05/04/2012 09:47 am ET Updated Jul 04, 2012

Whose Voice Is Screaming for Help? Zimmerman's or Martin's?

One of the key pieces of circumstantial evidence in the murder charge against George Zimmerman for killing Trayvon Martin is a cry for help moments before the fatal shot that was recorded while a witness who heard the struggle made a 911 call to the police. Identifying the voice of the person screaming, although not conclusive, would certainly be compelling evidence that the outcry was made by the victim of the physical encounter rather than by the aggressor. The recording, in other words, could be used by the prosecution to disprove Zimmerman's claim that he shot Martin in self-defense, or by Zimmerman to prove he was being beaten by Martin when he shot him. So, to prove it was Martin's voice, the prosecution likely will rely on family members and friends of Trayvon Martin. The prosecution may also try to rely on forensic experts to give their opinion that it was Martin's voice. Zimmerman, by contrast, will claim it is his voice, and he also will rely on his family and friends, and maybe experts as well. And a jury, if the case ever goes to trial, will have to decide this important factual issue.

But while there is no reason why the trial judge would bar family and friends from giving their opinion as to the voice of the person screaming, as long as those persons are familiar with the person's voice, there are lots of reasons why a court should exclude forensic experts from testifying to the voice. Indeed, the media has already reported that several forensic experts have already volunteered their opinions that the voice on the tape was not Zimmerman's, and that it was Martin doing the screaming. But are these opinions reliable? Are they supported by scientific analysis? Has the methodology of these experts been subjected to review by other experts in voice analysis? Indeed, has the subject of voice identification through scientific analysis been accepted broadly by the scientific community, and by the courts?

So, for example, one of these so-called experts used a technique called "biometric voice analysis" to compare Zimmerman's voice, recorded when he was calmly speaking to police dispatchers shortly before the shooting, and the screaming voice on the 911 tape. This expert found a 48 percent match between the voices, concluding "with reasonable scientific certainty that it's not Zimmerman." First off, what is biometric voice analysis? Is it similar or different from voice identification through voice spectrography, commonly called a "voiceprint," which has been studied for years but which most courts have not admitted into evidence because it lacks scientific reliability and acceptability. Another so-called expert gave the media his "strong opinion" that the voice was Trayvon Martin's "without a doubt." What was the basis for this expert's opinion? "The tone of the voice is a giveaway," he said. "That's a young man's voice." This from an expert?

Here's the concern. What is the basis for these conclusions? Are they based on scientific analysis? Or mumbo-jumbo? What is the scientific basis for biometric voice analysis? Would any court allow an expert to give his own subjective opinion as to the "tone" of a voice, and that the speaker is a young man? Or give an opinion about the gender or race of a speaker simply by hearing the voice? Are these examples of reliable scientific study and analysis, or junk science?

There are simply too many instances of prosecutors using fraudulent and erroneous scientific evidence to get convictions, sometimes convictions of innocent people, to allow so-called experts to step forward with suspect credentials and opinions. The fact is that juries typically believe experts more than any other witness. The expert is usually viewed by a jury with an aura of special reliability and trustworthiness. However, the records of contemporary criminal trials are replete with instances of "junk science" finding its way into courtroom through the testimony of so-called experts that contained false, exaggerated, and erroneous conclusions that lacked any scientific validity. Several of these experts have become notorious, and their pretentious opinions masking fraudulent and reckless quackery. These experts have testified to blatantly unscientific opinions: astronomical estimates of hair comparisons without any scientific basis, "unique" characteristics of feet without any scientific basis, weapons that "indeed and without doubt" caused certain wounds, opinions that an accidental fire was intentionally set, and other instances of faked fingerprints, faked autopsies, and faked breathalyzer readings.

Some of the opinions by voice experts raise similar concerns. For example, whereas fingerprint and DNA evidence have unique and distinguishable characteristics, there does not appear to be any scientific basis for claiming, as one of the above voice experts did, that each individual's voice is unique. And anyway, there are so many marked differences that may occur among various groups of talkers that giving an opinion "to a reasonable degree of scientific certainty" is blatantly misleading. Even using terms like "probable" or "inconclusive" would be problematic, but a much more accurate opinion. Also, comparing a speaker's voice in a calm state and an emotional state, as one of the voice experts did, makes any informed comparison even more hazardous. Finally, published studies of voice identification under experimental conditions show very high rates of error. Indeed, a report by the National Academy of Sciences on forensic voice identification concluded that the scientific basis for making reliable voice identifications is weak. The FBI, as a result of the report, apparently does not use forensic voice analysis for courtroom evidence, although as with polygraph evidence, it continues to use it for investigative purposes.

If George Zimmerman is brought to trial, the voice of the person screaming probably will be a major issue. There likely will be contradictory proof of whose voice it is. Such proof can readily be admitted from persons who are familiar with that person's voice. But such proof should not be admitted based on the opinion of a forensics expert. It's just not reliable.