Do Polygraph tests really work?
The problem is not that polygraph tests don’t work, it’s that they don’t actually measure whether someone is telling the truth, but rather they measure how anxious someone becomes in response to a stimulus (e.g. a question about committing an illegal act).
Polygraph tests are scientific if what you’re trying to measure is someone’s anxiety/nervousness when prompted by different questions, but the accuracy in determining whether someone is actually lying is not so great (at least not enough to be used as evidence in court). This is party because there are various reasons why you may become anxious when answering a question even when you’re being truthful (e.g. you may be embarrassed by a question, but decide to [nervously] answer the question honestly anyway). Also, not everyone who lies will necessarily become anxious when they do so…
One key issue is that there are known ways to ‘fool’ a polygraph which can make the test unreliable if the individual knows how to ‘cheat’. For example, if you intentionally apply stress/pain to your body during control/neutral questions, you can produce an anxiety-like response and minimize the difference in anxiety response from questions you are telling the truth to and questions you’re lying about (and this may produce inconclusive results or perhaps even a passing result).
Also, polygraph tests are ineffective when you ask people questions about matters they really couldn’t care less about (e.g. they really don’t care about getting caught and really don’t care about the potential consequences of being found guilty and thus don’t become anxious about being asked questions about their activities). Also, people who are psychopaths or sociopaths don’t typically show anxiety responses like the normal population and can often lie straight to your face with no physiological indications of anxiousness/lying/guilt.
Lastly, if you know how a polygraph works, you can simply be super-confident about all your answers (knowing that the machine is basically measuring confidence) and you may pass simply by not showing any anxiety (because you knew you could beat the test just by being confident in your lies).