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Forensic Polygraph Analysis

Forensic polygraph examiners often play a very important role in criminal investigations for local and national law enforcement. Polygraph analysis is part art, part science. In addition to maintaining and operating the polygraph machines, the forensic polygraph examiner is also responsible for administering the actual polygraph exam, and then interpreting the results to come to a reasonable conclusion on the truthfulness of the test subject.

Polygraph exams are often not considered sufficient proof on their own to warrant a conviction, or even to charge a person with a crime. Even in jurisdictions where they are not, the results can still point investigators in the right direction, and also to help them narrow down the list of potential suspects. There have been some criticism regarding the usefulness of forensic polygraph analysis. Considering that suspects and witnesses are only subjected to the test voluntarily, because it is illegal to force someone to submit to such a test against their will, investigators do not always get to examine all the suspects they would like in this manner. Despite the legal ability of suspects and witnesses to avoid a polygraph examination, many people view taking one as an opportunity to clear their name, while others will submit because they either think they can beat it or the results cannot be used against them.

Polygraph examiners must be trained in several specialties. Like forensic scientists, they must understand the principles that govern their analysis. But unlike other forensic disciplines, which often have straight forward results, the data resulting from a polygraph examination ca be much more open to interpretation. So in addition to knowing the general stress ranges of people when they are lying, the polygraph examiner must also establish each test subject’s individual range. This is the purpose of the “control” questions at the beginning and end of each polygraph test, because due to the uniqueness of each person’s physiology, the readings that indicate regular stress such as the heart rate, body temperature, and blood pressure changes are compared with those while being untruthful. Setting test subjects at ease in an environment where they are very much “on the spot” can be quite difficult, especially if the subject is a suspect in a serious crime. Most examiners draw out the control period for an extended period of time, to establish what the patient’s resting state.

The examiner must also be trained in how to spot the methods people can use to “beat” the polygraph test. It is not uncommon for test subject to try to control their breathing or attempt other disruptive behavior whenever they are asked a question. The idea behind this is to alter the patterns of the readings to make analysis impossible. It is the knowledge of how to spot such behavior and still get meaningful data that separates a forensic polygraph examiner from being a technician.

Another skill the forensic polygraph examiner must learn is how to interpret body language and behavioral clues. Before the subject is even “hooked up” to the polygraph machine, a successful polygraph examiner begins the analysis by observing the subject and reviewing with them the questions that will be asked during the exam. It is during this part of the process that a skilled examiner can actually gain the trust of a subject and get them to reveal clues or even confess to their actions.

  • Gaby says:

    One of the major problems with the poagrylph is that it is based on a false premise that stress, and consequent activation of the sympathetic nervous system, is evidence of guilt, rather than the stress innocent people feel from being placed as a suspect. In fact one of the major arguments against it is that psychopaths, who present a major problem on both sides of the polizio-judicial system (many criminals and rogue police and prosecutors alike), have a basic defect of part of the brain called the amygdala, which makes them uniquely resistant to anxiety! And of course uniquely convincing liars. So the poagrylph is weighted in favour of convicting the innocent and freeing the psychopath. It is small wonder it is so beloved of prosecutors and the systems they control, since they are paid to get convictions, and soft ones of innocent people are the easiest to go for. The poagrylph should be banned.

    July 9, 2012 at 1:18 am
  • Capatin says:

    A few years ago I’d have to pay sonmoee for this information.

    November 28, 2012 at 2:30 am
  • Tyler says:

    Excellent article. Police aalyws seem to say they use the polygraph as a tool to rule in or out a suspect, but this speculative method has been frequently used in the media IF the defendant failed, that is. In our case (Jamie Snow), the defendant passed a polygraph in 1994, but was never ruled out. We have been able to obtain the documentation of the tests of two jailhouse informants that took polygraphs well before Snow’s arrest, and the results are blocked out. Obviously they failed them the state would have been screaming it to everyone if they had passed them. It is frustrating that polygraph results seem to be used only if it is favorable to the state.

    November 28, 2012 at 6:24 am

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