My first experience with a lie detector happened when I was about four or five years old. Some older kids in the hood built one. It looked like a ordinary cardboard box except that it had two holes in it, one covered by red cellophane and the other by green cellophane. If you told the truth, the green one would light up, but if you told a lie the red one would light up.
Questioner: Do you have a girl friend?
Lie detector: Red light.
Turns out there was another kid inside the box with a flashlight. Doh!
The modern polygraph machine is a little bit more sophisticated, but it probably isn't that much better at distinguishing truth from fiction. There have been some famous cases of guilty people passing the poly. Spy Aldrich Ames, for example.
And no doubt there are many honest people who flunked it. Fortunately, the test results are generally not admissible in court.
The real value of the polygraph is if the test taker believes it will work. The technician tells the subject that the test says he/she lied. Faced with this scientific proof and a convincing technician, the test taker then confesses.
Oh, did I let the cat out of the bag? Nah. This is old stuff and came to us by way of thedailybeast.com in The Polygraph Has Been Lying for 80 Years.
P.S. This is barely related and didn't involve a polygraph. There's a very good talk on C-span in which Sandra Grimes talks about her book, "Circle of Treason" and her work in the CIA ferreting out the spy Aldrich Ames. It's a fascinating story, and her talk is worth a look and listen.