In the game Dungeons & Dragons, there is a magic spell called detect lie (or at least there was back in the first edition). A couple of my favorite high school gaming sessions revolved around the player characters flinging accusations at each other – and immediately finding out which one was lying.
According to psychologist Aldert Vrij’s excellent book Detecting Lies and Deceit, those of us without magical powers have a long way to go. Not only are laymen bad at detecting lies; so are policemen, customs agents, and other people who do it professionally.
How bad? There are many experiments where subjects are required to lie and tell the truth with equal probability. Observers then try to sort fact from fiction. A representative result: People correctly identify truths 70% of the time, but correctly identify lies only 50% of the time. If you know Bayes’ Rule, you can use this information to calculate the probability a statement is true given that it seems true:
[P(seems True|True)*P(True)+P(seems True|Lie)*P(Lie)]
which by my calculations=58.3%. A little better than random guessing, but not much.
A few other juicy morsels from Vrij:
Call me a liar, but Vrij’s book convinced me that I am almost pathologically honest. I would certainly lie to save an innocent person’s life. But the common sense moral truism that it is wrong to lie still seems compelling to me, and I adhere to it. (Yes, I just averted my gaze from the monitor, but that proves nothing!) Vrij argues that lies lubricate social relations, but there are honest ways to do the same thing. Most social pleasantries are non-propositional anyway; if someone says “Thank you,” you cannot coherently respond “False!”
And it is never false to smile and say “Mmm hmm.”