If you have ever seen a crime movie or TV show, you probably have at least some idea of what a lie detector is. Also known as a polygraph, it measures a person’s physiological responses to questions and records these responses. The lie detection technology is highly accurate in the hands of a well-trained examiner. It has many applications, not only in criminology but in various aspects of civilian life.

However, it doesn’t work quite the way it appears on movies and in TV. In fiction, the machine is able to tell the examiner right away when the subject is lying. In reality, the examiner needs to analyze the data and recognize patterns that demonstrate the subject’s truthfulness.

Origin and History

The scientific name for a lie detection device comes from two Greek roots. “Graph” means “writing” and “poly” means “many.”  Therefore, a polygraph is a measure of multiple physiological functions together into one written record. Today, however, it is likely that the record is on the computer rather than on paper.

The lie detector as we know it today is nearly 100 years old. It was preceded by other devices that only measured one bodily indicator at a time. Examples include an 1904 Italian device to measure breath rate and an abandoned American concept measuring a person’s blood pressure. The theory behind each was that a person’s breathing and/or blood pressure increases when lying.

Stress Response

The theory behind a lie detection machine is that deception causes physical stress on the body. When a person is under stress, the sympathetic nervous system produces specific and predictable responses in the body. In particular, the following physiological functions all tend to increase:

  • Heart rate
  • Respiration
  • Perspiration (i.e., sweating)
  • Blood pressure

When performing a lie detection test, the examiner hooks up multiple sensors to the subject’s body to record these changes. The physiological data is tracked and reproduced on a paper graph or on a computer. Significant increases in one or more of the functions measured could indicate that a person is lying. However, it requires a skilled examiner to correctly interpret the results.

Test Process

The examiner will require the subject to sit in a chair with feet flat on the ground and arms resting on arm rests. It is then necessary to apply a number of sensors to the subject’s body, usually on the arm, one or more fingers, and the upper body. Each sensor measures a particular physiologic function. For example, a cuff on the arm measures blood pressure, sensors on the fingers measure perspiration, and the sensors on the upper body measure pulse and breathing rate.

On movies and in television, the subject doesn’t know what any of the questions are going to be beforehand, making it more dramatic when the examiner catches the subject in a lie. In reality, it is actually counterproductive to surprise the subject with a question because the subject’s response could result in a false positive. Therefore, the subject knows what the questions are going to be because examiner asks the subject all the questions beforehand.

The test includes three types of questions:

  • Relevant questions
  • Diagnostic questions
  • Irrelevant questions

Relevant questions are those that relate directly to the matter that the examiner wishes to know about. Irrelevant questions do not relate directly to the matter at hand. Diagnostic questions are those that the examiner asks to determine whether or not the subject will produce a physiological response when lying.

Once the questioning is complete, the examiner(s) measure the results. A person who demonstrates a greater physiological response to the relevant questions than to the diagnostic questions fails the polygraph. However, a person who reacts more strongly to diagnostic questions than to relevant questions passes the test and is deemed to have told the truth.


Today many, if not most, of the purposes to which lie detectors are put to use are private matters rather than criminal investigation. Examples include the following:


Polygraphs can determine whether one’s spouse or significant other has been unfaithful.

Employment Screening

Polygraphs determine whether a job candidate is trustworthy and has been forthcoming about his or her background. This is especially useful when the position is one involving great sensitivity. Testing can also be useful when suspecting theft, fraud, or breach of confidentiality or when considering candidates for a promotion.

Defense Attorney Support

Even if the results are not admissible in court, lie detection testing can be useful to defense lawyers in filtering cases, identifying false accusations, and developing a defense strategy.