04
Aug
09

How accurate is a Polygraph Test?


Was watching this series a couple of days back called “Sach ka Samana”, a desi version of “Moment of Truth”. The serial asks “probing” questions to the player in the hot seat and verifies the truthfulness using a polygraph test and then make  a big deal out of it.

What I am here trying to figure out is how efficient Polygraph Tests are and how do we beat them?

This educational site first describes the types of lying and then goes to describe how would polygraph tests work.

There are different types of lies that we are capable of expressing. One of the more subtle types are lies of omission For example, you meet your best friend and her sister for lunch, and you can’t help but notice that her sister has a very strange habit that annoys you. When you talk about the lunch later but don’t mention her sister’s annoying habit, you’re omitting an important piece of the puzzle, and that is considered a lie. A lie of commission is the daily placating statement of “Just what I needed!” when you receive a useless present, or the automatic “Fine” response to “How are you?” These everyday lies generally aren’t malicious enough to merit a polygraph test, and leaves the question of whether or not these statements look the same in the brain as lies that threaten a marriage or National Security.

This is the reason why the polygraph machine method of lie detection is so inaccurate and outdated. The polygraph’s purpose is to measure the physiological characteristics that come with lying, as previously mentioned. According to the American Polygraph Association, they test the sweat glands and cardiovascular activity of the subject, but deny that voice stress testing is accurate.

However there have been various neurological tests using a functional-MRI scanner that are simply better. Lying requires a change in behavior, and therefore the brain as well. Assumed signs of a liar can be easily trained away, especially in cases of important secrets regarding National Security. Very few people can identify a liar just based on sight, and the polygraph machine would identify extreme anxiety as lying.

Myths that surround lies mostly concern the definition of a “liar”. It is assumed that when a person lies, there is a physiological change in their demeanor. Specifically, a liar may squirm, sweat, or fidget. Strangely there are assumptions of just the opposite – that a liar will hold very still and make direct eye contact. Everyday we lie, and we do it well, whether we lie to a jury of our peers or simply tell a friend that their hair looks nice when we think the contrary. If we started to sweat and fidget every time we faked a compliment, our relationships would quickly be sabotaged.

The Indian Law does not accept the outcome of the Polygraph Test like in this case. However, it helps investigators get hands on important leads.

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3 Responses to “How accurate is a Polygraph Test?”


  1. 1 Ruchika Mehresh
    August 4, 2009 at 12:43 pm

    Polygraph tests are not at all accurate. Except for the things that you mentioned in here, polygraph tests are susceptible to be wrong for the kind of questions that these people ask on the show. For the questions like if a person will do such a thing ever, or how do they feel about a certain thing – these questions fall into a grey region and whatever the person will answer, it will most likely be caught as a lie. Besides, no polygraphic test is known to have 100% efficiency.

    I don’t mind them using the test for the show, but they also should talk about the probability of inaccuracy in this case. It is sheer ignorance the way the host conveys to the audience that this test is the absolute test of truth and lies.

    The show must definitely warn about this, so the people who come on the show do not suffer the consequences long after they are done on it.

  2. August 6, 2009 at 4:14 pm

    When you see a polygraph test given on a television show, you must remember that the show’s primary interest is entertainment. A polygraph test is conducted far differently in a “real” situation. For example, scientific research has shown us that a single-issue exam is the only type of test that yields high accuracy rates. Some examples of single issues are, “Did you rob that bank?” or “Did you shoot John Doe?” or “Did you have sex with Jane Doe?” A serious professional polygraph examiner would never fire off question after question about numerous unrelated issues. You should also know that a polygraph test takes about 2 hours.

    More than four decades of high quality scientific research has shown that properly administered single-issue polygraph tests are highly accurate. And by the way, research has also shown that no one can beat a polygraph test that is given properly by a skilled professional examiner.


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