Brain-Scan Lie Detectors Just Don’t Work

Perpetrators can suppress “crime memories,” study finds

It sounds just like something out of a sci-fi police procedural show—and not necessarily a good one.

In a darkened room, a scientist in a white lab coat attaches a web of suction cups, wires, and electrodes to a crime suspect’s head. The suspect doesn’t blink as he tells the detectives interrogating him, “I didn’t do it.”

The grizzled head detective bangs his fist on the table. “We know you did!” he yells.

The scientist checks his machine. “Either he’s telling the truth … or he’s actively suppressing his memories of the crime,” says the scientist.

“Dammit,” says the detective, shaking his head, “this one’s good.”

But it isn’t fiction. Some law enforcement agencies really are using brain-scan lie detectors, and it really is possible to beat them, new research shows.

The polygraph, the more familiar lie detection method, works by “simultaneously recording changes in several physiological variables such as blood pressure, pulse rate, respiration, electrodermal activity,” according to a very intriguing group called the International League of Polygraph Examiners. Despite what the League (and television) might have you believe, polygraph results are generally believed to be unreliable, and are only admitted as evidence in U.S. courts in very specific circumstances.

The brain-scan “guilt detection test” is a newer technology that supposedly measures electrical activity in the brain, which would be triggered by specific memories during an interrogation. “When presented with reminders of their crime, it was previously assumed that their brain would automatically and uncontrollably recognize these details,” explains a new study published last week by psychologists at the University of Cambridge. “Using scans of the brain’s electrical activity, this recognition would be observable, recording a ‘guilty’ response.”

Law enforcement agencies in Japan and India have started to use this tool to solve crimes, and even to try suspects in court. These types of tests have not caught on with law enforcement in the U.S., though they are commercially available here. That’s probably a good thing; the researchers of this study found that “some people can intentionally and voluntarily suppress unwanted memories.”

The experiment was pretty straightforward, and the participants were no criminal masterminds. Ordinary people were asked to stage mock crimes, and then were asked to “suppress” their “crime memories,” all while having their brains scanned for electric activity. Most people could do it, the researchers found: “a significant proportion of people managed to reduce their brain’s recognition response and appear innocent.”

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