Lie detector tests, or polygraph examinations, turn up in police dramas constantly, but for every fictional detective who forces a suspect to strap into a lie detector, there is a very real element that often comes up—admissibility.  The problem with law enforcement using

lie detectors is that very often courts won’t recognize it as a legitimate piece of evidence. There are several historical and scientific reasons for this, but at the same time some courts have come around and will allow polygraphs in certain situations.


What Is A Polygraph?

Before discussing when a polygraph can and can’t be used, it’s important to understand how they work and why they sometimes create so much controversy. A polygraph is a machine with multiple sensors that are attached to a person’s body. The polygrapher then asks a long serious of questions, first control questions that should always be true and then the real questions the police might be after (Did you murder your wife? Were you at home on this night? Etc.). The sensors can’t determine truth from lie, but instead tell the polygrapher detailed information about the person’s physiological reaction to the questions: blood pressure, heart rate, and perspiration. Trained polygraphers can then interpret this to make an educated guess about whether the person was answering honestly or not.


When Do Police Most Use Polygraphs?

In a legal zoom article, attorney Michelle Fabio points out that, “People do confess before, during, and (most frequently) after polygraph tests, and so their use in police investigations is fairly common.” In this sense, polygraphs are more of an interrogation tactic for police and attorneys than a real piece of evidence intended to be brought before a judge and jury. It also comes in handy for defense attorneys trying to convince police and prosecutors to abandon a suspect since it’s reasonable that anyone willing to take a polygraph and then pass it is most likely not afraid of being convicted (hopefully because they’re innocent).

For the most part, the results of a polygraph will never be brought to a trial, but some states have laws that allow for a polygraph to be discussed at trial as long as everyone has agreed to it. There are currently very few circumstances when a person can be forced to take a polygraph and the results would then become evidence against them.


Special Circumstances

In many parts of the US, and as Express reports, also now in the UK, one instance where someone can be forced to take a polygraph is for sex offenders and parole violators. These already convicted criminals sometimes must submit to a polygraph in efforts to put their reform to the ultimate test, sending them back to jail if they fail or crack under the pressure and reveal that they have not followed their parole guidelines.