The Fundamentals of Lie Detector Testing

Lie detector testing is a method that is commonly used to arrive at the truth when something is being investigated. While the validity of the polygraph tests is still up for debate, modern technology has made it possible for these lie detector tests to yield more accurate results.

Polygraphs record and measure several physiological indices during testing. These indicators include blood pressure, blood volume, pulse rate, respiration, and skin conductivity among others. Results are evaluated and compared against accepted standards. Professional opinions are based on the discrepancies noted on the recorded data.

Historical Backgrounder

In 1921, John Augustus Larson developed an instrument that measured a diverse amount of physiological indices. A medical student at the University of California Berkeley and a police officer assigned to the Berkeley Police Department, Larson used the instrument for forensic investigations.

Larson’s instrument provided continuous readings of physiological data which improved on the prevailing system at the time. His instrument helped to solve numerous murders, robberies, sex-offenses, and thefts that would have baffled the police officers for a length of time.

Larson called his device a “cardio-pneumopsychogram,” while the mediamen lapped it up as “Sphyggy,” which is short for sphygmomanometer. Police Chief August Vollmer called it “lie detector” after it helped to uncover the truth behind a priest’s murder in San Francisco.

Leonarde Keeler improved on Larson’s instrument by replacing the smoke paper drum. Keeler incorporated ink pens to increase the efficiency of the machine. Keeler continued to incorporate ways to improve the instrument and in 1935, he added galvanic skin resistance to the polygraph.

John Reid then developed the Controlled Question Technique (CQT) which improved the methodology for polygraph testing. The CQT revolutionized lie detection testing as it emotionally aroused non-deceptive subjects and record less emotionally arousing answers from deceptive subjects.

Instrument Components

A polygraph instrument is made up of three main components. These are sensors attached to different body parts that collect and record data during the lie detector testing phase.

These components are the cardio-sphygmograph, the pneumograph, and the galvanometer.


The cardio-sphygmograph measures the heart rate and the blood pressure rate of the subject in question. It is the blood pressure cuff that is attached to the upper arm of the examinee. During the lie detector testing phase, the cuff is inflated. The air in the inflated cuff transmits the sound generated by the veins to a bellows which records the blood pressure. The frequency on the changes in the recorded sound is related to the heart rate.


The pneumograph records the subject’s respiratory rate. It is composed of two tubes. One is attached to the chest area and the second is wrapped around the abdomen. The tubes are filled with air and are ready to record data about the subject’s breathing.


The third component of the polygraph is the galvanometer. These galvanometers record the amount of perspiration of the subject. The electrical sensors are attached to the fingertips which are dense in sweat glands. As the amount of sweat increases, the electrical current resistance decreases and the changes are recorded.

Result Interpretation

The critical component in polygraph testing lies in the interpretation and analysis of the recorded physiological data on the polygraph charts. Examiners usually rely on numerical scoring and/or computerized algorithms for the test scoring.

Numerical Scoring

In numerical scoring, examiners use the 7-Position Numerical Analysis Scale. This scale relies on spot analysis where relevant questions have a position. Examiners look for changes in the amplitude, baseline, duration, and frequency of the signals in each identified spot. He then compares it to the activities in the nearest control question. He assigns values based on a 7-point scale (between -3 and 3). Spot scores are calculated by putting together the scores across charts for each channel. The grand total is the sum of all spot totals.

A grand total score of +6 and greater indicates that the subject expresses non-deception. A score of -6 and less indicates deception. Any score between -6 and 6 may be considered as an inconclusive result.

Computerized Scoring Algorithms

Computerized scoring algorithms are used to give a more accurate polygraph result. The Stoelting polygraph instrument relies on the Computerized Polygraph System developed by Scientific Assessment Technologies based on test results at the University of Utah. Axciton and Lafayette instruments utilize the PolyScore algorithms developed by the people at John Hopkins University Applied Physics Laboratory.

The polygraph instrument takes digitized polygraph signals and output the estimated probabilities of deception. PolyScore uses logistic regression to estimate the probability of deception. It also has algorithms used for detection and removal of artifacts and outliers.

Result Accuracy

A lot of people seem to doubt the accuracy of lie detector testing results. Forensic psycho-physiologists say detecting deceptive behavior is easier done than that of truthful behavior. Critics say that there are no real physiological standards for deceptive and truthful behavior.

The American Polygraph Association reports that polygraph test results are accurate 85 to 95 percent of the time. Some detractors say that the accuracy should be closer to 60 percent to 70 percent. The Congressional Office of Technology Assessment has found that false positive reports can be as high as 75 percent.


Polygraph tests are reasonably accurate and provide results that can help uncover the truth. Years of research, development, and use of lie detection equipment and methodology have installed the polygraph as the go-to method to verify the truth and detect deception.

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