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Brain Scan Can Beat Polygraph at Detecting Lies

Brain Scan Can Beat Polygraph at Detecting Lies

When it comes to lying, our brains are much more likely to give us away than sweaty palms or spikes in heart rate, according to new research.

The study, from researchers at the Perelman School of Medicine at the University of Pennsylvania, found that scanning people’s brains with fMRI, or functional magnetic resonance imaging, was significantly more effective at spotting lies than a traditional polygraph test.

It has been demonstrated that when someone is lying, areas of the brain linked to decision-making are activated, which lights up on an fMRI scan for experts to see.

While laboratory studies showed fMRI’s ability to detect deception with up to 90 percent accuracy, estimates of polygraph’s accuracy ranged wildly, between chance and 100 percent, depending on the study.

The Penn study was the first to compare the two technologies in the same individuals in a blinded and prospective fashion. The approach adds scientific data to the long-standing debate about this technology and builds the case for more studies investigating its potential real-life applications, such as evidence in criminal legal proceedings, according to the Penn researchers.

Researchers from Penn’s departments of Psychiatry and Biostatistics and Epidemiology found that neuroscience experts without prior experience in lie detection were 24 percent more likely to detect deception using fMRI data than professional polygraph examiners reviewing polygraph recordings.

Polygraph, the only physiological lie detector in worldwide use since it was introduced more than 50 years ago, monitors a person’s electrical skin conductivity, heart rate, and respiration during a series of questions. It is based on the assumption that incidents of lying are marked by upward or downward spikes in these measurements.

Despite being deemed inadmissible as legal evidence in most jurisdictions in the United States or for pre-employment screening in the private sector for almost 30 years, polygraph is widely used for government background checks and security clearances.

“Polygraph measures reflect complex activity of the peripheral nervous system that is reduced to only a few parameters, while fMRI is looking at thousands of brain clusters with higher resolution in both space and time. While neither type of activity is unique to lying, we expected brain activity to be a more specific marker, and this is what I believe we found,” said the study’s lead author, Daniel D. Langleben, M.D., a professor of psychiatry.

To compare the two technologies, 28 participants were given the so-called “Concealed Information Test” (CIT). CIT is designed to determine whether a person has specific knowledge by asking carefully constructed questions, some of which have known answers, and looking for responses that are accompanied by spikes in physiological activity.

Sometimes referred to as the Guilty Knowledge Test, CIT has been used by polygraph examiners to demonstrate the effectiveness of their methods to subjects prior to the actual polygraph examination.

In the Penn study, a polygraph examiner asked participants to secretly write down a number between three and eight. Next, each person was administered the CIT while either hooked to a polygraph or lying inside an MRI scanner. Each of the participants had both tests, in a different order, a few hours apart.

During both sessions, they were instructed to answer “no” to questions about all the numbers, making one of the six answers a lie. The results were then evaluated by three polygraph and three neuroimaging experts separately and then compared to determine which technology was better at detecting the lie.

In one example, fMRI clearly shows increased brain activity when a participant, who picked the number seven, is asked if that is their number. Experts who studied the polygraph counterpart incorrectly identified the number six as the lie. The polygraph associated with the number six shows high peaks after the participant is asked the same questions several times in a row, suggesting that answer was a lie.

The scenario was reversed in another example, as neither fMRI nor polygraph experts were perfect, which is demonstrated in the study, the researchers noted. However, overall, fMRI experts were 24 percent more likely to detect the lie in any given participant, they added.

Beyond the accuracy comparison, the researchers made another important observation. In the 17 cases when polygraph and fMRI agreed on what the concealed number was, they were 100 percent correct. Such high precision of positive determinations could be especially important in the United States and British criminal proceedings, where avoiding false convictions takes absolute precedence over catching the guilty, according to the researchers.

They cautioned that while this does suggest that the two technologies may be complementary if used in sequence, their study was not designed to test combined use of both and their unexpected observation needs to be confirmed experimentally before any practical conclusions could be made.

“While the jury remains out on whether fMRI will ever become a forensic tool, these data certainly justify further investigation of its potential,” Langleben said.

The study was published in the Journal of Clinical Psychiatry.

Source: Perelman School of Medicine at the University of Pennsylvania 

Photo Credit: Perelman School of Medicine at the University of Pennsylvania/Journal of Clinical Psychiatry.

Brain Scan Can Beat Polygraph at Detecting Lies

Janice Wood

Janice Wood is a long-time writer and editor who began working at a daily newspaper before graduating from college. She has worked at a variety of newspapers, magazines and websites, covering everything from aviation to finance to healthcare.

APA Reference
Wood, J. (2018). Brain Scan Can Beat Polygraph at Detecting Lies. Psych Central. Retrieved on December 3, 2020, from
Scientifically Reviewed
Last updated: 8 Aug 2018 (Originally: 5 Nov 2016)
Last reviewed: By a member of our scientific advisory board on 8 Aug 2018
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