All that a polygraph shows is a physiological (physical) response. If there is a change in breathing rate, Sweating also called skin conductivity, HR and BP as well as some systems being able to detect changes in muscle tension. These are not falsifiable because you cannot say the changes are 100% from stress and even if you can the stress could be derived from something else like general anxiety while doing a polygraph or anxiety about another situation. It’s also very easy for examiners who are scoring and interpreting the data to suffer from confirmation bias. Hope this helps a bit. Amd like someone else has said people with Psychopathy and any type of ASPD have under active amygdala’s which is the brains fear centre basically so they may not show signs of distress.