In the movies and on TV shows someone is hooked up to a lie detector and asked the critical question and the needles jump all over the place and the suspect breaks down and confesses. In movies and TV, it is infallible, but results aren't used in court because of unreliable results.

John Larson invented the modern polygraph, or lie detector, in 1921, and the same basic concept is widely used today. Polygraph machines are used for criminal investigations, and part of some employment interviews.

Polygraph Examinations

The operator attaches equipment to monitor heart rate, pulse, breathing, blood pressure and perspiration rates. These record lines on a roll of paper and needles move back and forth in response to changes in the body. The modern methods record data to a computer. The interrogator then asks a series of questions for a baseline. After the baseline questions, the examiner asks a series of questions that may or may get to the point of the examination. The questions are repetitive and asked in different forms. The test will last from an hour and a half to three hours, or longer. It is often given after the subject has been questioned for several hours and under unfamiliar stress.

The Tests

The examiner uses one of three basic methods to ask questions. The Control Question Test, CQT, Directed Lie Test, DLT, and the Guilty Knowledge Test, GKT.

The CQT examiner asks a series of questions to establish a baseline to compare with the others. Some of these will be of other possible misdeeds. With the DLT system, the interviewee is told to lie so that the following answers can be compared with it. The GKT test involves questions only the guilty person would know. The GKT is the most accurate method, but the CQT test is the most used.


The results depend on the one giving the tests, and the results are subjective. Some operators have less than 14 days training on the machine and evaluation. Results are from the questions asked, how they are answered and how they are interpreted. Different operators may interpret the same data differently.


The American Polygraph Association asserts that a polygraph is 98 percent accurate. This number can be juggled to different percentages as it includes inconclusive results.

The Popular Culture and Polygraph

The movie, Call Northside 777, was a popular movie that had the polygraph as its center. The 1948 movie starred James Stewart and was based on a true story. Stewart played a reporter that researched an old crime and concluded the man imprisioned for it was innocent. A big point of the movie was that the polygraph proved his innocence absolutly. The movie had the effect of setting the infalibility of the machine in the public's mind.

Modern talk shows such as The Maury Povich Show and The Steve Wilkos Show used lie detectors as a large part of their programs. Other similar programs, such as The Moment of Truth, use the lie detector as a focus for the program. The polygraph associations have issued statements about some of these programs the the machine is being used incorrectly to get valid results. Polygraph use is also shown in tv detective programs on a regular basis.

Polygraph Skeptics

Several organizations and people feel the polygraph isn't based on a valid premise. Among the skeptics are the American Medical Association and the American Psychological Association.

Doug Williams advertises himself as a police polygraph expert, and has given polygraph exams. He claims the test is inaccurate, and because the subject is nervous, may fail the test. Williams points out some polygraph tests are given after 8 to 12 hours of questioning. Being stressed and tired can skew the results. He offers manuals and DVDs for sale over the internet with instructions to pass the test.

Several years ago the TV newsmagazine, 60 Minutes, did a story on polygraph examiners. They hired three polygraph companies and told them someone stole a camera. They interviewed the same people each time. They told each polygraph company they thought a different person was guilty. The polygraph examination found the suspected person guilty even though nothing was stolen. The examiners missed that all those they examined were lying about something being stolen and telling the truth when they said they hadn't stolen anything. The operators didn't pick up on the fact that nothing was taken.

Unfortunately, the polygraph isn't as dramatic and conclusive as shown in the movies and on TV. Both sides has supporters and detractors.

If given a polygraph test, approach it like the examiners probably know all the tricks to beat the test. When taking one, get plenty of rest beforehand, relax and tell the truth.