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Matthew Radefeld & Dan Juengel
Matthew A. Radefeld and Daniel A. Juengel

3 reasons not to take a lie detector test

On Behalf of | Jan 2, 2024 | Criminal Law |

If you’re under investigation for any kind of serious crime here in Missouri, the police may ask you to take a polygraph test and “clear your name.” They can’t force you to comply with their request, but there’s no question that you’ll feel some pressure over the issue.

If you’re anxious to prove your innocence, it can be tempting to agree – but you shouldn’t make that call without obtaining legal guidance. Here’s why:

1. A polygraph doesn’t really measure truthfulness

Polygraph machines don’t actually have the capacity to sort lies from truth. All they can do is measure your physiological responses to the questions you are asked and the answers you give. Since lying causes most people a certain amount of stress, the underlying assumption is that your body will basically betray any deception through:

  • An increase in your heart rate 
  • A rise in your blood pressure
  • An irregular change in your rate of respiration
  • Increased sweat production

While it’s true that all of these things can be associated with the stress of lying, the mere fact that you’re in a police station, hooked up to a handful of wires from a device, being asked intrusive questions and clearly suspected of a crime is also stressful. That means you can fail the test just because you’re nervous. Some people simply have test anxiety that takes over any time they’re put on the spot, so they may “read” as deceptive on the test no matter how truthful they are.

2. Polygraph results aren’t admissible in court

The vast majority of states don’t permit polygraph evidence to be admitted in court, and Missouri is no exception. Passing the test is no guarantee that the police will believe you and back off with their investigation, but failing it just about guarantees that you’re going to be subjected to intense scrutiny for the foreseeable future. 

There’s simply no benefit to you if you take one, and a lot of potential drawbacks. 

3. You could incriminate yourself on accident

Again, taking a polygraph is stressful. The officer who administers the polygraph knows that you’re anxious, and they may capitalize on that. They’re well-versed in how to get suspects talking both before and after the polygraph is taken in order to try to tease out statements that they may later use in their investigation against you. There’s simply no safe way to have a conversation with the police, even under controlled conditions like a polygraph exam.

When you’re anxious and not thinking clearly, you may inadvertently say something that makes you appear guilty. The police can seize on that and use it to try to pressure you into a confession or spin it into enough evidence for a warrant to conduct a search of your property.

What’s the bottom line?

Under this country’s criminal law system, you don’t ever have to prove your innocence – and you shouldn’t try. Polygraph tests are basically junk science masquerading as a law enforcement tool, and they have a tremendous capacity to put you firmly in the crosshairs of an investigation. You’re far better served putting your faith in experienced legal guidance.