Asking them unexpected questions dramatically raises the chance you’ll be able to detect their lies:
We hypothesised that the responses of pairs of liars would correspond less with each other than would responses of pairs of truth tellers, but only when the responses are given to unanticipated questions. Liars and truth tellers were interviewed individually about having had lunch together in a restaurant. The interviewer asked typical opening questions which we expected the liars to anticipate, followed by questions about spatial and/or temporal information which we expected suspects not to anticipate, and also a request to draw the layout of the restaurant. The results supported the hypothesis, and based on correspondence in responses to the unanticipated questions, up to 80% of liars and truth tellers could be correctly classified, particularly when assessing drawings.
Source: “Outsmarting the Liars: The Benefit of Asking Unanticipated Questions” from Law and Human Behavior, Volume 33, Number 2, 159-166
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