[mtrimpe]

From what I've understood polygraphs aren't actually lie detectors but 'emotional distress' detectors.

That means you don't know whether what's being said is true or not, but you do know when you've found something worth digging deeper into...

[TDL]

Pretty much. They've been sold as lie detectors based on the notion that stress == lies, this is obviously incorrect. The same notion is also what make voice stress analysis unreliable as well.