Right. The point is: it absolutely does NOT measure what it claims to measure, i.e. truthfulness.
You can detect indicators of stress... or hot weather... or stage-fright (admittedly a form of stress)... or too much caffeine... or an underlying (maybe undiagnosed) medical condition, etc. So it does not even necessarily measure "stress".
It's about as useful as the so called "fruit machine" which they used to test for homosexuality[0], in that it is utterly useless while at the same time can be quite ruinous for people. People have been fired over polygraph "fails", and while not admissible in courts, people probably have been fingered for crimes after they failed polygraphs. Also, criminals have gone free after passing polygraphs[1].
>But everyone knows that it's not very reliable in almost every circumstance it's used.
You and I may know that. But a lot of people actually do not. That's why it's still used. Either because people administering those tests think it's "good science", or because those people administering it know that while it's all bullshit the person they are testing might not know that and break down and admit to things. Remember that fake polygraph on the show The Wire, which was just a copier they strapped to the suspect. If I remember correctly that was based upon true events.
A quick google shows e.g. you can hire "polygraphers" to e.g. "test" if your partner was unfaithful, making claims such as: "However, assuming that you have a good polygrapher with a fair amount of experience in working with betrayal trauma, you're going to get results that are at least 90% accurate or better."[2]
The US (and probably a lot of other) government(s) like their polygraphs very much, too[3].
> you can often tell (i.e. with better accuracy than chance) whether or not a subject is able to give a confident, uncomplicated yes-or-no to a straightforward question in a situation where they don't have to be particularly nervous
Uhmm, if somebody sat me down in a room, strapped all kinds of "science" to my body and then asked me questions, I'd be quite nervous regardless of whether I am truthful or not. In fact, I'd be even more nervous knowing it's a polygraph and bullshit, because I cannot know if the person administrating it would know that too.
If that somebody then asked me "Have you ever killed a prostitute?", or "Have you ever colluded with the enemy?", or "Have you ever cheated on your partner?", or "Have you ever stolen from your employer?", for example, my stress would certainly peak despite being able to confidently and truthfully answer "No!" to all of those questions. And I am sure the polygraph would "measure" my "stress".
[0] Yes, that was a real thing too. https://en.wikipedia.org/wiki/Fruit_machine_(homosexuality_t...
[1] E.g. the Green River Killer Gary Ridgway passed a polygraph, so the police turned their resources to another suspect who failed the polygraph. That was in 1984. Ridgway remained free until his arrest in 2001. He killed at least 4 more times after the investigation stopped focusing on him after that "passed" polygraph.
[2]
https://www.affairrecovery.com/newsletter/founder/use-abuse-...
[3] https://support.clearancejobs.com/t/the-differences-between-...