How Does A Lie Detector Work?

A "Lie Detector," which is actually a Polygraph test, is a method used to detect deception in someone suspected of a serious crime.

 

A polygraph cannot actually "see" a lie. What it shows is body reactions when the person is stressed. A number of sensors are attached to the person being questioned, and when the person's body reacts, that reaction shows up on either a piece of paper (the graph) or, more recently, on a computer screen, or a digital polygraph. The term "polygraph" means 'many graphs,' or papers.

 

When you arrive to have a polygraph done, you will be told to sit in a chair and there will be several items placed on your body:

 

• An air-filled rubber or flexible tube on your upper body, and one around your abdomen. These will detect changes in breathing.
• A fingertip plate, or galvanometer, that will detect perspiration (sweating)
• a blood pressure cuff to measure changes in blood pressure, particularly spikes (the blood pressure rises suddenly)

All of these things will detect changes in your body that are mostly involuntary; that is, most people cannot control them. An increase in any of the above may mean you are upset or are deceiving the questioner.

The questioner will ask a few 'control' questions, ones you know the answer to and will not be deceptive. Such as, do you have brown hair? Are you wearing black shoes today? Is your name Alex Jamison?

 

The questioner then will ask a series of questions that have 'yes' or 'no' answers, including some 'control' or 'safe' questions mixed with some direct questions about the crime: Did you do this? Have you told the truth about it to detectives? Do you know who did this? Were you involved?


Here is a video of a Bloomington Police officer demonstrating how it works:

http://www.bing.com/videos/search?q=how+does+a+lie+detector+work&&view=detail&mid=A285FFCE7D12B5FF85EDA285FFCE7D12B5FF85ED&FORM=VRDGAR

 

The reactions from your body show up in graph form, either on paper or on the computer, and look like this:

 

 

 

 

The only state allowing a polygraph test in court as evidence is New Mexico. Other states do not consider them reliable enough to prove a person's guilt or innocence and are aware that a false positive or negative result can jeopardize the wrong people.

 

This is why a polygraph is almost never allowed in evidence: It is possible to 'outsmart' a polygraph test. People have tried using sedatives before the test so they are not 'ruffled' by any questions asked of them. They have also tried to self-inflict painful stimulus during the test, so that they are always agitated; such as biting the tongue or putting something painful in their shoes. This could raise the blood pressure throughout the test.  It is possible that someone with severe mental illness, such as a psychopath, would be able to lie about anything and feel no guilt or upset whatsoever and indicate no physical reaction. Hence, that person's test would not be a good source of the truth. It is also possible that an innocent person would be so upset by the test that they give 'false positive' reactions, which do not truly indicate guilt.

 

People who are expert at polygraph testing are known as Forensic Pschophysiologist. Training to be certified in performing the test can be completed by law enforcement workers as well.

 

What law enforcement careers might involve the use of a polygraph:

• Detective
• Attorney
• Forensics Investigator
• Police Officer
• Homeland Security
• Fraud investigator
• FBI

Degrees would be in Criminology, Social Studies, Law, Law Enforcement, Criminal Justice, Psychology.