How Things Work: Lie detectors

Lie detectors have existed for centuries in one form or another. In ancient China, a suspect would chew dry rice during questioning. The rice would be examined after the questioning to determine if the suspect had lied. Because the stress caused by lying inhibits saliva production, dry rice would imply a high chance that the suspect was lying.
Lie detectors today are much more complex than the dry rice method used by the ancient Chinese. A lie detector test, called a polygraph, measures someone’s physiological reactions to questions.

Created by psychologist William Marston, a polygraph test begins with simple questions to get “baseline” measurements. Further questions are asked, and the polygraph measures involuntary deviations from the baseline values. These deviations, while not a sure indication of a lie, show stress that suggests that the person is lying.
There are several different questioning methods used to test if someone is lying. Psychologists consider the Guilty Knowledge Test (GKT) to be the most scientific. The GKT compares the responses of multiple people to a list of multiple choice questions about the crime. Some of these questions contain information that only the criminal would know, and therefore a response would be expected from the criminal, but not from an innocent suspect. Another commonly used test is the Control Question Test, which compares responses to questions about the crime to other questions that could make the suspect uncomfortable.

A polygraph monitors several changes such as heart rate, blood pressure, respiratory rate, and electro-dermal activity — or sweat on the fingertips. Lie detector results were initially displayed by needles scribbling lines on paper. Now, analog polygraphs have mostly been replaced with digital versions where polygraph results are displayed on a computer monitor.

Changes in respiratory rate are measured using two rubber tubes filled with air, called pneumographs. Expansion of chest muscles causes air inside the tubes to be displaced, which causes a mark when the person takes a breath. Anxiety due to lying can cause an increased respiratory rate.

Blood pressure and heart rate are taken using a sphygmomanometer, worn around the upper arm. Blood pumping through the arm makes a sound, which displaces air in the tube and creates a changing signal on the polygraph. The stress caused by lying raises blood pressure.
Fingerplates, called galvanometers, measure the ability of the fingertips to conduct electricity. They measure electro-dermal activity, or galvanic skin resistance, on the fingertips. Stress causes perspiration, which results in increased conductivity.

The data from these different devices is analyzed, and the responses to each question are compared. Data for all three devices typically spikes when the subject gives a dishonest answer to a question. Lie detectors are thought to be accurate about 80 to 90 percent of the time. However, they are more accurate in identifying innocence than guilt; in other words, a large percentage of innocent people test falsely positive.
Additionally, it is possible to fool a lie detector. One method is to make oneself tense during initial questioning so that the baseline is inaccurate, either by thinking about something stressful or by flexing muscles without the examiner noticing. Lie detectors can also be thrown off by the use of drugs such as valium.
An entry in the NationMaster Encyclopedia gives an account of Aldrich Ames, a counterintelligence officer working for the CIA who was actually a spy for the Soviet Union. Ames managed to pass two polygraph tests without being caught.

Due to the inaccuracy of the machine, the polygraph is highly controversial. Regardless, the FBI and the CIA continue to use lie detectors. In the United States court system, polygraph results can be used as evidence in some states.

Polygraphs cannot be used by employers for pre-employment screening or during employment, due to the Employee Polygraph Protection Act, passed in 1988.

Methods aside from polygraphs exist to detect lies, but none are foolproof. One method, developed during the Vietnam War, is a voice stress analyzer. Stress affects muscles controlling the voice, causing vocal tremors when someone lies. These tremors are subsonic components of voice and are not audible to humans. When the laryngeal muscle is relaxed, the pitch of these tremors is about 12 Hz. Stress causes the muscle to tense, causing the pitch to become as low as 8 Hz. This method is far easier to use than the polygraph, since it can be used on any voice recording. The FBI and CIA use voice stress analyzers in their investigations, and private use of the device is not regulated in most states.