Lie detectors are widely used by police and corporations, but their accuracy has been called into question. Some experts even claim that master criminals can beat them.

Invented nearly a century ago, the first polygraph machine monitored blood pressure, heart rate and respiration to identify reactions linked with lying. This article explains how the test works.

Physiological Measurements

Invented nearly a century ago, the polygraph (poly = many; graph = writings) records a person’s heart rate, blood pressure, respiration, and galvanic skin response (the amount of moisture secreted by the body) while they answer questions. The basic theory is that liars’ responses will differ from truth tellers’.

The examiner will ask “control” questions to establish baseline levels of physiological response, then administer a series of ‘probable lie’ and’relevant lie’ questions. If the subject’s response to the ‘probable lie’ questions is significantly greater than that of the’relevant lie’ questions, the examiner will conclude that the test subject is lying.

But scientists have found that certain specific physiological reactions do not distinguish between truth-tellers and liars. Furthermore, the results of a polygraph are open to subjective interpretation by the examiner, making them inherently unreliable. In addition, the use of countermeasures to manipulate the tests can alter their results. Aldrich Ames, the infamous Soviet spy who passed polygraph tests while working for the FBI and CIA, used such countermeasures as taking sedatives to minimize anxiety and applying antiperspirant to avoid sweating.

Detection Techniques

Lie detector tests are staples of police dramas and are used as part of the interview process for many jobs. However, most psychologists agree that liar detection relies on involuntary physiological responses to questions that are not controlled by the examinee. In fact, even when the test is administered correctly (with the right control questions, neutral questions and core questions), the results are not very reliable.

Over the years, lab and field studies have shown that most liars show similar patterns of changes in their body while answering questions. These are recorded by sensitive equipment such as polygraphs, which are based on Marston’s idea that deception is accompanied by certain physical signs like a rise in blood pressure and sweating. However, these responses can also be caused by anxiety and other factors. Therefore, they are prone to errors. Many liars are also able to fool the machines by using countermeasures, such as a mental exercise or taking drugs, which can affect their physiology.

Interpretation Techniques

You see it on every crime TV show — the suspect sweating nervously in a police interrogation room as the detectives use a polygraph machine to determine whether they’re telling the truth. The lie detector is also used by businesses to evaluate employees’ loyalty and in many government jobs. But is it as reliable as we’re led to believe by pop culture?

Invented in the 1920s, the polygraph (or physiograph) measures typical stress responses to questioning, including spikes in heart and breathing rates. These reactions trigger certain brain activity that can be measured by electroencephalography or functional magnetic resonance imaging.

To interpret the results, polygraph examiners look for peaks of physiological response that correlate with specific questions or events – known as control questions – and for reductions in responses that are uncorrelated within a reasonable timeframe. It’s not that simple, however, and some people can beat the system by controlling their responses to the control questions.I recommend this website for more Lie Detector Test.


Despite being used by law enforcement and government agencies to interrogate criminal suspects and screen applicants for jobs, lie detector tests remain controversial. There are many factors that can affect the accuracy of a polygraph test and, even when they’re conducted properly, it’s not always easy to distinguish between physiological responses that are actually caused by lying and those which are simply the result of anxiety during questioning.

A typical test starts with a pre-test interview to establish the normal patterns of responses. Then the examiner asks questions designed to trigger the response a person would experience when they’re telling a lie. The machine records reactions like blood pressure, heart rate, and perspiration, as well as arm and leg movements.

It’s important to note that the court will only allow a lie detector test as evidence if it meets certain standards. For example, the person taking the test must be free from any mental illness, drug use, and alcohol abuse which could interfere with his or her ability to respond truthfully.