Module 5 - Polygraph Testing & Forensic Document Analysis

The Invention of the Polygraph Instrument

In 1915, Dr. William Marston, an American lawyer and psychologist, found that the blood pressure of people increased consistently when they were lying.  This fact lead Marston to design an instrument that could be used during interrogations to monitor the blood pressure of the person being interrogated.  Dr. Marston"s device served as the basis for the modern polygraph instrument or what many commonly call the lie detector.

John Larson, an American medical student working for the Berkeley Police Department, invented the modern polygraph instrument in 1921. Larson"s device was able to measure blood pressure, heart rate, and respiration rate simultaneously.

John Larson also developed an interview technique that was used during a criminal polygraph test called Relevant/Irrelevant (R/I) questioning.  During R/I questioning, a mixture of questions relevant to the crime (for example, Do you own a handgun?) and irrelevant questions are asked (for example, Are you forty years old?).  The basis for this type of questioning was that an innocent person has a similar physiological response to both types of questions, but a guilty person would react more strongly to the crime-relevant questions.  The development of Larson"s polygraph instrument lead to many major police agencies in North America using the polygraph instrument in their criminal investigations in 1924.  Currently, all major police agencies across North America have polygraph units.

In the 1926, Leonarde Keeler modified the polygraph instrument designed by John Larson by adding a device that measured electrical skin conductivity or electrodermal response.  The measurement of electrical skin conductivity is based on the fact that skin becomes a better conductor of electricity when a person perspires because of the high concentration of sodium chloride (NaCl) in sweat.  Electrical skin conductivity is monitored using electrodes attached to the fingertips because a high density of exocrine sweat-secreting glands are located  in the hands.   Keeler also devised a new questioning procedure that replaced the R/I questioning procedure during a polygraph test. (You will learn about this in the next lesson.)  In the 1930"s, Leonarde Keeler helped establish a school that provided specialized polygraph training.

In general, the polygraph instrument and related questioning techniques developed by Leonard Keeler has surprisingly seen only minor modifications since the mid-1920s. In the last ten years, the advent of computer technology has transformed the polygraph instrument into a digital computer-based device; however, all the same measurements are still analyzed.

The Invention of the Polygraph Instrument

The polygraph instrument is designed to monitor four involuntary sympathetic nervous system responses (blood pressure, heart rate, breathing rate, and skin conductivity) that occur when a subject is given a polygraph examination.  The polygraph instrument not only monitors these sympathetic responses; it records this physiological data so the polygraph examiner can anlayze and interpret the results.

Polygraph instruments are either analog or digital.  The analog polygraph instrument converts the physiological data it collects into electrical signals that cause ink-filled needles to record data on scrolling paper.  The digital polygraph instrument monitors the same physiological data as the analog polygraph; however, the data is converted by computer software into electrical signals displayed on a computer monitor. Some polygraph examiners still use the analog polygraph instrument; however, the majority of polygraph examiners (especially in large police departments) use computerized digital polygraph instruments.