Lie detector machines work based on the theory that a person’s body involuntarily reacts differently when they lie than when they tell the truth.  The lie detector or polygraph machine observes and records these changes.  First, the person administering the lie detector test asks the person in question a series of control questions in order to gauge his/her body’s normal reactions to true or false statements.  Then the real questions are asked and the reactions to these responses are compared to those of the control questions.  The first machine was created in 1921 by John Larson and it recorded the changes in a person’s breathing, blood pressure, and pulse rate.  Later models, such as the psychogalvanometer were able to measure electrical responses on the skin and others looked at perspiration.  Now, a recent innovation called the psychological stress evaluator investigates the validity of a statement based on voice modulations.  Although lie detectors are used in police investigations and sometimes in courts, they are not believed to be foolproof, and thus cannot be used as concrete evidence.  Because of the complexities of a person’s emotions, especially in a high pressure setting such as a courtroom, the readings will not necessarily always be accurate.

*Pictures from www.polygraph.com/ and www.truthproof.com/ polygraph.cfm