are more common than the group predicts. Prelec justifies this scoring system with his theorem which says that a true belief in a particular opinion is a clue that there are more people with that opinion than the group as a whole will predict. He feels that people will be motivated to tell the truth by their desire to get high scores.

Say, for example, that out of a group of 100 people, 90 claim to like Bleu II, but the average prediction is that 95 people will like the painting. In that case, the scoring system would give higher scores to the minority of 10 who express dislike, because their answers are twice as common as the group had predicted. (This is particularly true in situations where people believe there is a socially “correct” answer.) So, people who desire to get high scores should tell the truth, whether or not they think their opinions are widely shared.

Even though the people who truthfully answer that they like Bleu II receive a lower score in this instance, their chances of earning high scores rise as they continue to answer additional questions truthfully. Conversely, “those who are lying will find themselves more often in the loser camp,” says Prelec, as they will misrepresent their group’s predictions.

Prelec says his method would work for gathering opinions about art, humor, products, and political candidates. He envisions a Web-based game or application that would teach people to distinguish between what they truly believe and what they think they should believe.

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