Saturday, December 19, 2009

Why Polygraphs Don't Work

Polygraph tests are often portrayed as a reliable method for determine if someone is lying, but their usefulness is highly overrated. While polygraph defenders claim a 90% reliability rate, independent psychiatrists found it to only work 61% of the time. Obviously, if the polygraph test only has a slighter better than a 50/50 chance of detecting a lie, it's totally worthless. However, what's not so obvious is that even if it worked 90% of the time, it's still useless. I'll explain why.

If you have 1,000 court cases, then 100 defendants will give a false positive or false negative if the polygraph only works 90% of the time. That's way too risky to be used in a court of law; the stakes are too great. Imagine being thrown in jail, potentially for life, for a false positive on a polygraph. One person falsely put in jail is too many, let alone 100.

Another use for polygraph tests is for companies or government bodies to screen for potential spies. If you have a large enterprise of 10,000 employees and 20 employees are spies, 2 spies would pass the polygraph and 998 innocent employees would be marked as spies. With so many false positives, either a good portion of the company is going to be fired or the entire results of the polygraph test will be thrown out.

Despite the unreliability of polygraph tests, they are still being used today. Massachusetts, for example, uses polygraph tests to obtain warrants. Luckily most courts will not accept polygraphs as evidence for the reasons above unless both sides agree to submit it as evidence. However, some courts, such as the justice system of New Mexico, still accept lie detector results. The Debunkey Monkey is not pleased. .


  1. Lie detectors should not be used as a evidence.

    Instead, they work much better as a harmless psychological tool of law enforcement. Lie detector tests, even fake ones, have been used to effectively gain confessions. Of course, this only works if the suspect believes in the effectiveness of lie detectors. I imagine refusal to take a lie detector is also seen as a tip-off that the suspect is guilty (whether or not that is true), which can encourage the police to dig deeper in other, more reliable ways.

  2. "This only works if the suspect believes in the effectiveness of lie detecors."

    *whispers* It's a secret to everybody.

  3. Damn psychology... never works when people realize we are using it on them.

    Lucky for us, most criminals don't understand psychology, so we just need to portray lie detectors as flawless on TV and in movies while eliminating them as a measure of honesty in the real-world situations you mentioned.

    I wonder how many cases actually hinged on the results of a lie detector. That would be interesting to find out, especially in this era of conviction reversals.

  4. Yeah, I seriously doubt I can find the number of court cases in which a polygraph test was submitted as evidence, let alone the number of court cases that hinged on the polygraphs results.

    I'll try digging around this afternoon to see what kind of numbers I can come up with.