In short, polygraph tests record a number of different bodily responses which can then be used to determine whether someone is telling the truth. They usually measure things like blood pressure, changes in a person's breathing, and sweating on the palms.

When and why are polygraphs used?

In addition to criminal investigations, they are used to conduct pre-employment screenings. The most common polygraph is the Comparative Question Test (CQT; Reid, 1947), which consists of monitoring the variations of physiological parameters, such as blood pressure, breathing, and dermal responses during an interview.

How accurate is the lie detector test?

There have been several reviews of polygraph accuracy. They suggest that polygraphs are accurate between 80% and 90% of the time. This means polygraphs are far from foolproof, but better than the average person's ability to spot lies, which research suggests they can do around 55% of the time.