How I Use Experts in a Massachusetts Criminal Case

By Attorney Joe Serpa | Georgetown University Law Center | Massachusetts Criminal Defense Since 1995

Most of the expert work in my cases is never heard by a jury. An expert who reads a laboratory file and tells me the analyst skipped a step has changed the case, and has done it without putting a witness in front of the jury to be cross-examined. Clients expect the opposite, so it is worth explaining what these people are for.

An expert does three separate jobs in a criminal case: reviewing the evidence, shaping the defense, and testifying. Those are different assignments, they call for different people, and confusing them is expensive. This page covers each of the three, what the rules permit an expert to say, what they forbid, and which opinions need no expert at all.

The Expert Who Reviews the Evidence

This is the use that comes first, and none of it is visible at trial. I send the discovery to someone who does the work for a living and ask a narrow question: is this right, and does the file show the method was followed.

What comes back is rarely a conclusion. It is usually a list of what is absent. The calibration record for the period covering this test. The bench notes behind the summary report. The chain of custody entry between two dates. The validation study for the software version used. None of that appears in a report, and asking for it changes what the Commonwealth can prove.

The subjects come up repeatedly: breath and blood testing, drug analysis, digital forensics and cell site records, firearms and toolmark comparison, forensic pathology, and eyewitness identification. In each of those the Commonwealth is presenting an opinion dressed as a measurement, and a lawyer who does not understand the method cannot tell the difference.

A reviewing expert also tells me when to stop. A great deal of money is wasted attacking evidence that is sound. Hearing that the testing was done correctly is worth the fee, because it moves the case out of one category and into another.

The Expert Who Shapes the Defense

The second job is different. Here I am not testing the Commonwealth’s evidence, I am asking whether a different account of the same facts is scientifically possible. Could this injury have occurred in a fall. Could this phone have been in that location without the person carrying it. Could this level of alcohol be consistent with drinking after driving rather than before.

The answer determines what defense exists. It also determines what I can say in an opening statement, because a theory that an expert will not support is a theory I cannot promise a jury.

This work happens early, before the motion schedule is set. A defense discovered two weeks before trial is a defense the Commonwealth has had no reason to prepare for, which sounds like an advantage and is not. It means I have had no time to develop it either. The full sequence appears in how I prepare a case for trial.

The Expert as Evidence

The third job is testifying, and it is the one I use least. Calling an expert puts a witness in front of the jury who can be cross-examined about his fee, his prior testimony, and every case in which he has said something different.

I call one when an element of the defense cannot be established any other way, which usually means the jury has to understand something outside common experience before the defense makes sense. Absent that, the same points can generally be made through cross-examination of the Commonwealth’s own expert, at no cost and with no exposure.

That choice is made witness by witness rather than as a general policy. The mechanics of testing an opposing expert appear on the cross-examination page.

What an Expert Has to Establish First

Qualification and reliability are separate requirements, and the second is the one more often overlooked. An expert opinion must rest on a reliable foundation, and the party offering the opinion bears the burden of establishing it. Commonwealth v. Lanigan, 419 Mass. 15, 26 (1994).

That applies to the Commonwealth’s expert exactly as it applies to mine. A witness with impressive credentials who is applying a method with no established reliability is offering an inadmissible opinion, and the objection is to the method rather than to the person.

An expert may also base an opinion on facts or data that are not in evidence, but only where those facts are independently admissible and are a permissible basis for an expert to consider. Department of Youth Services v. A Juvenile, 398 Mass. 516, 531 to 532 (1986). Massachusetts did not adopt the broader federal position that the underlying data need not be admissible.

That limit is worth using. Where I think an expert is resting on material that could never come in, I ask for a voir dire to determine the basis of the opinion. Id. It is a short hearing and it occasionally removes the opinion entirely.

