How does polygraph test work? Is it accurate in detecting lies?

Polygraph is a scientific term for what is popularly known as a lie detector. It is an instrument used in criminal investigations to find out if the person being prosecuted is telling the truth. Many employers, too, use lie detectors during job interviews to check the integrity of job applicants’ answers to the interview questions.

An examiner in a polygraph test starts with very simple questions (“What is your name?”, “Do you reside at…?”) and moves on to slight stress-inducing questions such as those involving personal details. This is necessary to establish the baseline of physiological responses in the subject because different people experience a different level of stress while answering a particular type of question. By running this routine of escalating questioning, the examiner would know what the normal level of stress experienced by the subject being examined is, which is then compared with the unusual spikes in the polygraph.

Accuracy of polygraph tests is still a subject of debate as the opinions are widely divided. There are ways in which polygraph can be “fooled”. These involve:

1) Using drugs while under the test to neutralize emotions.

2) Biofeedback training.

3) “nail-in-the-shoe” technique in which the subject would press his foot against a sharp nail hidden in the shoe to cause pain which would skew the polygraph results, etc.

Not to mention, the polygraph test is completely ineffective on psychopaths (people with the inability to feel emotions). Polygraph’s accuracy also varies by the type of equipment used and proficiency level and subjectivity of the examiner. When conducted without manipulation on the subject’s part, polygraph tests are fairly accurate. According to the American Polygraph Association (APA), the accuracy rate of polygraph tests is 80-90%.

What is The Procedure of The Polygraph Test?

It works by measuring certain physiological responses of a subject. The subject who is being tested for the truth is connected to the polygraph via various sensors applied to his body. The sensors detect the different responses of the body, and the apparatus collects them and draws a graph, which shows the peaks and troughs of those emotions or responses.

The signals recorded are usually the person’s breathing rate, pulse rate, blood pressure, and perspiration. When a person lies, he is bound to experience some emotion like fright or excitement. Without the subject even being conscious of this, the body has a natural way to respond to these emotions. Adrenaline is a hormone that is released in these instances and peaks the breathing, pulse blood pressure, and also brings about other changes in the body, which can also be detected.

However, the polygraph can be deceived by people who have perhaps mastered the art of lying or know other methods to fake emotions as described above. These factors were put to the test, and it turns out that most of the times, the polygraph test is accurate.

Here is a real-world example of the effectiveness of polygraph test: A private investigator in the US, named Scott Lewis (of Scott Lewis Private Investigation), took a polygraph test to test its accuracy. The test was conducted by Neil Myres, who runs the company called Forensic Polygraph Services. Myres asked Lewis to choose a number between 1 and 7, and then he would ask him, one-by-one, whether the number chosen is 1, or 2, or 3, and so on. Lewis was asked to say “no” each time. This means he would be lying to one of the questions. It was a simple test involving irrelevant questions. Lewis expected to pass the test as he did not think the questions would be stressful.

Moreover, he already knew the questions. However, he was surprised by the results of the test. The number he had mentally chosen was 5. He saw on the graph paper that as Myres asked if it was the number 4 the graph showed an upward movement; and it peaked on the fifth question when he was asked if it was the number 5 and Lewis had to lie.

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