The Lie Detector: Its Use in North Carolina Criminal Cases

The polygraph, or lie detector, is a device that combines several different medical measuring tools. It measures such factors as blood pressure, pulse, breathing rate, and the electrical response of the skin—called galvanic skin response—that detects sweatiness. The concept behind the device is that when someone is being deliberately deceptive, his body will register the stress of lying in subtle and uncontrollable ways. Those telltale charges will be registered on the graphs of the lie detector, and a skilled polygraph operator will be able to tell conclusively if the subject is lying.

It makes a nice story. Too bad it’s almost 100 percent untrue.

Can a machine actually detect the truth?

Since the earliest days, a dispute has raged about the accuracy of the polygraph. Proponents of the tool like to say that lie detectors can achieve “between 96 and 98 percent accuracy” in detecting deception.

Most scientific researchers who have looked into the issue, though, have concluded that the accuracy is probably closer to 60 percent—little better than flipping a coin.

It’s important to remember that the polygraph isn’t actually scanning the subject’s mind to look for falsehoods; instead, it’s looking for body reactions—faster pulse, sweaty hands, rising blood pressure—that might signal increased stress because the subject has lied. The indirect route by which a polygraph gets its results explains why critics complain about accuracy problems:

  • False positives. Being attached to a polygraph machine is itself a stressful and unusual experience. It’s quite easy for a subject to experience nervousness, and that may be enough for the operator to conclude the subject is lying.
  • False negatives. On the other hand, sociopaths—people without a well-developed conscience—may be able to pass a lie detector test with ease, simply because they do not show a physical reaction to some kinds of stress.
  • “Beating” the test. Popular culture (and the Internet) is replete with methods for controlling emotional states and preventing arousal in order to beat a polygraph exam. Some of these work, some of the time.

Lie detectors in North Carolina courtrooms

Law enforcement officers and criminal prosecutors were quick to decide that the lie detector was a great tool for their jobs. Even as evidence began to mount that polygraphs were not particularly reliable, investigators continue to use them.

Courts, however, were not so easily led astray. Today, in most jurisdictions, evidence from polygraph examinations can only be admitted in a trial if both sides agree to admissibility before the test is given. North Carolina follows a different path. Since 1983, the North Carolina Supreme Court has ruled that lie detector results may not be introduced as evidence in a trial. By coincidence, the Fourth Circuit—the federal appeals court that includes all of North Carolina—also rejects the use of polygraph evidence in its District courts.

Simple enough, then: If you’re facing criminal charges in North Carolina, either in state or federal courts, you don’t have to worry about a polygraph test, right?


Why lie detectors still matter in North Carolina criminal cases

Even though the results of a lie detector test can’t be introduced as evidence in a trial, police and sheriff’s departments are still permitted to use polygraphs in their investigations—if a person under suspicion agrees to be tested.

As Wilmington criminal defense lawyers, we know of a number of cases in which a person thought he had nothing to lose from a polygraph test. In the first place, he reasoned, the test would prove his innocence. And even if the test was unclear, the results would never be known in the courtroom. In too many of these cases, the test subject ends up making a statement that increases suspicion, or the results of the polygraph test persuade the prosecutor to file felony charges.

If you have been asked to take a lie detector test, preserve your legal rights and refuse until you have consulted with your North Carolina defense attorney. Then call Speaks Law Firm at 910-341-7570 or toll-free at 877-593-4233 to get legal representation that can give you sound advice on how to proceed. Your future freedom may depend on it.

R. Clarke Speaks
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Trial Lawyer and Founder of Speaks Law Firm, P.C.