By Virginia Hughes | National Geographic Phenomena: Only Human | September 30, 2014

You’ve no doubt heard about the polygraph and its use as a lie detector. The homely box records physiological changes — such as heart rate and electrical skin conductance — that are indirect signatures of emotion. Because these biomarkers tend to change when people tell lies, criminal investigators have long used the polygraph as a crude tool for detecting deception.

But there’s a huge problem with the polygraph: it’s all-too-frequently wrong. Truth-tellers may show a strong physiological response to being questioned if they’re nervous or fearful, which they often are — particularly if the target of a hostile interrogation.

“You end up with a lot of false positives,” says John Meixner, a law clerk to Chief Judge Gerald Rosen of the United States District Court for the Eastern District of Michigan. The traditional polygraph suffers not only from false positives, but missed positives: With a bit of training, liars can pass the test by intentionally turning down their emotions. Continue reading »