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Why Do Expert Witnesses Say They Hold Their Opinions “to a Reasonable Degree of Scientific Certainty”?

Posted October 06, 2017

By Jonathan A. Karon

I’m a big fan of the comedian John Oliver’s show Last Week Tonight. In the most recent episode he did an in-depth piece on the use of questionable forensic expert witnesses to obtain convictions in criminal cases. He made some very valid and important points. But one of his criticisms was off-target. He mocked the experts for uniformly testifying that they held their opinions “to a reasonable degree of scientific certainty”. While Oliver was quite right that the standard doesn’t exist and the phrase is meaningless in the scientific community, he was wrong in blaming the experts. This one is all the fault of the lawyers.

In almost every trial the expert witnesses will be asked if they hold their opinions “to a reasonable degree of scientific certainty” (or if they are doctors “to a reasonable degree of medical certainty”). They will all testify, as they have been coached, that they do. This is because lawyers are afraid that the judge will not allow the expert witness to present their opinions to the jury, unless the expert witness uses that precise phrase. So, to insure that the jury gets to hear the expert witness’ testimony, all lawyers ask them if they hold their opinions “to a reasonable degree of scientific certainty.”

This is, of course, silly, at least in Massachusetts, where our appellate cases have specifically held that expert witnesses are not required to use these “magic words”. See, for example, Bailey v. Cataldo Ambulance Service, Inc., 64 Mass. App. Ct. 228 (2005). Although I can’t absolutely rule it out, I’m not aware of any other jurisdictions that actually have a rule or court cases that require the use of this silly phrase. This makes sense as what the law really requires is that the expert use sound methodology, believe that what they are testifying to is “probable” not merely “possible” and they hold their opinions with “sufficient firmness”. To the extent that “reasonable degree of scientific certainty” means anything at all, it seems to mean that the expert, applying the principles of their scientific discipline, believes that their conclusions are more likely true than not true (although you won’t see that written down anywhere except here).

So why do lawyers make their expert witnesses testify that they hold their opinions “to a reasonable degree of scientific certainty”? As far as I can tell, sometime long ago, a lawyer had their expert witness testify that they held their opinions “to a reasonable degree of medical certainty” and an appellate court held that this was sufficient to make the testimony admissible. Since then, the legal profession, in lemming like fashion, has uniformly used these magic words rather than run the risk of having the testimony excluded.

I’m not proud that in an abundance of caution, I also routinely ask my experts if they hold their opinions “to a reasonable degree of scientific (or medical) certainty.” But, I am proud of the following, which occurred in Federal Court here in Boston, when I was asked to try a case that had been prepared by another attorney. One of my witnesses was an expert psychiatrist who was asked at his deposition (before I was involved in the case) if he held his opinions to “a reasonable degree of medical certainty” and answered “no”. My co-counsel thought this was a tremendous problem and I wasn’t too happy either, but I thought we still had a chance. At trial I asked him the following questions:

Q: Doctor, in the practice of psychiatry do you use the phrase “reasonable degree of medical certainty”?

A: No

Q: In psychiatry, what term do you use to describe how confident you are in your opinions?

A: We discuss how confident we are.

Q: And doctor, in this case, how confident are you in your opinions?

A: I’m very confident.

The judge overruled the defense lawyer’s objection and let our expert present all of his testimony to the jury.

John Oliver’s show Last Week Tonight is on Sunday nights on HBO

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