Techniques to trap liars

R. Edward Geiselman, a UCLA professor and the guy who wrote the book on cognitive interviewing (employed by trained law officers) suggests these techniques to ferret out liars:

• Make them tell their story backwards, starting at the end and systematically working their way back. 
• Tell them to be as comprehensive and detailed as they can. This cognitive interview technique increases the cognitive load to push them over the edge. They're busy making up stuff, masking their lying behaviors, and monitoring you. Even professional liars will break under the cognitive load. 
• Ask open-ended questions to get them to provide as much details as possible. For example, ask:   Tell me more about...?", "Tell me exactly...?" "Give me the complete details. 
• Start with general questions, and then get more specific.
• Don't interrupt, be silent, and let them talk. 

For more details on what behaviors are signs of deception, see also the article on Spotting Liars. 

Spotting liars

R. Edward Geiselman, a criminal psychology professor at UCLA best known for his work on interviewing techniques and witness testimony thinks he can spot liars based on studies with students and prisoners.

Here's how you can tell if someone might be deceiving you under questioning:

• They say as little as possible. They keep their stories basic and barebones.
• They justify their statements without prompting. 
• They tend to repeat questions before answering them (perhaps to buy more time).
• They tend to monitor your reactions to see if you are buying their stories.
• They often slow down their speech at first, possibly because they are making up  their stories and watching your reaction; and when they think they have you, they speed up their speech (because they're afraid slow speech is suspicious. In contrast, honest people are OK with slow speech).  
• They dramatically change their speech rate, sometimes within a single sentence. 
• They tend to use sentence fragments more frequently than truthful people; often, they start to answer, back up and not complete the sentence.
• They are more likely to press their lips when asked a sensitive question.
• They are more likely to groom themselves (for example, playing with their hair). 
• They tend to gesture towards themselves. Truthful people tend to gesture outward. 
• When you challenge them about details, they generally won't provide more specifics.
• They tend to not want to look away (except briefly), unless your questions require intense concentration.
• Their attempts to deceive becomes even more obvious when they try to mask these normal reactions to lying. 

But don't start leaping up and accusing people of lying just because they meet just one or two of these suspicious behaviors. Consider the whole pattern. 

Here's how you can tell when someone is probably being truthful. 

• They don't change their rate of speech. 
• They gesture outwards or away from their body. 
• When you challenged them about details, they will often deny that they are lying and explain even more. 
• When you ask them a difficult question, they will often look away because the question requires concentration.