There's an old interview on C-SPAN's BookTV with a CIA polygrapher. He seems to genuinely believe in the validity of the polygraph, but watching the interview, I was convinced that the only value comes from intimidation and stress.

(all-caps bad transcription)

> THE ESSENCE OF A POLYGRAPH TEST IS IF YOU HAVE SOMETHING TO LOSE BY FAILING A POLYGRAPH TEST IF YOU WILL, OR SOMETHING TO GAIN BY PASSING IT, THAT IS WHAT MAKES THE POLYGRAPH EFFECTIVE. WITHOUT THE FEAR OF DETECTION IT IN A SIMPLE WAY AS I CAN PUT IT THAT IS WHAT MAKES IT WORK. YOU HAVE TO BE AFRAID. IF YOU HAVE NOTHING TO LOSE BY TAKING THE POLYGRAPH TEST THAN THE PRESSURE IS NOT ON YOU. BUT AS I SAID THAT IS WHAT MAKES YOU WORK. IT HAS TO BE PROTECTION MORE THAN GILTS. NOW YOU MAY FEEL GUILTY, BUT FEAR OF DETECTION IS THE OVERRIDING CONCERN IN IN A POLYGRAPH TEST

https://www.c-span.org/program/book-tv/gatekeeper/180053

It sounds like religion; it only works if people believe in it.