system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are polygraph tests still widely used in law enforcement investigations even though they are so unreliable?

I have been watching Forensic Files a lot lately on Netflix.  Almost every case they mention suspects taking a polygraph test.  Many times a suspect will fail one only for the DNA evidence to prove he/she wasn't lying.  Why are these highly unreliable polygraph tests still used?
gpt
To scare people into making a confession, basically. If you threaten someone with the fact you can tell if they're lying and then inform them that they can make it easier on themselves to confess now to lighten their sentencing you can get a confession out of them faster. It's like playing good cop/bad cop. "Make it easier on yourself kid, tell us what we want to know, otherwise if we find out later you were lying to us its only gonna make it harder on you, see?" Only the bad cop in this case is your heartbeat, which you have no control over if your nervous and guilty, or even not guilty. But if you're innocent and tell the truth, they wont need the lie detector to tell that based on body language and tone of voice. Hell, pretty sure the damn thing hardly ever gets any use these days anyway.