I was wondering if anyone else has a suspicion that polygraphs might not be used to test whether or not a person is telling the truth for agencies like the FBI.  

Is there any evidence/writings that would indicate that there are other things that a polygraph examines that are unbeknownst the test subject and possibly a majority of the polygraph administers out there?

Could polygraphs have been used "legitimately" (with a high level of confidence that they were accurate) at one point time, but after years of testing some agencies determined that they were not good for detecting whether or not someone was lying, but better as some sort of psychological test?  Telling the public that they're undergoing a psychological test instead of a lie detector test might ruin the validity of the results.

Instead of worrying about whether or not the person is lying about something, is there a possibility that they want to find out how the person reacts to certain types of questions and holds up under pressure?

I'm just curious, a lot of the stories I've read seem to point to something else going on that we're not being told.