George Zimmerman and Trayvon Martin: Judge orders voice testimony to be excluded

23 June 2013

The Zimmerman ruling

A ruling has been handed down on the voice evidence in the Zimmerman case we have been following. Judge Debra S Nelson gave high praise for Prof Peter French’s evidence (in her words, ‘The Court found the testimony of Dr. French to be the most compelling of the witnesses presented.‘). That is an important endorsement for the role of genuine expertise in relevant branches of phonetics in the legal system.

Hear the 911 audio (preferably before reading about the case) and the testimony in our earlier post. Link to the full text of the ruling below.

Here is the conclusion:

Based upon the above, it is ORDERED: That the opinion testimony of Mr. Owen and Dr. Reich are hereby excluded from trial. This order does not prevent the parties from playing the tapes at trial or from calling witnesses familiar with the voice of the Defendant or Martin to testify regarding the identity of the person(s) making the screams.
DONE in chambers at Sanford, Seminole County, Florida this 22 day of June, 2013.

Note that while this judgment is useful in disallowing the evidence of the poorly qualified expert with the ineffective methods, it still allows the audio to be played for the jury to reach its own decision. In my opinion, this is an unfair burden to place on a jury, who really require guidance from an appropriately qualified expert to help them evaluate material of this nature, and especially to help them avoid the pitfalls of priming and other issues to which human speech perception is prone.

The problem of circular reasoning

The problem with priming in this context is that beliefs about who is guilty can affect judgment as to whose voice is heard in audio evidence (in this case, in the background of the 911 call).

That is really the opposite of the intention, which is for evidence to affect beliefs about who is guilty. Worst of all, with speech even more than other kinds of evidence, it is very difficult to disentangle effects on perception that come from the audio from those that come from contextual beliefs. In other words, a jury, or anyone without specific training in relevant branches of phonetics, can genuinely believe they are listening to the audio objectively, when in fact they are being influenced by their subjective beliefs.

We see this effect strongly in the public discussion of the Zimmerman case, where those who already hold beliefs about the case tend to hear the screaming in the background of the 911 call as belonging to the person they already believe produced it – even though the experts agree it is not possible to tell from this sample who is screaming.

Full text of Judge Debra S Nelson’s ruling on the admissibility of voice evidence in State of Florida vs. George Zimmerman (case number 12-CF-1083-A) is now publicly available. It gives a good summary of the evidence.

A few news stories covering the ruling (beware some have errors of fact):

Sign up here for occasional news about Forensic Transcription (no spam!)