How to Catch a Liar

Listening to the podcast Criminal (episode 2) I came across some interesting information. While CIA and FBI agents often pride themselves on their ability to detect lies, their ability to do so never reaches much above 55% in controlled tests. This is barely above what you would get by guessing randomly. Lie detecting machines are better, but nowhere near as good as cop dramas or Jeremy Kyle would have you believe.

Criminal interviewed some researchers who proposed two strategies for catching liars when they were trying to tell a story. In their tests they would have people either try to tell the truth or tell a lie and they were asked something along the lines of “tell us about the best concert you have ever been to.” Strategy number one was to keep asking about what the person saw, heard, smelled etc as they narrated it. For instance they would be talking about their drive to the concert, you would ask “what could you hear?” Liars tend to keep repeating their main story without embellishing on the details. Someone telling the truth tends to put in little bits without effort, ie I was listening to Muse on the radio or I could hear a noisy lorry in the next lane for ages. The liars will keep rushing back to the main narrative.

Part of the strategy was also to ask them walk you through it backwards. Starting at the end isn't much harder for those telling the truth, but for liars it is hard to get all the cause and effect in order. “So talk me through it starting with when you got home from the concert and working backwards.”

Strategy number two is much more elegant but is hard to do in real time. What you do is count how many words there are in the story and then you count how many unique words there are in the story. For example, “the quick brown fox jumped over the lazy dog” has 9 words, but it only has 8 unique words. Stories with a high ratio of unique words to total words tend to be more truthful. False stories repeat themselves a lot and are unadventurous in they synonyms, usually as an attempt to seem consistent. I love how simple a metric this is and the researchers had a success rate of about 80% with it: much higher than humans going on gut instinct.