There is a false belief that polygraphs (the so-called ‘lie detectors’) can detect if someone is lying. Quite wrongly, the press have often called for its use whenever they need to see if a politician is lying or not.
There was a frequent call for its use in the Monica Lewinsky saga,for example. Take a look here:
Basically, medical personnel will tell you that when someone’s under stress, the body develops a ‘flight or fight’ reaction which causes the heart-rate to rise, blood pressure to climb, breathing to accelerate and palms to sweat. The polygraph assumes that psychology drives physiology and attempts to measure the above changes when a person is lying. Hence, it was thought that investigators could tell if a person was lying if the response to the above changes were excessive.
But criminals and even terrorists have gotten wiser : they would train controlling their body response at all times to questioning and even resort to tongue-biting and anal-tightening exercises to artificially create changes in body response. So much so, law enforcement agencies now consider these lie-detectors as “useless” nowadays.
Detecting when someone is lying also becomes difficult with pathological liars. As a medical student, we were taught that this was not a clinical diagnosis. You cannot find it in the ICD classification of diseases. A pathological liar can be defined as someone who often embellishes his or her stories in a way that he or she believes will impress people, often to the extent that they will believe in their own (false) story.
Here are some ways to tell someone is a pathological liar (contributed by WikiAnswers)
- They lie about even the smallest things. For example, saying “I brushed my teeth today,” when they didn’t.
- They add exaggerations to every sentence.
- They change their story all the time – fools people at first but once they get to know him/her, no one believes anything they ever say.
- They act very defensively when you question their statements.
- They believe what they say is true, when everyone else knows it isn’t.
- They tend to be extremely manipulative and may have a personality disorder.