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They use them on TV shows like Jerry Springer which is pathetic so why can't the police use them. It would be as simple as just asking on a murder case, "Did you kill Mr.Smith?" and if they said no, the police would know straight away wether they're lying or not? Am I right?
Because they are not 100% accurate. They have flaws. DNA evidence is much more reliable.
All a lie detector really does is judge your heart rate. Hell, if someone you found arousing was in the room then you'd fail the test easy. If there is anything around to increase your heart rate then you'll fail the test, including drugs in your system or anything like that.
Point is, they are not that reliable and they are pretty expensive and only use them when DNA evidence doesn't suffice I think (not too sure about this last part, though).
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They're easy to cheat. They measure things like heart rate, perspiration level and various involuntary muscle movements. It's possible to control these to a certain degree in such a way that the test becomes inaccurate.
The most reliable lie detectors are rated at about 96% accurate I think but even that's not enough. For 4% of people, the test will come back wrong and if you think how many people go to court everyday, that's a lot of people who have an unfair trial.
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Point is, they are not that reliable and they are pretty expensive and only use them when DNA evidence doesn't suffice I think (not too sure about this last part, though).
I don't know how often they use them as far as interrogation goes, but polygraph tests are never admissible in court. They're just too unreliable, due to the factors that have already been mentioned. It can give an idea of whether a person is guilty or not, and it can push them towards a confession if they are, but they're never used as an actual indicator of guilt.
A lie detector is only about 70% accurate, if I'm correct, and can be cheated easily. It's much more realiable to use forensic evidence, and even then you need a motive, witnesses, etc.
Everyone already pretty much tackled this one, but food for thought. Story time.
When I was younger, I was like a little thug. And there was one particular kid (looking back, he was about twice my height, so this story doesn't even make sense XD) whom I had gotten into a number of skirmishes with. Popped his jaw out of place on one particular occasion where we were in a fist fight.
Anyway, I came to school one day and I got brought to the Dean's office. Now, I had been warned the whole day that this kid was accusing me of having hit him with a stick after school ended the day before by my classmates. So, I must have seemed really guilty when they only just started asking me the question and I was like "Wasn't me." Haha.
Anyway, I got brought down to the precinct and had to spend the whole day there for something I didn't do. Eventually, I got pissed off and started demanding a lie detector test, so I could get out of there. Mind you, at the time, I didn't know how they worked. I just knew that I didn't do it and it might absolve me. Eventually, the officers there were like "wtf" and let me go because it didn't make sense that I was pretty much challenging them to prove me guilty.
My mom was actually going to sue the school for illegally questioning me. Turns out that the kid's dad hit him with a belt, which left the welt on him.
Case and point: They could have easily used the polygraph (lie detector) test upon my request, but the realization that it's not infallible would make them skeptical about it. I could, afterall, know how to cheat it and be guilty or get nervous and be innocent.
Because they are not 100% accurate. They have flaws. DNA evidence is much more reliable.
All a lie detector really does is judge your heart rate. Hell, if someone you found arousing was in the room then you'd fail the test easy. If there is anything around to increase your heart rate then you'll fail the test, including drugs in your system or anything like that.
Point is, they are not that reliable and they are pretty expensive and only use them when DNA evidence doesn't suffice I think (not too sure about this last part, though).
This ^
But even then DNA is not 100% reliable either, DNA evidence is only 98% accurate so there is still room for error even then. But yeah, Lie Detectors can have up to a 70% accuracy rating. As Ray said, almost anything can effect you're heart rate, hell people may even go to extremes as inflict sharpe pain onto themselves by some method as that also increases heart rate and can crap up a reading.
Today's Toppers
Why don't police use lie detectors?
