A polygraph machine is a device that attempts to predict whether someone is lying or not based on physical information about them at the time of testing. When people lie, they tend to become more nervous and their rate of breathing, their blood pressure and the amount they perspire changes as a result. These factors increase when someone is probably telling a lie.
Although is recognized as a valid piece of evidence in a criminal trial, polygraphs have a number of unreliable factors and as a result there are a few criteria that must be met before they can be used:
- There can’t be any valid scientific claims that the test might be unreliable
- The test must be performed by a polygraph expert
- The procedures must be recorded to provide proof that they were followed
Polygraphs can sometimes give an indication of the truth, but their results are not definitive. The first factor which throws them into question is that they examine a nervous response. The suspect taking the test might be nervous for a number of reasons other than telling a lie. The test itself is stressful, tubes hooked up to the chest, blood pressure cuff on the arm, and fingers lying on cold metal plates while being asked a series of questions by a stranger. However, the topic of conversation might be stressful as well, if the questions involve a traumatic event.
The American Polygraph Association emphasizes that the test’s results are no wrong, but just inconclusive. They can give an indication of the truth, but should not be taken as the truth itself. In order to take out some of the extraneous factors, polygraph procedures now involve specific questioning techniques, such as questions with ‘yes’ or ‘no’ answers, and questions which are off-topic to reduce the stress of the conversation.
Do Polygraphs Really Work?
Polygraphs strongly rely on one theory – that everyone has a stress response in their body while telling a lie. By measuring many different types of physical responses, the polygraph can accurately pick up on some of those stress responses and print the results out on a graph. Polygraphs have both reliable and unreliable aspects in their use. They can be considered reliable because hardly anyone would be able to control all of their physical responses at the same time while telling a lie. Also, the polygraph experts find out what their natural physical response is to a lie before the actual test begins. The only way to ‘beat’ the detector would be to have a different response in the pre screening than in the actual test. This is almost impossible for anyone to do.
The main factor which makes a polygraph unreliable is that there is no way to distinguish why someone is having a stress response. Just because heart rate or perspiration might increase does not necessarily mean that someone is telling a lie. As a result, many courts do not allow polygraphs to be admitted as evidence. Under some circumstances, they are allowed when both the prosecution and the criminal defense agree before the test is taken that it will be admitted as evidence.