Bryan Caplan  

Detect Lie

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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(True|seems True)=

[(P(seems True|True)*P(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:

  • "[O]bservers overestimate the likelihood of being able to detect deceit by paying attention to someone's behavior, and... underestimate the possibility of catching liars by paying attention to their speech content."

  • Contrary to popular opinion, gaze aversion does not predict lying. This may be because almost everyone believes it does, leading even inexperienced liars to try not to avert their gaze.

  • It is easier to tell if someone is lying if you are familiar with their ordinary (non-lying) speech and behavior. But actually meeting or intimately knowing the suspected liar does not give an additional benefit. In other words, you are more able to detect lies in your friends because you know how they normally act, not because people look guiltier when they lie to their friends.

  • Men and women lie equally often, but men tell more self-oriented lies and women tell more "other-oriented lies, particularly with regard to other women." (Self-oriented lies are designed to gain an advantage for the liar; other-oriented lies are designed to help someone else - usually the listener).

    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."


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    COMMENTS (5 to date)
    SBPete writes:

    Why am I not shocked to find out that BC played DD in HS? :)

    Brad Hutchings writes:

    TMI, Bryan. ;-)

    Timothy writes:

    Responding to "thank you" with "False!" is likely just to convince others that you have tourrette's.

    Lancelot Finn writes:

    A deliberate, conscious lie to another person is one thing. Other cases of lying are subtler and more ambiguous.

    (1) Is it lying to go through the motions of one's born religion, going to church, reciting the creeds, when in fact you have lost your faith in God?

    (2) Is it lying to say "I know the Bible is the Word of God," if the evidence you have in support of that proposition falls radically short of the evidential standards you ordinarily require in order to refer to one of your beliefs as "knowledge"?

    (3) If so, is it also lying for a materialist, whose ontology apparently acknowledges the existence only of particles, energies and forces, to speak of "moral" principles at all, or to call behaviors "right" and "wrong?"

    (4) Is it lying for a salesman to present only the upside of a product, and to tell every potential customer that this product is worth their money when, actually, he thinks that most of them would do better to spend their money on something else?

    (5) If so, then what if the salesman believes that every customer would do well to buy his overpriced trash, but he believes this only because he has deliberately suppressed the mental faculties by which his mind ordinarily distinguishes true from false propositions, for the sake of economic gain? Is that lying?

    Telling the truth is not an on-off switch. It's more like a staircase. I wonder how a more nuanced account of lying and truth-telling would affect Vrij's study.

    Randy writes:

    The purpose of the brain is to reason a way to obtain what it needs or wants.

    A lie, like all the other methods the brain may choose, is good if it works, and bad if it does not. And morality? - morality too is a method of the brain.

    The ends do justify the means. Those who say otherwise have simply not yet accounted for all the ends.

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