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Lie Detectors: Silly, stupid, nonsense…

I had a debate with a friend at lunch today about lie detectors.  He argued that lie detectors, even if they’re mostly nonsense, are at least valuable because they “sometimes” detect a lie.  I said lie detectors are not just nonsense, but actually really powerful tools in the hands of police departments.  It’s not because lie detectors can tell if you’re lying or not.  They can’t.  It’s because most people think that lie detectors can tell whether they are lying, and so they will tell police things!

Most people who swear up and down that polygraph machines are accurate are themselves polygraph analysts.  Of course an analyst will say that the polygraph machine is accurate, just as a witchdoctor will say that voodoo medicine actually cures people.

But most scientists agree that polygraph machines are not at all effective at detecting lies or truth.  In fact there are numerous reasons to doubt that polygraph machines can tell the truth, most of them are described on

The important thing to remember is that the results of polygraph exams are not admissible in court because polygraph exams are not reliable.

So why do police agencies use polygraph machines?  For two reasons: First, the average citizen believes that the polygraph is accurate.  Consequently, it’s a well known fact that people will admit to things during a polygraph examination, believing that if they lie, the detector will reveal the lie.  As a result, even though the polygraph machine is a piece of junk, it’s effective because it creates the illusion that it can detect a liar. So it gets suspects talking, and a police detective would much rather have a talking suspect, than a silent suspect who requests a lawyer.

Second, polygraph machines are used because they offer another opportunity for the police to question the suspect.  Police can ask all kinds of questions, the suspect can say that he’s innocent, and the police officer can say: “Well, how about you take a lie detector test,” holding out hope to the suspect that if the suspect “passes” the test, he’ll be able to go home free.

The problem is that in many cases, the suspect never “passes.” That’s because the polygraph exam is designed so that the suspect will “fail” even if the suspect has told the absolute truth.  At the end of the exam, the detective can come in to the suspect, say that the suspect has failed the exam, and say “Well, if you’re telling the truth, why did you fail the exam.”  And the police can point to squiggly lines on the “exam,” which may indicate lying, but also may indicate a nervous suspect.  And who isn’t nervous talking to police?  Everyone is nervous being questioned by police, even the Pope himself.

The person then will panic and try to think of some reason why the lie detector said he was lying, and make additional statements that sound incriminating, even when they aren’t.  I have worked with individuals who, in their desperation to explain why the lie detector exam found that they lied, have written out “explanations” that read like confessions.

At that point, even though the lie detector test is a bogus test, it has achieved its purpose for the police: it has gotten the suspect to “confess.”

The truth of the matter is that lie detectors are junk, and just as someone under investigation should refuse to talk to police unless an attorney is present, someone under investigation should refuse to take any polygraph exam unless an attorney is present.

Refusing to take a polygraph exam is NOT an admission of guilt.  It is your right to refuse to talk to police without a lawyer present.  It is also your right to refuse to take a polygraph exam.  And you should say, “Thanks, but no thanks. I’d like a lawyer, please.”



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