Good news – Marvel's Agents of SHIELD has been renewed for a second season. In a later episode (no spoilers) the programme includes 'the most advanced polygraph in the world'.
Polygraphs are used in real life, and labelled (incorrectly) as 'lie-detectors'. It's easy to see why they're so popular in fiction (in Homeland, it's been used a few times), but a magic box that tells truth from deceit is just that: magic. It doesn't have a basis in science.
So, how is it meant to work, and why doesn't it?
The polygraph measure autonomic physiological responses. They actually vary quite a bit with some models being more advanced than others (if you're doing one, for fun, which has a belt around the chest try leaning back in your chair. It may alter the signals and make the tester think you're a chronic drug user). The basic operation, however, is identical.
Aspects measured include things such as how much you sweat, heart rate, and that sort of thing.
Initially, you'll be asked a couple of Yes/No questions. I believe (I did my research on this about a decade ago, so small errors are possible) that they'll be mostly truthful for the set-up phase, to establish a baseline of what your readings are when you're honest.
After this, they'll ask you a series of Yes/No questions about the matter at hand (have you been having an affair, do you know who stole the nukes, why did you watch Eurovision last night, and so on).
There's just one small flaw in an otherwise ingenious device: it doesn't work.
A report by an American intelligence agency (which, I hasten to add, was freely available when I was at university) found that the polygraph was barely better than tossing a coin, and that its only real value was to frighten idiot criminals who believe in magic into confessing.
We all know some people are better liars than others. Some people sweat more. Some are very calm. The polygraph can't tell between sweating due to nervousness because you're a serial killer and sweating due to nervousness because you've just had a shitload of wires attached to you and have been asked by a stern man in a suit whether you've killed several people.
There are also various ways to deliberately fool it. For example, entering a Zen Buddhist trance. That's quite tricky, but easier methods include a copper coin under the tongue, or a drawing pin in your shoe (just prick your toe when giving an honest answer. It'll cause your readings to spike, and then, if they do when you're lying, they won't look beyond normal).
This is actually a serious issue. Leaving aside the artistic licence of fiction writers to exaggerate the polygraph to make it something that actually works, it's used in the US and, sadly, has started to be used in the UK.
Paedophiles let out of prison undergo polygraph tests to 'prove' they have not reoffended. Brilliantly, paedophiles are up there with psychopaths as the best liars in the world. Terrifyingly, this gives a stamp of 'scientific' approval that they've not abused any children lately, even though the polygraph is worthless as a lie-detector.
The polygraph is a triumph of marketing over science. If policymakers paid more attention to science and a little less to fiction we wouldn't use the damned things at all.