If a criminal can pass a lie detector test, watch out...sociopath alert!!!
That’s right. A polygraph works by measuring your physiological responses to stress, i.e. heart rate, breathing changes, etc on the theory that telling a lie crests stress, albeit maybe very small changes.
A sociopath or pathological liar actually enjoys lying and is not going to manifest the normal signs of stress. One of the reasons polygraph results are not admissible in court.
You don’t even need to be a sociopath. Just feel strongly about a subject and be resolute and clear. The test is meant to work with an interview where the interviewer causes stress and catches slip-ups. 90% of the polygraph is the interview and the mechanical aspects of it are just a minor tool.