Lie detection is one of the most misunderstood tools of law enforcement. There is no such thing as a lie detector machine. That technology does not exist. Lies are a normal and even necessary part of human communication. Some lies are socially acceptable and expected in a polite society. They are complex psychological processes that serve to spare hurt feelings, expedite interactions or avoid conflict. However, some lies cause great harm. There is good reason why “Thou shalt not bear false witness” made God’s top 10 list. In law enforcement, we waste a great deal of effort because of lies. The ability to detect deception has always been critical to obtaining justice. 

The polygraph instrument is not the magic box we see used so easily in movies. It simply records multiple channels of physiological data simultaneously that are affected by psychological processes. The goal of the detective is to obtain truthful information while motivating the suspect toward confession. Interrogators use a variety of techniques to elicit information. Some are simple conversational tactics, while others — such as kinesics and statement analysis — can be very technical. The best tool by far is polygraph. Hollywood has trained so many with bad information that people who should know better believe polygraph to be either an infallible panacea or worthless voodoo. Neither is true, of course. Done properly, it is a critical technology that should be part of every law enforcement agency. 

There are no cheap substitutes. Computerized voice stress analysis (CVSA) has no scientific validity; it is merely an interrogation prop that mimics polygraph to trick a subject into revealing information. Lots of research has been done by the federal government to determine whether it has any limited usefulness as a forensic science. It does not. It is true that the costs of training and maintaining a detective certified in polygraph are significant, but the cost of not doing so is even greater. Smaller agencies should consider hiring a part-time detective specialist or contracting with a qualified private examiner. Applicant screening, internal affairs investigations and major crimes are some of the areas where these specialized interrogators get information that may otherwise be impossible to obtain. Enhancing the credibility of informants for search warrants is another area often overlooked. Understanding this unique combination of the art of interrogation and the science of collecting psychophysiological data is key to appreciating its value. 

Polygraph best practices

A proper polygraph session will begin with the examiner obtaining all the information possible about the case and the subject. Then, in the interview, the examiner will elicit all the information the subject will disclose on the relevant issue. The relevant questions are reviewed, with the objective being to verify truthfulness rather than detect lies.

Testing consists of asking the reviewed questions several times. Data must be replicated and consistent in order to meet scoring criteria. The data collection utilizes a pneumograph to record inhalation/exhalation ratio and changes in breathing patterns; electrodes to monitor galvanic skin conductivity; a plethysmograph to measure blood flow; a sphygmomanometer to monitor mean blood pressure levels, pulse rate and blood volume changes; and a motion sensor to ensure the subject remains still during the collection of data. 

Once a sufficient quantity and quality of data is collected, the examiner evaluates and scores the charts. The scoring systems are well researched and standardized. Computer algorithms help with quality control, making results even more reliable. 

Ideally, the subject has disclosed all relevant information in the pre-test and will show no significant reactions. Passing the test confirms the information given is complete and accurate. If the subject fails, the examiner has a huge advantage to begin interrogation. First and foremost, polygraph examiners are interrogators.

Myths about polygraph

Let’s dispel some common myths: 

“Polygraph is not admissible in court.” Although most states give no blanket admissibility for polygraph as evidence (nor should they, in my opinion), this argument misses the real value of this investigative tool. Polygraph does find its way to court in a number of ways, but generally it is used in pre-trial negotiations and post-conviction probation issues, such as with sex offenders. 

“Nervous people will fail.” Everyone is nervous, but not everyone lies. There is a difference between being nervous and being deceptive. It’s normal to be nervous.

“People who habitually lie can beat it.” True pathological liars will show no deception on polygraph only because they actually believe they are telling the truth. These rare individuals are clearly mentally ill, not just good liars.

“Psychopathic people like Ted Bundy can pass a polygraph.” Psychopaths have no empathy or remorse, but they do have self-interest at stake and know when they are lying. If they lie, they fail.

“One can get confused by the questions.” During the pre-test interview, all the questions are discussed and reworded until they can be comfortably answered with “yes” or “no.”

“People can take drugs or put a tack in their shoe to fool the polygraph.” Any well-trained examiner can easily spot pain or discomfort and distinguish it from the subtle, genuine phasic reactions consistent with truth or deception. Drugs act uniformly to suppress or stimulate the system and cannot be selectively deployed for individual questions, thus making this attempted countermeasure futile.

“Polygraph results are 100% accurate.” Even DNA testing has an error rate. Validity studies show polygraph is about 85% to 95% accurate overall — very good and useful, but not perfect. Some situations can utilize a searching peak of tension format that has a less than 1% error rate.

The takeaway

Having certified forensic polygraph examiners available saves time in investigations, not only in getting confessions from criminals but also in gaining cooperation of witnesses and clearing the innocent. In applicant screening, there are many issues that will only be revealed with polygraph. There are good reasons why all of the three-letter federal agencies rely on it. The American Polygraph Association and the American Association of Police Polygraphists offer training, set standards for certification and are a great resource for establishing a polygraph program, including model policies for accreditation. 

Take care of yourself and stay safe!