The science of lie detectors

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.