From Mathematical “truth serum” promotes honesty:
Now, Drazen Prelec, a psychologist at the Massachusetts Institute of Technology in Cambridge, US, has devised a scoring system, or ?Bayesian truth serum? to encourage people to divulge their honest opinions.
First you ask a straightforward question — “Will you vote in the next presidential election?” or “Have you had more than 20 sexual partners in the last year?” Then you ask the respondent to estimate of how many other respondents would answer the same way.
Prelec says if people truly hold a particular opinion, they tend to give higher estimates that other people share it. So if someone did have more than 20 recent sexual partners — but lied about it — that person would probably assume a higher rate of such behaviour in general than someone who had not had so many partners.
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For example, he describes a situation where two paintings are viewed by a group of 10 people who are then asked, privately, to pick their favourite. Seven people say they prefer painting A, while three vote for painting B. If, on the second question, all 10 people said they thought everyone else would prefer painting A, then those three people expressing a personal preference for painting B might be thought of as a safer bet for having told the truth. That is because, argues Prelec, despite what they thought was more popular, those individuals still chose the other painting.
(Hat tip to Marginal Revolution.)