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Would you trust a lie detector test?

On Behalf of | Aug 5, 2019 | Criminal Defense

We’ve all heard of the lie detector test or polygraph. What many people don’t realize, however, is that polygraph tests are not admissible in court. Why not? They have never been proven to work.

When they are administered in test conditions, they don’t reliably indicate whether someone is lying. In large-scale screening tests, there have been significant “false positive” rates, meaning that many people will be labeled deceptive when they were not.

The fact that polygraph results are not admissible as evidence doesn’t mean that they are never used, however. Some employers — notably many federal security agencies — use the polygraph as a tool for hiring or in internal investigations. Moreover, some law enforcement agencies continue to rely on polygraph tests to vet suspects during investigations.

Basics of the polygraph

Polygraph tests were developed in the early 20th century, based on the assumption that people can’t control their physiological reactions to the stress of lying. The machines measure changes in breathing depth, blood pressure and the skin’s conductivity to electric currents.

Unfortunately, these physiological changes have never been shown to be reliable indicators of deception.

Not only that, but it’s unclear that everyone has a physical reaction when they lie — and some people are able to suppress such reactions using countermeasures such as controlled breathing. Even if everyone did have a physiological reaction to telling falsehoods, it’s not clear that they would all have the same reaction.

There are also problems with how the test results are evaluated. For example, in the context of employment screening, federal agencies have long required polygraph tests for hiring or certain security clearances.

Yet, although the agencies have insisted for decades that the tests provide useful information, they never actually came to a consensus about what polygraph result would indicate a model employee vs. one who should be denied employment, according to Smithsonian magazine.

Why are polygraphs still in use?

The idea behind a lie detector test is certainly attractive. It seemed to promise a fair, objective way to tell when someone is being untruthful — and that could be extremely useful in a variety of circumstances. Just as useful would be an objective test for truth-telling.

While we now know that polygraphs aren’t reliable, they can still be useful when expediency calls. According to the Smithsonian magazine’s review, they are often used as an excuse to dismiss unwanted employees or to vouch for people who are already trusted. In other words, they are frequently used to back up existing opinions.

Polygraph tests are not evidence, and they should never be used by law enforcement, even when the results aren’t intended to be used in court.