Should You Trust a Polygraph Test? The Truth About Lie Detectors.

We have all heard someone saying it on tv, etc. Someone gets accused of a crime. She or he shouts out “I didn’t do it; I’ll even take a polygraph test to prove it.” I always chuckle to myself when I hear this. The mathematics behind polygraph tests is very telling.

What does a polygraph measure? Polygraph machines measure blood pressure, pulse, respiration, and skin conductivity. Those that advocate for use of the polygraphs claim that deceptive answers will produce physiological responses that are different from responses with non-deceptive answers. Unfortunately for polygraph supporters, there has never been any evidence to correlate these specific physiological reactions with lying. In other words, there has never been any evidence to suggest the one’s blood pressure increases, heart rate increases, respiration changes, and/or skin conductivity increases simply because one is being deceptive. Furthermore, there is no evidence that these changes don’t just occur as a result of one being strapped into a machine.

What does the mathematics say about polygraph tests? Even the most fervent proponents of polygraph tests claim that the tests are about 90% accurate — critics claim that they are less than 70% accurate. However, what does that actually mean? Let’s assume that you work for a particular company and…