Many people assume polygraphs, which are more commonly known as lie detectors, are a fairly recent invention, but the technology was actually developed in the 1920s by a California police officer. Polygraph machines identify stress by monitoring an individual’s heart rate, perspiration, breathing patterns and blood pressure during a question-and-answer session, but there is no proof that they actually work.

A coin toss

The results of polygraph tests were once thought to be as reliable as fingerprint evidence, but few experts feel this way today. The American Polygraph Association claims that lie detectors accurately identify deception 90% of the time, but most psychologists and even many veteran police officers believe that figure should be no more than 50%. The American Psychological Association even believes that calling the machines lie detectors should be avoided as the term makes the technology seem more accurate and reliable than it really is.

Polygraph results in criminal trials

Another common misconception about polygraphs is the belief that the evidence obtained by using them is not admissible in a court of law. Polygraph test results were used in a criminal prosecution for the first time during a 1935 murder trial. The defendant was convicted. The U.S. Supreme Court weighed in on the issue in 1998. The justices concluded that polygraph evidence was unreliable, but they left the question of admissibility up to the states. California is one of 23 states that still allow lie-detector test results to be admitted in court.

Allowing defendants to take a polygraph test

While polygraph tests may be unreliable, there are situations where experienced criminal law attorneys may advise their clients to take one. The results of polygraph tests can only be admitted in court in California if all parties agree, which means defense attorneys can prevent this evidence from being used if their clients perform poorly. However, the results of a successful polygraph test could prompt police to explore other avenues of investigation or support motions to dismiss.