Polygraph test

Classic Projects: Polygraph

Can a polygraph tell if you're telling the truth?

Despite being regarded by much of the scientific community as 'pseudoscience' the polygraph – also known as a 'lie detector' – is used as an interrogation tool in law enforcement. The procedure assesses the quality of statement being given by a subject under examination by recording unexpected changes in vital signs.

The idea behind using a machine to tell if someone is lying dates back to the late 19th century when Italian criminologist Cesare Lombroso used a device measuring changes in blood pressure in suspects during police investigations. US inventor William Marston took matters a stage further in 1905 when he included monitoring breathing rates and concluded that women are more fundamentally honest than men.

But the polygraph as we know it is the brainchild of forensic investigator John Augustus Larson, the first American law enforcement officer to hold an academic doctorate, gaining a PhD in physiology in 1920. His key development was to include the physiological indices of pulse, respiration and skin conductivity into a monitoring device that gave continuous readings.

The first person to be convicted on the basis of Larson's technology was murderer William Hightower. Newspapers reproduced graphs from the polygraph, or as Larson called it the 'cardio-pneumo psychogram', pointing out the spikes that indicated the lies. Despite challenges to the authenticity of the polygraph, the technology has remained on the Encyclopaedia Britannica's list of inventions that have had "profound effects on human life for better or worse".

Statistical accuracy claims vary for the polygraph technique of lie detection depending on the data you read. Some manufacturers claim to be in the 80-99 per cent bandwidth, while a survey of more than 400 psychologists revealed that faith in the procedure was held at 61 per cent, which is little more than flipping a coin. One of the reasons for the lack of trust on the part of psychologists is the claim that experienced liars can beat the polygraph, while there is no guarantee against a false positive.

Counter measures employed by suspects (including known spies in control experiments) involve self-regulation of respiration, and deliberately increasing blood pressure by self-inflicting pain (tongue-biting for example) while telling the truth. The threat of a test can induce confessions, leading to further claims that the polygraph is a psychological prop rather than a scientific aid. We know that the technology can give accurate readings about physiology, but there can be no scientific certainty that these changes reflect genuine alterations in the monitored parameters related to lying. Recent tests in 2003 in the US reveal that polygraphic testing on subjects untrained in counter-measures could detect the truth as a "greater level than chance".

Polygraphic evidence is admitted in 19 US states, but not considered reliable or used in law enforcement in Europe. The device is used as an investigative tool in Canada, cannot be used in a criminal trial in Australia or Israel, while in India tests are legal if a defendant requests one.

Modern polygraph kits come with software, respiration transducers, EDA kit, disposable electrodes, blood pressure cuff, finger cuff, countermeasure sensor pad, scoring algorithms and laptop. Everything you need for your own interrogation, provided, of course, you accept that the technology will tell you about your subject's state of mind.

NEXT MONTH: Tower Bridge

Recent articles

Info Message

Our sites use cookies to support some functionality, and to collect anonymous user data.

Learn more about IET cookies and how to control them

Close