The science of lie detectors

Do polygraphs really reveal the truth, the whole truth and nothing but the truth?

(Image credit: Federal Bureau of Investigation)

Since the early 1920s ‘lie detectors’ have been used to evaluate whether or not someone is being deceitful by measuring their physical responses to a series of questions. Elevated blood pressure, a higher breathing rate and increased sweating are all indications that a person may be telling a lie.

A polygraph (lie detector) test uses these biological signals of lying to sniff out the truth. Hooked-up to a series of sensors, these signs are recorded during the test. The subject is initially asked baseline questions to establish a normal rate of blood pressure, breathing rate and perspiration.  The interviewee is then repeatedly asked questions targeted to evaluate whether the answer is deception or truth. The examiner conducting the interview monitors the information coming in from each sensor to see when a spike on the chart occurs – the sign that a deceptive answer may have been given. 

This form of lie detection, however, isn’t an exact science and has sparked controversy about its legitimacy. A polygraph’s ability to collect vital data hasn’t been called into question, but how that information is interpreted is debated. Sitting strapped to several wires while an interviewer asks you probing questions is enough to make anyone sweat. Therein lies the main problem with polygraphs. The experience of taking a test can cause enough stress and spikes in blood pressure and respiratory rate to suggest a lie.

Similarly, there are ways that people can suppress these changes during an interview. This doubt in its validity has led to most US and UK courts to ban polygraph data as evidence of guilt or innocence during a trial.

(Image credit: DUNKernel)

How a polygraph sniffs out a liar

Data

Blood pressure, respiratory rate and perspiration are simultaneously tracked on a line graph, spiking in unison when a deceptive answer is given.

Interrogation

Interviewees are asked a series of predetermined questions to evaluate deception, alongside control questions for comparison.

Breathing rate

Known as a pneumograph, two air-filled tubes are strapped across an interviewee’s chest to measure their respiratory rate. During breathing the chest expands and contracts to displace the air in the tube, which is then recorded.

Sweating

Galvanic skin response (GSR) is measured to monitor a person’s sweat levels. Sensors are placed on the fingertip to record how electrically conductive they are. More sweat equals more conductivity.

Blood pressure

Using a blood pressure cuff, the sound vibrations of blood passing through vessels displaces the air in the connecting tubes. Changes in the displacement are converted into electrical signals by the use of transducers.


This article was originally published in How It Works issue 129, written by Scott Dutfield 


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