Polygraph Test Result in Zimmerman Case is Unreliable and Useless

July 7, 2012

The Sanford Police Department concluded that George Zimmerman passed a voice stress analysis test, which is a type of polygraph test.

How much credence should we place on the result and will it be admissible in court?

Although police routinely use polygraph tests when questioning a suspect in order to confront the suspect, if the test indicates he is attempting deception while responding to a question, the United States Supreme Court and many other state supreme courts have held that polygraph test results are not admissible in court because they are not sufficiently accurate and reliable.

For background information on polygraph testing, go here.

Florida excludes polygraph test results. Therefore, the results of Zimmerman’s test will not be admissible at Zimmerman’s trial.

Nevertheless, many members of the public now know that he passed a polygraph test, so let us take a look at his test and see what conclusions we can draw about it.

Yamiche Alcindor and Marisol Bello reporting for USA Today said:

Zimmerman was asked nine questions, including two related directly to the shooting: “Did you confront the guy you shot?” the tester asked. “No,” Zimmerman responded. “Were you in fear for your life, when you shot the guy?” the tester asked. “Yes,” Zimmerman said. The examiner concluded that Zimmerman “told substantially the complete truth.”

Ron Grenier, a former FBI agent and lie detector expert, said the voice stress analysis test is not as reliable as a polygraph test. Also, he said, it’s unclear what the examiner meant by “confront.” Further, such tests don’t measure a person’s state of mind or fear at some other time, he added.

“He may have convinced himself that he was in fear of his life, but whether or not he was is not definitive,” Grenier said.

Zimmerman’s responses would be more meaningful, he said, if he had been asked, ” ‘Did Trayvon Martin attack you and knock you to the ground?’ Or ‘Was Trayvon Martin on top of you hitting you before you shot him?’ ”

Joe Navarro, a former FBI agent who teaches interviewing techniques at Saint Leo University, agreed. “You have to ask precise questions,” he said. “You want to know at what point you feared for your life.”

I agree with Ron Grenier’s criticism that the question, “Did you confront the guy you shot?” is an improper question.

The question is vague because it is too subjective to be of any use in determining whether GZ followed TM and was the aggressor. Note that the legal test for self-defense depends on the objective circumstances of the encounter between GZ and TM and whether a reasonable person in that situation would have concluded that it was necessary to use deadly force to prevent imminent death or grievous bodily harm.

A confrontation can be verbal or physical and GZ may have believed or convinced himself, without any rational basis for doing so, that TM’s presence in the neighborhood and his attempt to elude GZ justified GZ in attempting to prevent him from getting away. Therefore, GZ might have perceived any resistance by TM as a confrontation, even if TM were merely standing his ground and using force to defend himself.

Similarly, the test result does not help us determine if GZ was the aggressor.

I also agree that the question, “Were you in fear for your life, when you shot the guy?” is improper because whether he was in fear of losing his life when he shot and killed TM does not help us determine if a reasonable person in his situation would have been in fear of losing his life.

The test result is basically useless because to decide this case the ultimate finder of fact will have to decide what each person did and whether a reasonable person in GZ’s situation would have done what he did.

Finally, even if proper questions had been asked, there are any number of examples of guilty people who passed polygraph tests. One person with whom I am familiar is my former client Gary Ridgway, the Green River Killer, who passed a polygraph in 1987 when he denied being the Green River Killer. He was released and continued killing until he was arrested in the fall of 2000, after DNA test results implicated him in several homicides.

Due to their proven inaccuracy and lack of reliability, I do not support using polygraph tests for any purpose and I think police departments and all employers should be prohibited from using them.

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