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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Medical Myths Medical Myths

The Lie of Lie Detectors

October 23, 2012

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

Recently, two studies announced effective ways to determine whether a person was telling the truth — one used a brain scan while the other detected heat around the face. Since you probably tell the truth all of the time, it is likely that these reports will have no direct bearing on you. But, for those who perform lie detector tests or for those who might be asked to submit to one, these techniques could someday change how these tests are performed.

Brain Activity To Detect Lies

The first report describes how a specific part of the brain (the anterior cingulate gyrus) becomes active when healthy volunteers were asked to lie about a card they were holding. It used a scanning technique that tracks changes in where blood flows in the brain; blood flow, in turn, is an indicator of brain activity. This report adds to other recent studies linking specific areas of brain activity and such activities as gambling or drug ingestion. These studies are likely to improve our understanding of brain function and how it relates to human behavior.

Companies have already begun to spring up using a type of MRI called "functional MRI" that analyzes brain activity in different parts of the brain. The site of activity varies depending on whether someone is being deceptive or truthful. Although their reliability is still uncertain, companies (such as No Lie MRI) are hoping to convert this technology into a profitable enterprise.

Facial Warmth To Detect Lies

The other report described a technique called "thermography" — that is, it measured heat. It turns out that humans tend to have increased blood flow around the eyes and other parts of the face when lying. The researchers found a good correlation between increased heat among volunteers who were asked to lie compared with those asked to tell the truth. The results were available more quickly and did not require as highly trained technicians for proper testing and interpretation, as with standard polygraph testing.

The Myth of Lie Detectors

It is a myth that current lie detectors (polygraph tests) detect lies; the fact is that the term "lie detector" promises more than can be delivered. What they detect is the body's response to lying, such as sweating of the palms or an increase in heart rate. The brain scans and thermography tests described here also do not detect lies, but seem to detect an increased blood flow in the brain or around the face when telling a lie. While some people might be able to control their bodily response to lying (and "pass" a polygraph while telling a lie), brain scans or thermography could turn out to be more reliable and harder to fool.

The Brain and Behavior: Chicken or Egg

If a certain part of the brain is active when lying, is there an abnormality in that part of the brain among habitual liars? Might other undesirable or deviant behaviors be linked to specific brain areas? And is the behavior (such as lying) the result of voluntary actions that result in one area of the brain becoming more active, or is there an abnormally overactive brain region that causes the behavior? Either way, how the brain contributes to behavior and is affected by it might be reconsidered based on reports such as these new studies that link behavior to brain function. In addition, "automatic" responses in the body — such as facial warmth or heart rate — might turn out to become more controllable with training, a key feature of biofeedback techniques.

The Bottom Line

Look for more studies similar to these, linking human health, disease, behavior and even thought with specific areas of brain activity or other changes in body function. In these recent reports, brain scans and thermography accurately distinguished lies from truth — but they were not perfect. Larger studies with high accuracy rates will be needed before these tests will replace current polygraph testing. Simply adding more imperfect tests to the process could do more to blur the truth than to find it.

Not only are these findings interesting, but they point out a trend in research studies of the brain and other organs: to study not only the details of how an internal organ looks (as is the case with X-rays or CT scans, for example), but also to assess how part of the body is actually functioning in real time. If future studies verify the reliability of the brain scanning or thermography techniques, not only will "lie detectors" change but the way we evaluate the brain and other organs in health and disease may change as well.

Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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