According to the American Psychological Association, “There is no evidence that any pattern of physiological reactions is unique to deception. An honest person may be nervous when answering truthfully and a dishonest person may be non-anxious.”
Polygraphs don’t actually detect anything meaningful except that the person is under stress – and simply being given a polygraph is sufficient to trigger the stress.
Even the creator of the machines stated that they were bunk. They’re purely a prop for interrogators.
Their real value is to work as a placebo to trick the suspect into thinking that they will be caught in a lie. This in turn amplifies behaviors for concealing lies – which makes the lies much easier to catch by a trained observer. Also if the criminal believes the lie has been ‘found out’ they are much more likely to confess.
There are also all sorts of cool tricks that can be used to amplify the placebo effect and convince the individual that they have been caught in a lie.
For instance an individual interviewing for a security clearance at an agency with a high level for security clearance mentioned the following.
He was taking a polygraph, about half way through they had asked a question about ‘Have you ever engaged in criminal behavior’ to which he answered ‘No’.
The technician showed him a big squiggle on the polygraph and explained that it had detected a lie. The interviewee maintained his innocence. The technician then called his supervisor, who came in wearing a white lab coat, and closely examined the squiggle, confirming that it indeed showed evidence of a lie.
So the guy thought about it for awhile, and finally confessed to something – which resulted in him being disqualified for the position.
Of course the reality is that it was almost certainly just a random squiggle, that the machine hadn’t detected anything, that the first technician lied, and that the supervisor was a planned action simply to amplify belief in the placebo using the expert authority trick – to give the exact result – people confess since they were certain that they were caught.
In the US, polygraph tests are not admissable evidence, neither is whether the suspect refused to take one. SCOTUS said something to the effect that polygraph tests are totally unscientific, and not better than tossing a coin.
Cops know this, and will use them only when they think it’ll intimidate the suspect into confessing something he/she wouldn’t otherwise confess.
So if the cops ever ask you to take a polygraph, the only correct answer is “Lawyer!”.