Its not used for diagnostics, its used as an interrogation tool. The idea is that you grill someone much easier if they're connected to a strange machine that can seemingly read their thoughts. Decent article about this from 2000 here:
Polygraphs don’t have to work to be a deterrent. People just have to believe that they work and can reveal whether they have committed crimes. The DOE doesn’t have to believe they work, either.
More important, polygraphs are an immensely effective interrogation tool; they need not detect lies. Lykken tells an anecdote of two cops interrogating a suspect at a time when copy machines were not familiar objects. Lacking a lie detector, the cops put a piece of paper in the copier that said “He’s lying!” They made the suspect place his hand on the strange machine while they asked him questions. When they didn’t like his answers, they’d hit a button on the machine. It would groan, whir, stink and shoot out a piece of paper that read “He’s lying!” Realizing that denial was useless, he confessed.
“If I was in the police business I would use [the] polygraph,” says Lykken. “It’s a powerful inducer of confessions, and you don’t have to hit ’em with any clubs. I can’t blame the police for using it; I only blame them for believing it.”
A 1983 report from the Office of Technology Assessment says, “It appears that the NSA [National Security Agency] (and possibly CIA) use the polygraph not to determine deception or truthfulness per se, but as a technique of interrogation to encourage admissions.”
edit: I am not advocating for this. Personally, I think they should be illegal to use.
We've been led to believe that NSA/CIA hire smart people. Smart people probably know things that were common knowledge in 1983, so what is the use of a polygraph administered to NSA/CIA staff thirty years later?
As the article mentions, the polygraph results are still occasionally used in court. Regardless of its utility as an interrogation tool, there are serious concerns about the scientific validity of its output. The use of polygraph output to bar people from jobs and as evidence in court deserves scrutiny.
Is that a good excuse to maintain a pseudo-science complete with "professional organizations" and "journals" and other publications? It pollutes a space that should be for the pursuit of truth.
http://www.salon.com/2000/03/02/polygraph/
Polygraphs don’t have to work to be a deterrent. People just have to believe that they work and can reveal whether they have committed crimes. The DOE doesn’t have to believe they work, either.
More important, polygraphs are an immensely effective interrogation tool; they need not detect lies. Lykken tells an anecdote of two cops interrogating a suspect at a time when copy machines were not familiar objects. Lacking a lie detector, the cops put a piece of paper in the copier that said “He’s lying!” They made the suspect place his hand on the strange machine while they asked him questions. When they didn’t like his answers, they’d hit a button on the machine. It would groan, whir, stink and shoot out a piece of paper that read “He’s lying!” Realizing that denial was useless, he confessed.
“If I was in the police business I would use [the] polygraph,” says Lykken. “It’s a powerful inducer of confessions, and you don’t have to hit ’em with any clubs. I can’t blame the police for using it; I only blame them for believing it.”
A 1983 report from the Office of Technology Assessment says, “It appears that the NSA [National Security Agency] (and possibly CIA) use the polygraph not to determine deception or truthfulness per se, but as a technique of interrogation to encourage admissions.”
edit: I am not advocating for this. Personally, I think they should be illegal to use.