Liars have to do an awful lot of thinking to lie effectively. Here's how to push them into cognitive overload and catch them in a lie.
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When you're telling the truth, all you have to do is remember what happened. If you're a liar, you have to do quite a bit more thinking. If a liar has too think too much it causes cognitive overload and it becomes a bit easier to tell whether or not they're lying. Dr John R. Schafer, over at Psychology Today, suggests that restating the lie and then asking if it's true is an effective way of bringing about cognitive overload in a liar. Here's an example:
Kermit: Miss Piggy, when you told me you weren't having an affair with Beeker... were you telling me the truth?
While Miss Piggy would probably just throw a fit, in the non-Muppet world a liar would have to take a minute to think about this statement. They have to recall the lie, compare it with your reiteration of the lie, and then figure out if they should answer yes or no. People telling the truth don't need to do this because they're already sure and can answer you quickly. Liars generally cannot. So, if you need to weed out a liar, just give them something to think about.
Poor Man's Polygraph Part 5 | Psychology Today