Lie detector tests, also known as polygraph tests, are probably the most well-known kind of test that private investigators will use when obtaining information for a client. However, while we've all seen them in the films, many people don't actually know too much about them. So with that in mind, here's three truths about lie detector tests, so that you can get your facts straight!

1. Lie detector tests have been used since the 1920s

It is believed that the modern lie detector test was invented in 1921 by John Augustus Larson, a medical student at the University of California at Berkeley and a police officer at Berkeley Police Department. However, Larson began working on the concept much earlier, writing papers on polygraphs as early as 1915. Since its invention, the lie detector test has been used throughout the years by police departments, intelligence agencies, and private detectives.

2. Lie detector tests are used the most in the United States of America

Lie detector tests are used more in America than anywhere else in the world, which might explain why they appear in so many Hollywood movies! Usage, however, varies from state to state; sometimes the lie detector tests are taken in front of the judge and jury of a particular court case, whereas in other states they are administered by the police forces or the FBI during interviews in order to gather evidence. They are used in UK, however, unlike America, they are used mainly by private investigators and not by the police force or intelligence agencies. A lie detector test is not seen as reliable evidence in courts in the UK.

3. They start with a pre-test interview

Many people believe that lie detector tests start with the interviewee being hooked up to the polygraph machine, but in fact in most cases it starts with a pre-test interview, in which the person administering the tests will get to know the subject. This helps them to evaluate the results of the polygraph test, which usually leads to more accurate results. After this interview, the test then begins.