Published on Jun 05, 2023


A Polygraph (popularly known as a lie detector) is an instrument that measures and records several physiological responses such as blood pressure, pulse, respiration, breathing rhythms, body temperature and skin conductivity while the subject is asked and answers a series of questions, on the theory that false answers will produce distinctive measurements. The polygraph measures physiological changes caused by the sympathetic nervous system during questioning.

Description of Polygraph

When we lie, our blood pressure goes up, our heart beats faster, we breathe more quickly (and our breathing slows once the lie has been told), and changes take place in our skin moisture. A polygraph charts these reactions with pens on a moving strip of paper. The result is jagged lines that don’t convey a lot to you. But…an examiner can tell from those mechanical scribbles whether or not you’ve spoken the truth.

The underlying theory of the polygraph is that when people lie they also get measurably nervous about lying. The heartbeat increases, blood pressure goes up, breathing rhythms change, perspiration increases, etc. A baseline for these physiological characteristics is established by asking the subject questions whose answers the investigator knows. Deviation from the baseline for truthfulness is taken as sign of lying.

For as long as human beings have deceived one another, people have tried to develop techniques for detecting deception and finding truth. Lie detection took on aspects of modern science with the development in the 20th century of techniques intended for the psychophysiological detection of deception, most prominently, polygraph testing.

The polygraph instrument measures several physiological processes (e.g., heart rate) and changes in those processes. From the charts of those measures in response to questions on a polygraph test, sometimes aided by observations during the polygraph examination, examiners infer a psychological state, namely, whether a person is telling the truth or lying.

The machine measures changes in blood pressure, breath rate, and respiration rate. When a person lies it is assumed that these physiological changes occur in such a way that a trained expert can detect whether the person is lying. Is there a scientific formula or law which establishes a regular correlation between such physiological changes and lying? No. Is there any scientific evidence that polygraph experts can detect lies using their machine at a significantly better rate than non-experts using other methods? No. There are no machines and no experts that can detect with a high degree of accuracy when people, selected randomly, are lying and when they are telling the truth.

Although the polygraph instrument is the centerpiece of the technique, the ability of the polygraph test to detect deception also depends critically on other elements of the process. One is the interpretation of the polygraph chart. Interpretation normally involves comparison of physiological responses.

Interpretation is often done by the examiner, who reviews the chart and may code it according to a standard protocol. People other than the examiner may also use such a protocol to code a chart.