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"Describe, in simple words, only the good things that come into your mind. About your mother."

If being a PI has taught me nothing else, it's this Single Truth of Human Nature: people lie. And, being a PI, people lie to me.

A lot.

They do it for all sorts of reasons, some nefarious, others not. Sometimes people lie because they think it's what a friend should do.

Remember Crystal?

Well, she didn't change her mind and decide to talk to me after all. No. But a friend of hers, Dan (not his real name), volunteered to provide a statement as he was the first person she saw after her accident.

I really only had a few questions I really wanted to ask Dan, but padded out the interview with some minor details. As he was a friend of the POI (Person of Interest), I was mindful that he was not exactly about to incriminate her in any wrongdoing.

I got the patter going, adopting my usual mostly-informal interviewing style. I ran through the questions of where and when and he answered all in an easy and relaxed manner.

Until I asked: "In your opinion, was Crystal intoxicated?"

Unbeknownst to him, I'd already established that Crystal had a history of driving whilst intoxicated (and had one charge to that effect), had crashed - and written off - at least 2 other vehicles in the past few years, had been seen drinking the night before the accident, and had been thrown out of the bar for being too drunk.

Dan's reaction to the question was the best part.

He closed his eyes heavily for a moment then looked away to the right before meeting my gaze and saying 'No'.

A classic tell, or non-verbal cue.

Not only the pronounced pause before answering the question, but also that he looked away; atypical to his behaviour while answering the other questions.

Dan was lying.

Do I know that for a fact? Despite what crappy TV programmes like Lie To Me or The Mentalist may try to portray, no, I just have a firm belief based on balance of probabilities.

Does that mean there's nothing to non-verbal cues or body language etc?

No.

Understanding basic principals of nonverbal communication is fundamental to an investigator who deals with people and there are several good books available on the subject. My favourite two would be Joe Navarro's What Every Body Is Saying and Allan Pease's The Definitive Book of Body Language. Navarro is an ex-FBI counterintelligence agent and his book primarily deals with detecting deception. The Pease book is more of a basic guide to body language, not focused specifically on lying but a general grounder.

And at the other end of the spectrum, you have crap like this from a 'Relationship Coach' who sagely advises that you can tell, 'spot-on', he's a married man cheating on his wife if he never runs out of cash.

Riiiiight.

Well, it's going to be another few years before we'll be able to carry out Voight-Kampff tests but I look forward to the day.

"My mother? Let me tell you about my mother"