• The Truth about Polygraphs

    This post is part of a series of guest posts on GPS by the undergraduate and graduate students in my Science vs. Pseudoscience course. As part of their work for the course, each student had to demonstrate mastery of the skill of “Educating the Public about Pseudoscience.” To that end, each student has to prepare a 1,000ish word post on a particular pseudoscience topic, as well as run a booth on-campus to help reach people physically about the topic.


    The Truth about Polygraphs by Kelsie Witt

    Leonarde_Keeler_1937Polygraphs are more commonly known as lie detector tests.  They are used on television shows and movies unmasking the truth and assisting in putting away criminals.  In reality polygraphs are widely used in four different sectors: law enforcement agencies, the legal community, government agencies, and the private sector.  Some of the major uses of polygraphs today are for interviewing suspects, interviewing potential job candidates, and monitoring convicted sex offenders.  It all sounds glamorous… a machine that can tell when someone is lying and essentially do a detective’s job for them, right? Wrong.

    Polygraphs are not lie detectors but are only used as such.  Polygraph machines measure physiological changes in the body.  These changes include blood pressure, heart rate, galvanic skin response, and respiration.  Generally a polygraph exam will begin with questions that are neutral to obtain a baseline.  Essentially, if the difference between the physiological changes that occur when the baseline questions are answered and the physiological changes that occur when the target questions are answered are extreme enough the polygraph examiner would determine that individual to be lying.  This type of measurement results in an extremely high false positive rate (up to 50%!).  In essence this would be the same as someone flipping a coin every time an individual made a statement, heads for truth, tails for lies.  The false positive rate would be about the same in the coin toss as in polygraph examinations.

    Why are the results so often inaccurate?  Because these physiological changes do not only happen when someone is lying.  I’m sure you sweat at times other than when you are not telling the truth, like when you are uncomfortable, or you are extremely nervous. And would be more nerve wracking than taking a polygraph exam whether it is for a job interview or because you are being accused of a crime you may or may not have committed? There are a variety of things that can influence the results of a polygraph – nervousness, needing to use the restroom, pregnancy, drugs and alcohol, lack of sleep, blood pressure conditions, tight and restrictive clothing, sickness, having a headache, and many more! So how do you know that there is not something happening, besides your answers, that is causing the machine to give false positives? You don’t…and neither does the polygraph examiner.

    As mentioned previously, it is plausible that a polygraph exam could be used during a job interview, especially if that interview is for a law enforcement agency.  Polygraphs are not typically used for employment outside of law enforcement because of the Employee Polygraph Protection Act (EPPA).  This prohibits business from not hiring an individual due to a polygraph alone or firing someone on the basis of a polygraph alone.  It is important to note that the EPPA does NOT apply to government workers, which is why a polygraph exam is often used for law enforcement job interview purposes, and can be the reason why someone does not receive that job.

    When it comes to using polygraph exams in court things are even less clear and defined. Some states, such as California, Arizona, Georgia, Nevada, and Florida, allow polygraph exams to be admitted as evidence in court as long as all parties agree to it. Florida even requires some previously convicted sex offenders to submit to polygraph exams but, because of their unreliability, these results are not used in court but for counseling purposes. Other states, such as New York, Texas, Illinois, Pennsylvania, and the District of Columbia, prohibit the use of polygraph exams as evidence in court.  A few of these will allow them to be used in gaining probable cause for search warrants but New York and Texas go so far as completely banning them from law enforcement.

    Federal courts allow judges discretion in their own court room as to whether or not they will allow polygraph exams but President George H.W. Bush banned polygraphs from military trials due to their unreliability.  Before 1993, polygraph exams were inadmissible in all courts according to Frye v. United States.  In this case, tests like the polygraph exam had not gained enough scientific evidence to be consider credible in court.  When the Daubert Standard came into effect in 1993, polygraphs started to become more widely used because it was no longer based on scientific evidence but on two core elements – whether it is relevant and reliable. Rather than being determined by weight of empirical evidence, these are determined by the judge. The error with allowing judges to determine the credibility of polygraph exams is that they do not often have any scientific background or knowledge and, even if they do, they do not necessarily have any background knowledge about polygraph exams themselves. How can someone that has no knowledge of a scientific, or in this case, a pseudoscientific technique make and educated decision about whether the technique should be admissible in court?

    The results of polygraph exams are first and foremost a pseudoscience. They are not allowed in all courts because there is no evidence that what they do is any better than chance. There is a law that prevents employers from using them against employees or potential employees because there is no evidence that what they do is any better than chance.  Polygraph examinations in no way pass scientific muster and cannot begin to pass through the scientific method.

    Category: PseudoscienceScienceSkepticismTeaching


    Article by: Caleb Lack

    Caleb Lack is the author of "Great Plains Skeptic" on SIN, as well as a clinical psychologist, professor, and researcher. His website contains many more exciting details, visit it at www.caleblack.com