Last Updated on August 27, 2024
Everyone has heard of polygraphs (often called “lie detectors” by the public) being used by police or prosecutors, but are these tests even admissible in court in California? As it turns out, California law about evidence admissibility makes it very unlikely that a lie detector test’s results will be admitted in court. However, police still ask defendants to take them even if they can’t be used as evidence.
How do Polygraph Tests Work?
Polygraphs measure changes in a person’s heart rate, breathing patterns, blood pressure, and sweat levels to determine whether or not that person is telling the truth. In theory, a person’s body will demonstrate signs of stress when they lie, and these are the physical attributes a lie detector tests for.
How Accurate Are Lie Detector Tests?
In practice, a person may fail a lie detector test because they are stressed for any number of reasons unrelated to lying —including being concerned about the outcome of the test. Alternatively, some people can pass polygraphs while telling nothing but lies because they can maintain their composure no matter what questions have been asked.
Because tests can be easily be manipulated and frequently say nervous individuals are lying, polygraphs are known to be unreliable. How reliable they are varies based on who you talk to, but studies show they have an accuracy rate of between 50 and 90% —with the higher number being touted by the understandably biased American Polygraph Association.
Why Are Lie Detector Tests Not Admissible in Court in California?
Even using the American Polygraph Association’s alleged 90% accuracy rate, polygraphs are inaccurate one out of ten times at best. Courts have overwhelmingly agreed this is not an acceptable margin of error regarding a person’s guilt or innocence, which is why California limits their use in court. Under state law, these tests can only be admitted in court if both the prosecutor and the defense lawyer agree it is ok to use the test as evidence.
Although California technically allows for lie detector results to be admitted with both parties consent, it is very rare for this to occur. “I have never seen one admitted in over 40 years of practicing law,” says attorney Peter Liss. “A defendant wouldn’t want a negative report introduced, and a prosecutor wouldn’t want a positive report admitted.”
Realistically, prosecutors are very skeptical of defense-administered lie detector tests because they both question the examiner’s impartiality and the results’ reliability.
Under state law, if the two sides do not agree to admit a polygraph test into evidence, then the following information also may not be used in court:
- the opinion of a polygraph examiner
- the fact that someone did take a polygraph test
- the fact that someone offered or refused to take a lie detector test
Why do Police Ask Defendants to Take Polygraphs?
Lie detectors are commonly used in criminal investigations to allow a detective to get a suspect to confess after taking a real or even sometimes a fake test. In these cases, investigators don’t care about the admissibility of the evidence because they have no intention of using the results in court. Instead, investigators just want to get the suspect talking. Always have your defense attorney present while speaking with police and when considering whether or not to take a polygraph test, as he will advise you about the applicable California laws.
When Witnesses are Asked to Take Lie Detector Tests
The defendant isn’t the only one who may be given a polygraph test. In some cases, witnesses may also be asked to take a lie detector test. If a witness for the prosecution was given the test without the defense attorney giving prior approval that the results can be used in court, the District Attorney must tell the defense if the witness failed the test. At this point, neither the DA nor the defense will be able to use this information in court, but both sides may use it to strengthen their case.
If you have any questions about the specifics of California law regarding polygraph admissibility, please call (760) 643-4050 to schedule a free initial consultation with criminal lawyer Peter M. Liss.
