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Police Polygraph Test

John Augustus Larson, a student at the University of California Berkeley and a Police officer, invented the polygraph, famously known as the lie detector, in 1921.

Although still controversial among psychologists, the name polygraph is derived from the function of the machine that records several different body responses at the same time during the interview process of the subject.

An earlier lie detector was invented in 1902 by James Mackenzie, but was less successful. Later in 1914 Vittorio Benussi, an Italian psychologist discovered a method of calculating the differences in inhalation and exhalation as a means of verifying the truth, giving great insights into the emotional changes that occur when a person lies.

In 1915, there was more groundbreaking work on the lie detector with Dr. William Moulton Marston, an American Psychologist and attorney developing the discontinuous systolic blood pressure test that was later added to the modern day polygraph. His technique was based on measuring the intermittent systolic blood pressure readings of a suspect during questioning by attaching a standard blood pressure cuff and stethoscope to the suspect.

John Larson’s addition of the respiratory rate to these developments produced the first lie detector, which was used successfully in many criminal investigations. The first person to simultaneously and continuously measure respiration rate, blood pressure and pulse rate, he produced a chart of the responses on a revolving drum of smoked paper- a welcome opportunity for record keeping and analysis.


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After gaining experience under John A. Larson, Leonarde Keeler, in 1926, refined the lie detector by introducing inked pens that recorded the relative changes occurring in the suspects pulse rate, blood pressure and respiratory rate. In 1938, further developments of the Keeler polygraph followed including a third psychological measuring component for detecting deception. This was measured through changes detected in the subjects skin resistance, signaling the birth of the polygraph as it is used today.

In 1947, a lawyer from Chicago, by the name John E. Reid, developed the Control Question technique, which included questions that were designed to create emotional arousal for non-deceptive subjects and less emotional arousing for deceptive subjects. The Reid control question technique was a major breakthrough, replacing the relevant/irrelevant question technique, which was less effective. Leonarde Keeler founded the first polygraph school in 1948, in Chicago Illinois.

Polygraph tests are designed to identify involuntary responses occurring in a person’s body when the person is subjected to stress. The trained examiners who monitor the polygraph tests are known as forensic psycho-physiologists. They look for significant amounts of fluctuations in the various physiological activities monitored by the polygraph. For example:
Respiratory rates
Two pneumographs measure the inhalation and exhalation rates of the subject

Heart Rate
The blood pressure is measured using a blood pressure cuff placed around the subjects arm. When the blood pumps through the arm it makes a sound that causes pressure changes and air displacement in the tunings that run from the cuff to the polygraph. These are measured and converted into electrical signals.

Galvanic Skin resistances
Also known as electro-dermal activity is a measure of the sweat on a person’s fingertips. Fingertips are the most porous areas on a human body, and probes placed here detect slight change in sweatiness. When the fingers are sweating, galvanometers placed on the skin conduct electricity more easily that when the skin is dry. Some polygraphs also measure the movements of arms and legs.

From a person’s breathing rate, pulse, blood pressure and perspiration, the examiners can conclude a suspects lying and truth patterns which can be used when real life questions are presented.

When the polygraph test begins, the examiner asks three of four questions to establish the norms of a person’s signals like, “Are you married?” Then the true questions are asked. During and after the examination, examiners can refer to the graphs to ascertain the vital signs, noting where vital signs changed substantially. Changes such as a faster heart beat, higher blood pressure and increased breathing or perspiration are usually indications that the person is lying. A well-trained examiner can detect lying with high accuracy.

Polygraphs use changes in involuntary body responses to predict lying behavior. Using the control question technique, interrogators ask the various questions both related to the incident the suspect may be involved in and others outside the scope. The interrogators assume that guilty people are more likely to be stressed when interrogated. The effectiveness of lie detectors is still in question in various parts of the world. However, it is used in the U.S. and some countries in Europe.

Taking a polygraph test is considered an important part of the police recruitment process. The process begins with a pretest interview where candidates are informed about the examination, and the proficiency of the process. By informing the subject about the effectiveness of the polygraph test, examiners can sharpen the differences between deceptive and non-deceptive subjects as they react to the information. This takes about 20-30 minutes and is usually voluntary. Candidates are then provided with a waiver form, which they sign agreeing that in case of employment, these results will be provided to the police department. The pretest interview also advises the subject on the nature of questions to expect including questions about previous employment, driving record, drug use and criminal history.

The polygraph is inadmissible in court but is a great way for police academies to screen applicants. Effective polygraph testing requires the skill of the examiner, because it solely depends on the interpretation of the operator. The most commonly used interrogation technique is to draw the subject into trusting the examiner by showing empathy. Candidates should be focused, avoid divulging too much unnecessary information. Candidates looking for placements as law enforcements officers should avoid making additional irrelevant statements, which can raise red flags.

Polygraph examinations are governed by laws of the Equal Employment Opportunity Commission and hence are regulated as to the types of questions they can ask. Candidates are not subjected to any discrimination on the basis of gender, race, sexual orientation, etc. Typical questions asked on the police polygraph test touch on honesty and peddling of lies, petty cheating, stealing, basic drug use, crime conviction, and withheld information regarding the employment opportunity.

Questions in pre-employment interviews are derived from John E. Reid’s approach using various control questions. Though it is still not clear how Reid’s pre-employment control questions are different from relevant questions, it is a safe assumption that both truth and non-truthful subjects will be similarly concerned about subject matter and control questions. Candidates for the position of police officers should be aware of the basic equipment and the intention of the examination. As long as the candidate is open, honest and relaxed, they will easily pass the test.

The 10 questions asked by the examiner are usually graded in the categories of truthful, untruthful and inconclusive. For candidates in the hiring process, getting an untruthful result in any sensitive question for the employer can be a problem, but employers are also known to conduct follow-up checks to collect more information and clarify the information.

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