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Feature Article: Polygraph and the Des Moines Police Department

Daniel Defoe wrote in 1730:

"Guilt carries fear always about with it; there is a tremor in the blood of a thief, that, if attended to, would effectually discover him; and if charged as a suspicious fellow, on that suspicion only I would always feel his pulse, and I would recommend it to practice. The innocent man which knows himself clear and has no surprise upon him; when they cry ‘stop thief’ he does not start; or strive to get out of the way; much less does he tremble and shake, change countenance or look pale, and less still does he run for it and endeavor to escape."

And so it goes, though many years have passed since the author Defoe wrote of using human physiology in the detection of deception, the basic principles still apply. Human beings respond in subconscious actions that were developed over thousands of years by our ancestors. When early man perceived a threat to his well being, his body responded, his hands became sweaty- allowing him to grip objects better, his breathing rate increased- providing essential oxygen to fuel his muscles, his heart rate increased- pumping blood to critical areas at a more rapid rate, and his hair even stood-up straighter, projecting a larger physique to the threat.

While telling the truth and lying may not be the same level of threat as our ancestors were confronted with in the past, our instincts still identify the threat and respond accordingly. This reaction is the "fight or flight" syndrome. The polygraph simply measures the physiological reactions to a stimulus within the body. The sweat response can be measured by conductance or resistance across the skin, the blood volume and rate is measured the same way a physician would- with a blood pressure cuff, and the changes in breathing can be monitored by measuring expansion and contraction of the chest and abdomen.

The Des Moines Police Department has used the polygraph instrument to detect deception since 1960. Throughout the years there have been several improvements to the instruments used by the department. In the 1970’s, Electro-mechanical instruments replaced the mechanical instruments of the 1960’s. Both of these instruments recorded physiological changes of the person being tested on a paper graph to be analyzed by the polygraph examiner. Today the polygrams are recorded on a computerized instrument, the polygraph software enables the examiner to easily make adjustments, store the polygrams, analyze the results, and print a written document.

As important as the instrument is the polygraph examiner, all examiners are required to attend a recognized polygraph school and complete extensive on-the-job training. The polygraph examiner must continually work to enhance professionalism by attending annual seminars and maintaining membership in national and state polygraph associations. The examiner must interpret the recorded physiologic reactions and determine if the examinee is being truthful or deceptive. The polygraph is used in virtually every type of criminal investigation, providing an invaluable investigative aid to detectives in solving homicides, rapes, robberies, kidnappings, assaults, burglaries, and child abuse.

The polygraph has been used extensively as a requirement for police applicants to assist in selecting the most desirable candidates for employment. The polygraph has been utilized to clear many falsely accused suspects as well as to determine guilt in the perpetrator of the crime.

The Des Moines Police Department will continue to use the polygraph as a supplement to professional investigation as it has for the past forty years.