|Federal Bureau of Investigation Educational Internet Publication|
One of the most misunderstood tools of law enforcement is known as the polygraph or "lie detector." The polygraph is based on the scientific theory that when telling a lie, a person's body will respond in a certain way (for fear of detection). Internal changes measured by the polygraph include breathing, blood pressure, pulse rate, and galvanic skin response (amount of moisture secreted by the skin).
The Polygraph, Past and Present
Scientific methods to uncover lies were developed in the early 20th century. In 1914, scientists experimented with all sorts of devices including pneumographs, which detected lies based on a person's breathing patterns. Later, the galvanometer came into use for its ability to measure change in the amount of perspiration on a person's fingertips.
The two researchers responsible for the modern lie detection test are John Larson and Leonard Keeler. Through their efforts they developed the polygraph, an instrument capable of continually recording changes in blood pressure, pulse, respiration and perspiration while the subject responded to questions.
The Polygraph Process
The polygraph is a scientific instrument used to measure how a person's body responds to various questions in a controlled setting. The test is based on the theory that when telling a lie, the individual being examined, known as the examinee, will fear being caught in the lie and his or her body will react to reflect that fear.
The polygraph consists of three phases: the pretest interview, the test and the post-test phase which may include the examiner questioning the examinee and his or her responses to the questions. Before beginning the first phase in the polygraph process, the examiner will gather information about the case from the investigator. Collecting this information allows the examiner to create appropriate questions for the actual examination. Having created his questionnaire, the examiner is ready for the pretest interview.
To begin the pretest interview, the examiner introduces himself or herself to the person being examined and also describes what will happen during the test. The examiner then gets the individual to explain his or her version of what happened and then discusses the questions that will be asked during the polygraph. Once the examiner has discussed this information with the examinee, phase two of the polygraph begins.
Within the test, the examinee will face a variety of questions. All the questions require only a simple "yes" or "no" answer. Through most of these questions, the examiner is trying to measure the individual's knowledge, participation and involvement in the crime under investigation . The idea is that, throughout the polygraph, the examinee's attention will focus on the questions that he or she finds most threatening. These questions are usually those which the examinee will likely answer with a lie. By giving these questions so much mental attention, the examinee is likely to have a similar physical reaction to these questions. For example, increases in perspiration, blood pressure and pulse or changes in breathing patterns may be telltale signs of a lie.
To establish how an individual would physically respond when telling the truth or lying, the polygraph includes questions that develop baseline readings for what examiners call known truths and probable-lies. For example, a question targeting a known truth for a 35-year-old male examinee might be "Are you 35 years old?" to which the examinee would truthfully answer "yes". The probable-lie questions are ones to which the examinee will likely respond with a lie because the truth is less personally or socially acceptable. For example, a probable-lie question might be "Have you ever lied to someone who trusted you?" Both known-truth and probable-lie questions allow the examiner to record the blood pressure, pulse, perspiration, and respiration of the examinee, and then compare those results to the results recorded for questions regarding the crime.
Once the examiner finishes with all of the questions, he or she evaluates the results to find out whether the individual told the truth or not. If the polygraph results indicate that the examinee was telling the truth, the individual is thanked for participating in the test and is leaves. But, if the polygraph shows that the individual was lying, the examiner will begin the third phase of the test.
The questioning, or interrogation, allows the examiner to try to persuade the examinee to tell the truth. While acting in a professional and understanding manner, a skilled examiner will use effective questioning techniques to make the examinee comfortable with telling the truth. After all, that is the purpose of the entire process.
The polygraph is one of many tools that law enforcement agencies, including the FBI, use when conducting investigations. Besides identifying guilt, the polygraph can eliminate suspects, verify witnesses' statements or informant information, and determine the truth of an individual's statement. When used properly by trained examiners, the polygraph can reduce the cost of investigations by saving time and can increase the number of convictions by encouraging suspects to confess if they have been dishonest prior to the exam.