In 2013, Chad Dixon was imprisoned after being convicted of threatening national security. Did he aid ISIS, copy Timothy McVeigh, or act as a courier for North Korean agents? Nope. Rather, he taught government job applicants how to pass polygraph exams. Prosecutors never questioned the validity of his techniques, but focused only on the fact that he taught them. This amounted to a de facto admission that the techniques work and that polygraphs do not.

At least they don’t work as far as determining dishonesty. The polygraph is able to record a subject’s blood pressure, respiration, pulse rate, and skin resistance. But this means it only that is able to detect rattled nerves, not prevarication.

In most polygraph sessions, the examiner compares responses to relevant questions with answers to control questions. The latter are intended to mitigate for the anxiety of being suspected of serious crime. Control questions concern misdeeds that are similar to those being investigated, but are broad and take such forms as, “Have you ever stolen anything?”

A person telling the truth is assumed to fear control questions more than relevant questions since control questions are designed to arouse a subject’s discomfort over abstract misdeeds as opposed to focusing on a specific crime. Greater physiological response to relevant questions leads to a being suspected of deception. Greater response to control questions leads to a judgment of no deception. If no difference is found between the two, the result is considered inconclusive.

However, deception is a cognitive function and there are no scientific studies that show the emotional response linked to prevarication can be measured. The National Academy of Sciences issued a comprehensive report on polygraph research, and concluded that it was “unreliable, unscientific, and biased.” It found 57 of the 80 research studies it looked at were significantly flawed. Some studies showed a success rate of no better than chance, and others performed still worse.

Despite this, polygraphs are still used on prospective employees of the FBI, CIA, and the Secret Service, even though needing to pass a polygraph test to land a prestigious job would make almost anyone nervous.

Another shortfall is that reactions to lying and being suspected crimes vary with each person. Aldrich Ames and the Green River Killer, Gary Ridgway, both passed their polygraph exams. This is because lying with no physiological change is standard for sociopaths. Others, by contrast, would get freaked out if being questioned about assassinating James Garfield. In Ridgway’s case, an innocent suspect failed the polygraph when asked the same questions that the perpetrator had been given.

Such false positives are common, which is unsurprising. Being falsely suspected of a crime, then being questioned by police detectives while hooked up to wires is going to be very stressful. Hence, the polygraph often causes the result it is meant to be determining.