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The Truth About Lie Detectors

Updated on November 6, 2015

Not Reliable

The polygraph or lie detector as it is commonly known is nowhere near as reliable as some say. And according to many experts, can easily be beaten. Psychologists surveyed on the validity of polygraphs say test results should never be admitted into a court of law.

The American Medical Association has even testified before Congress in support of the 1988 Employee Polygraph Protection Act. Their reasoning was since the machines can’t accurately distinguish between truth and falsehood with healthy people, why expect them to be accurate with individuals having serious medical conditions. There are many reasons someone can fail a polygraph. Nervousness, health problems, surprise and fear top the list.

Easy To Trick

There are thousands of people every year tricked into submitting to a polygraph. NASA has even published studies on why they don't work. These tests can be a source of tremendous anxiety, even for those telling the truth. Simply put, polygraph examinations are nothing more than interrogations and it’s not uncommon to fail them for no reason. The devastating results can be denial of employment or unfounded criminal accusations. Many experts consider them unreliable and virtually all scientists and polygraph administrators admit they are seriously limited. For this reason they are also easy to trick.

A polygraph is a recording instrument which simultaneously monitors changes in a person’s physiological and metabolic processes. Heartbeat, blood pressure and respiration are printed out on a graph. Polygraphs are used by police departments, FBI, CIA, federal and state governments, and many private agencies.

The theory behind the polygraph is when people lie they get nervous about it. Their heartbeat increases, blood pressure rises and perspiration increases. A basis for these characteristics is established by asking questions whose answers are known facts. Any changes from the baseline are taken as an indication of lying.

Is there any proof the polygraph is able to detect untruths? Since the machine measures deviations in blood pressure, respiration rate etc., it is assumed these changes occur in ways trained experts can tell if a person is lying.

There are no scientific formulas or laws which can establish a matching correlation. Is there any scientific proof polygraphs can detect lies using their machine at a significantly better rate than non-experts using other methods? No. There are no machines or experts that can accurately detect when people are lying or telling the truth.

The reason a polygraph can’t be considered a true lie detector is that what it measures can be caused by a variety of things. Nervousness, anger, sadness, embarrassment, can all be factors in altering one's metabolic rates. Even having to use the restroom could be a cause. Other medical conditions such as colds, headaches or neurological and muscular disorders can also cause physiological changes. So, the claim someone can tell when changes are due to a lie or due to other factors has never been proven.

Some people may fear a polygraph test will indicate they are lying when they aren’t. Furthermore, even supporters of the polygraph machine must admit liars can sometimes it.

In many states, polygraph results are inadmissible in a court of law. This is because polygraph tests are known to be unreliable. Skeptics consider polygraphs no more reliable than statements elicited under hypnosis, also not allowed in a court of law in many states.

So, the question becomes, why do government, law enforcement agencies and private sector employers want to use the polygraph? They use them because in some cases the polygraph appears to work. There have been cases when people see they are failing the test and suddenly confess.

On the other hand, it is possible one reason so many use the test although they know it’s unreliable, is because the people taking it think it does.


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    • profile image

      adeonline 5 years ago


    • JY3502 profile image

      John Young 6 years ago from Florence, South Carolina

      Sorry to hear you were another victim Rubbish. A job interview is stressful enough without throwing a polygraph into it also,

    • profile image

      Rubbish 6 years ago

      I recently failed a polygraph for employment. I'm crushed about this because I told the truth. It's a feeling you can't describe being called a liar because of readings on a machine. A poly is up to the interpretation of the tester. They can absolutely sway it anyway they choose. It justifies their exhistence when you fail a poly. Emotions will definitely make the machine react, but it will never be able to tell if your truthful or not. Needless to say I'm getting a good lawer..

    • JY3502 profile image

      John Young 7 years ago from Florence, South Carolina

      Thank you very much.

    • Pamela99 profile image

      Pamela Oglesby 7 years ago from United States

      JY, That is really interesting. I could see where it might not work on a sociopath and they have no remorse. I have also heard that people like CIA operatives are trained to beat the polygraph. I enjoyed your article.