If I was a suspect or witness testifying in a trial that could result in myself or someone I know getting the death penalty or going to jail for a long time, I'd be really nervous and maybe fail the lie detector even if my answers were completely truthful.
OTOH a police officer who's used to lying to suspects (it's the main tool police have in countries where torture or police brutality isn't allowed), or a hardened psychopath criminal who doesn't see anything wrong with lying to protect his own interests, may be so cool that he/she can easily pass the lie detector.
If I was a suspect or witness testifying in a trial that could result in myself or someone I know getting the death penalty or going to jail for a long time, I'd be really nervous and maybe fail the lie detector even if my answers were completely truthful.
OTOH a police officer who's used to lying to suspects (it's the main tool police have in countries where torture or police brutality isn't allowed), or a hardened psychopath criminal who doesn't see anything wrong with lying to protect his own interests, may be so cool that he/she can easily pass the lie detector.