Explanation of How the Polygraph Works

The polygraph test, or lie-detector, is one of the most misunderstood tools used for legal purposes. Many people think the polygraph is fail-proof. The truth is, the polygraph test is often not even reliable, much less an end-all, be-all. This is a description of the polygraph and how it works, to aid in understanding this device.

The polygraph test measures physiological arousal in effort to detect deception. Three physiological thresholds are measured and compared to changes throughout questioning during a polygraph examination. These physiological measures are blood pressure, respiration, and Galvanic Skin Response (GSR). The polygraph test operates under the assumption a suspect or person undergoing testing will experiences measurable changes in these physiological responses if they are lying. It is assumed a liar will have increased blood pressure, increased respiration (leading to a faster heart rate) and increased electrodermal activity (measured as the GSR). Electrodermal activity is a measure of sympathetic activity, or the electrical resistance of the skin. The theory behind this is if a person being tested is lying, they will sweat and their skin will be less resistant to electrodermal activity.

The polygraph test does a wonderful job of measuring physiological changes in a suspect. However, it can not be assumed every suspect will have these changes when lying. As with any other biological aspect, each person differs in their physiological reactions to situations. Although this is controlled for somewhat by obtaining a baseline measure from the suspect, it should not be assumed each person will have the same significant changes when lying. Some persons may have significantly less changes and be able to lie undetected, while others (especially those who are particularly nervous) may have extreme physiological reactions to questions, despite guilt or innocence. As most people who are being questioned using a polygraph will be nervous, the polygraph is more likely to pick up responses from an innocent person than to not pick up responses from a guilty person. This is perhaps the most dangerous aspect of using polygraph testing, as the risk of false convictions is absolutely unacceptable. It is for this reason polygraph testing has not been generally accepted as evidence in the court system and will not be unless a more reliable model is created.