How Polygraphs Work

You have seen them in countless TV shows, movies, and heard about them on the news. Polygraph machines, also called lie detector tests, are common in courtroom dramas, and for many, part of their mental image of what a criminal investigation entails.  

The truth is, it would be great if lie detectors were 100% accurate. Imagine if the police were faced with two contradictory versions of an event, but the machine could identify which party was telling the absolute truth. That is how our modern-day polygraphs are supposed to work—but there are some doubts about the validity of these tests.  

Board-certified attorney Ben Stechschulte discusses how polygraphs work, and where they often fall short.  

About the Polygraph 

The polygraph was invented in 1921 by John Larson, while working as a part-time cop in Berkeley, California. With a Ph.D. in physiology, Larson aimed to make police investigations more scientific and less reliant on gut instinct. The belief was the act of deception and lying was often accompanied by physical tells like changes in a suspect’s breathing and blood pressure.  

Dubbed a “lie detector,” even Larson himself became skeptical about the machine’s true ability to reliably detect deception. Yet in 1935 the results of a polygraph test were admitted for the first time as trial evidence—and secured a conviction. 

How a Polygraph Works  

Polygraph tests measure changes in respiration, perspiration, and heart rate. When submitting to a polygraph test, sensors are strapped to a subject’s fingers, arm, and chest to report real-time reactions to questions during an interrogation. A spike in any of these parameters can indicate nervousness or lying.  

In an attempt to eliminate false positives, the test relies on a set of control or baseline questions. The interviewee will respond to basic questions answering with a “yes” or a “no” such as: 

Is your name John Smith?  

Were you born in Tampa, Florida? 

Do you currently work as a manager at X company? 

The answers to these questions are already known by the interviewer and are used as a baseline for the interview to follow.   

It is important to keep in mind that increases in blood pressure and sweating do not always point to lying or deception. Hands can get clammy when a person is nervous, and hearts can beat faster when stakes are high.  

The validity or lie detector results remain a subject of debate for that very reason. The reliability of test results are uncertain at best. 

Where Polygraphs Fall Short  

While the American Polygraph Association claims that polygraphs have almost a 90% accuracy rate, it should be noted that some police officers and psychologists believe that these tests are biased and even have a 50% chance of hitting a false-positive for honest subjects.  

Polygraphs may appear to be accurate because subjects believe that the test works, and they will be very anxious when questioned. If this assumption is correct, this machine should be called a fear detector instead.  

Further, some polygraph operators might attempt to extract a confession by convincing their suspects that they cannot beat the machine. By increasing detection apprehension, the operator hopes to make the guilty confess. But the caveat here is the innocent people who suffer false accusations and confess under the pressure of a polygraph to obtain relief from the interrogation. 

Are Polygraph Results Admissible in Court? 

Generally, polygraph results are not admissible in court. In the state of Florida, they are only admissible if both parties involved in a case agree to use any results as evidence.  

In any federal court, polygraph results are not admissible.  

Should I Take a Polygraph Test? 

If you are ever asked to take a polygraph test, decline it and ask to speak with an attorney. Even if you are innocent of a charge, there is nothing to be gained by passing a polygraph, but much to lose if your test results indicate dishonesty.  

In Florida, the only individuals who are required to take annual polygraphs are those on the sex offender registry. Still, these results are not admissible in court and are used during court-mandated therapy sessions.  

Asked To Take A Polygraph Test? Call Stechschulte Nell  

If you are facing a criminal investigation, we do not suggest you submit to a polygraph test. They can produce a false-positive result that may negatively impact you should a case go to trial. If the police call you for an interview or mention taking a polygraph, call our attorneys at Stechschulte Nell immediately.  

We are available 24/7 to discuss your unique case. Speak with a criminal defense attorney near you —813-280-1244. 

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