- The polygraph instrument is a legacy system with a long history of use in law enforcement.
- While it may be challenged at court, polygraph testimony can be allowed.
- There has been a decrease in the cost of these machines due to market competition.
- It is a well-known scientific investigation tool due to its high exposure in the media and popular culture.
- Due to its increased popularity, its reputation has been tarnished especially due to some shows that use the polygraph instrument ineptly.
- It is subject to countermeasures, false positives, false negatives, and inconclusive results. As already mentioned in a previous lesson, research has shown that its validity is immeasurable.
- The accuracy rates of the test can vary widely ranging from 50% to 87.5%.
- The examination can be very lengthy and requires the subject to remain still while hooked up to numerous sensors thereby potentially altering readings as the exam goes longer.
- Training to become a polygraph examiner can be a costly and lengthy process, yet there is no recertification program to determine continuing compliance of those already certified.
As mentioned in previous lessons, the polygraph instrument is not an absolute tool. Instead, it is but an investigative tool. A good investigator would do well to remember the following limitations of this machine:
- It is an investigative aid, never a substitute for an investigation.
- It is not a lie detector, but a scientific diagnostic instrument.
- It does not determine facts, only diagnostic reactions.
- It only records responses, not determines the truth.
The test must not be given until enough facts of the case have been established to permit an examiner to prepare a complete set of suitable questions. The test should not be given without voluntary consent of the subject nor should it be given before the accusations could be explained to the subject. No indication to any person or place in any report that a person will be considered guilty because he refused to take the test.
Additionally, the test should not be conducted on any unfit subject nor should it be used for the mental or physical evaluation of any person.
As already mentioned multiple times, the polygraph instrument is not a lie detector machine. Rather, it is simply a machine that senses and records the physiological changes in the subject. As such, the test and its interpretation are subjective in nature. Due to this, erroneous results can potentially lead to the incarceration of innocents if it is considered the sole reasoning behind a conviction.