It does not measure deception, which is the core problem, says Prof Aldert Vrij, who has written extensively on the subject.
![]() One of his accusers, Christine Blasey Ford, has already done so. But how accurate are these tests And how do they work Lets start with the basics. What is a polygraph test In short, polygraph tests record a number of different bodily responses which can then be used to determine whether someone is telling the truth. So polygraph tests do not measure deception or lying directly, but rather possible signs that a person could be deceiving the interviewer. ![]() Theres a fairly long pre-test interview that lasts for about an hour, says Prof Don Grubin, who has trained polygraph examiners in the UK. This focuses the individual on the questions theyre going to be asked and tries to remove any outside distractions. This is followed by a practice test, which usually involves a number of straightforward questions. The aim is to relax the individual so they are comfortable and able to understand how the process works. The equipment is then attached, and it usually includes a blood pressure monitor, electrodes which are placed on the fingers or palm, and two tubes which are wrapped around the chest and stomach. There may be something thats put on the tip of the finger that records blood flow and we also use something called a movement detector which is on the seat and picks up if youre trying to beat the test, Prof Grubin explains. Youll probably be attached to the equipment for 10-15 minutes but youll be in the room for about two hours, he says. Interviewers ask a number of control questions during the test and then compare the responses to the key questions. It finishes with a post-test interview, where the person will be able to explain any responses they showed. The curious history of the lie detector Can you cheat Yes, according to the experts. Theres no question that you can beat a polygraph test but you really need the training to do it, says Prof Grubin. You see websites telling you how to, but the reality is if you just go in and take a polygraph while hoping to beat it then youre not going to. But for those who dont have a qualified questioner to hand - what methods can work You might put a tack in your shoe which will cause, for example, a big increase in your sweating response, Prof Grubin says. Any sort of muscular activity or movement because you need to sit still. There are various drugs that people try but they tend not to be successful, he adds. But he cautions that most examiners will be able to spot any covert attempt to beat the test.
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