The Lie Detector

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How many of us have seen the movie ?Meet the Parents??  Don?t worry if you haven?t but for those who did and cannot quite remember the movie, you might probably do so when you see the photograph.  Alright in this movie, the girls father played by Robert DeNiro uses a lie detector to find out if his would be son-in-law played by Ben Stiller is lying about his identity.   Well, we have seen many such instances across in movies or on TV where someone takes a lie detector test.  When I first heard about a lie detector, I thought it must work on either chemical or physical response to the question that were asked.  How then does a lie detector really work? Are lie detectors efficient in detecting lies?  Let us find out what are the signs of lying before we learn how a lie detector works.  This is really interesting.  Who knows you may pick someone telling lies based on the certain behavior of an individual which are detailed below.


Body Language of Lies:



  • Physical expression will be limited and stiff, with few arm and hand movements. Hand, arm and leg movement are toward their own body the liar takes up less space.
  • A person who is lying to you will avoid making eye contact.
  • Hands touching their face, throat & mouth. Touching or scratching the nose or behind their ear. Not likely to touch his chest/heart with an open hand.

Emotional Gestures & Contradiction



  • Timing and duration of emotional gestures and emotions are off a normal pace. The display of emotion is delayed, stays longer it would naturally, then stops suddenly.
  • Timing is off between emotions gestures/expressions and words. Example: Someone says “I love it!” when receiving a gift, and then smile after making that statement, rather then at the same time the statement is made.
  • Gestures/expressions don?t match the verbal statement, such as frowning when saying ?I love you.?
  • Expressions are limited to mouth movements when someone is faking emotions (like happy, surprised, sad, awe) instead of the whole face. For example; when someone smiles naturally their whole face is involved: jaw/cheek movement, eyes and forehead push down, etc.

Interactions and Reactions



  • A guilty person gets defensive. An innocent person will often go on the offensive.
  • A liar is uncomfortable facing his questioner/accuser and may turn his head or body away.
  • A liar might unconsciously place objects (book, coffee cup, etc.) between themselves and you.

Verbal Context and Content



  • A liar will use your words to make answer a question. When asked, ?Did you eat the last cookie?? The liar answers, ?No, I did not eat the last cookie.?
  • A statement with a contraction is more likely to be truthful: ? I didn’t do it? instead of ?I did not do it?
  • Liars sometimes avoid “lying” by not making direct statements. They imply answers instead of denying something directly.
  • The guilty person may speak more than natural, adding unnecessary details to convince you… they are not comfortable with silence or pauses in the conversation.
  • A liar may leave out pronouns and speak in a monotonous tone. When a truthful statement is made the pronoun is emphasized as much or more than the rest of the words in a statement.
  • Words may be garbled and spoken softly, and syntax and grammar may be off. In other
    words, his sentences will likely be muddled rather than emphasized.

Other signs of a lie:



  • If you believe someone is lying, then change subject of a conversation quickly, a liar follows along willingly and becomes more relaxed. The guilty wants the subject changed; an innocent person may be confused by the sudden change in topics and will want to back to the previous subject.
  • Using humor or sarcasm to avoid a subject.


Wasn?t that interesting enough?  I can bet after reading the above points, you will be experts (probably an exaggeration) in identifying liars around you. 


Why would we need a lie detector device in the first place?  We lie for many reasons, some good and some bad.  For all those bad reasons which can either cheat a system or perform hurt to another person, when proof is not available to the law enforcement officials, the best way to find out if a person is innocent or guilty is to have a lie detector test taken.  Although this is one of the effective tools out there, it is certainly not the only tool to make a final conclusion on a crime event.  Let us see how the lie detector works.


The modern lie detector ? the polygraph records several different activities at the same time. Typically it involves a pneumograph, a sphygmomanometer and the galvanic skin response.



  • The pneumograph involves a hose, which is pleated like an accordion, that is put around the suspect’s chest. It records the depth and rate of respiration.
  • The sphygmomanometer is merely a blood pressure cuff.   This is nothing but our body?s physical response.
  • The galvanic skin response (GSR) is related to the electrical resistance of your skin. When most people lie, their anxiety increases and reduces the electrical resistance of their skin. The GSR measures these changes in resistance.

Lie detectors only indicate changes in arousal.  From the arousal changes, lying is inferred.


Essentially, the polygraph ? lie detector ? only measures changes in physiological arousal. The assumption is that lying increases anxiety that, in turn, is indicated by changes in physiological levels of arousal. The polygraph operator will first get a measure of baseline physiological levels ? the levels of functioning typical of the person when not under stress. Lies are identified when there are changes from this baseline level.


If a person is suspected of a crime, he will typically not be asked, “Did you do it?” This type of question may even cause arousal in innocent suspects. Usually the operator asks questions that would only create stress in a person who is familiar with the crime. If a stolen object was found in a locker, the mention of the locker will most likely show changes in arousal in a guilty person ? but not an innocent one.
The role of the polygraph operator is very important. He must read the charts and interpret which changes indicate lying.  There are only two people in the room during a polygraph exam — the person conducting the exam also called polygraph operator and the subject being tested. Today, some polygraph examiners prefer to be called forensic psychophysiologists (FPs). Because polygraph examiners are alone in the room with a test subject, his or her behavior greatly influences the results of the exam.


The forensic psychophysiologist has several tasks in performing a polygraph exam:


Setting up the polygraph and preparing the subject being tested 
Asking questions 
Profiling the test subject 
Analyzing and evaluating test data


How the question is presented can greatly affect the results of a polygraph exam. There are several variables that an FP has to take into consideration, such as cultural and religious beliefs. Some topics may, by their mere mention, cause a specific reaction in the test subject that could be misconstrued as deceptive behavior. The design of the question affects the way the person processes the information and how he or she responds.


In 1996, the journal of General Psychology looked at 41 criminal cases and found that control question tests were 95% accurate.  Polygraphy is therefore most effective when the psychophysiologist can structure questions around knowledge known only to the person who committed a crime.

Author: Allen Martis- USA


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