What an Expert May Not Say

The clearest prohibition concerns credibility. An expert may not render an opinion on the credibility of a witness, because the jury is capable of making that assessment without help, and such an opinion intrudes on the jury’s factfinding function. Commonwealth v. Ianello, 401 Mass. 197, 201 to 202 (1987); Simon v. Solomon, 385 Mass. 91, 105 (1982). Evaluations of credibility are within the exclusive province of the trier of fact. Commonwealth v. Montanino, 409 Mass. 500, 504 (1991).

The rule matters most in cases involving allegations of child sexual abuse, where the line is fine and frequently crossed. Expert testimony on the general behavioral characteristics of sexually abused children is permitted, because those characteristics are beyond common knowledge and may help the jury. Commonwealth v. Federico, 425 Mass. 844, 847 to 848 (1997).

The permission stops there. The testimony must be confined to a description of the general or typical characteristics, and the expert may not refer or compare this complainant to those characteristics. Id. at 848; Commonwealth v. Trowbridge, 419 Mass. 750, 759 (1995). Nor may the expert opine that the complainant was in fact subjected to sexual abuse.

Even without an explicit comparison the testimony can be inadmissible, where a reasonable jury would think the expert was implicitly vouching for the complainant. Commonwealth v. Quinn, 469 Mass. 641, 647 (2014). The risk is greatest where the same person testifies both as a treating clinician and as an expert, which is a combination worth objecting to before it happens.

The Opinions That Need No Expert at All

A great deal of opinion testimony in a District Court trial is lay opinion, and treating it as expert testimony is a mistake that runs in both directions. Whether a person appeared intoxicated is a lay opinion, admissible without any qualification. Commonwealth v. Canty, 466 Mass. 535, 540 to 541 (2013).

The limit on that opinion is where the objection lives. A witness, including a police officer, may not offer an opinion on the ultimate question of whether the defendant was operating under the influence, or whether his consumption of alcohol diminished his ability to operate a motor vehicle safely. Id. at 541, 544. Apparent intoxication is permitted. The conclusion the jury is there to draw is not.

The same principle covers a good deal of ordinary testimony that nobody thinks of as opinion: whether someone seemed frightened, how fast a car was moving, whether a voice was familiar. Those are within common experience and no expert is needed. Bringing one in on such a subject invites the response that the jury is equally competent to assess it.

Where the Line Moved: Marijuana

Marijuana is the clearest illustration that the lay and expert boundary is drawn by what is commonly known rather than by the subject matter. Because the effects vary greatly between individuals and are not yet commonly known, neither a police officer nor any other lay witness may offer an opinion that a driver was under the influence of marijuana. Commonwealth v. Gerhardt, 477 Mass. 775 (2017).

An officer may still describe what he saw. Bloodshot eyes, drowsiness, and lack of coordination are observations. Id. An officer may also describe a driver’s performance during roadside assessments, to the extent it bears on balance, coordination, and mental acuity. What he may not do on direct examination is say that the performance established influence, or that the driver passed or failed anything. Id.

Two further points from that decision are worth having in front of you at trial. Performance on those assessments, standing alone, is not sufficient to support a finding that the ability to drive safely was impaired by marijuana, and the jury must be instructed to that effect. Id. And introduction of horizontal gaze nystagmus results requires expert testimony. Id.

Gerhardt is a marijuana decision and should not be borrowed for alcohol cases, where the governing authority is different. The alcohol testing questions are addressed on the page about the science behind breath and blood testing.

What I Ask Before I Hire Anyone

Four questions, in order. What is the precise question I need answered. Is that question one the jury cannot answer on its own. If the answer comes back against my client, can I live with having asked. And what will this person look like on cross-examination.

The third question is the one lawyers skip. A consulting expert who reaches an unhelpful conclusion has cost money and nothing else. A testifying expert who reaches one in front of the jury has handed the Commonwealth a witness. That is a reason to consult before deciding to call, and never the reverse.

Whether the resulting opinion comes in, and on what terms, is usually settled before the jury arrives. That is a motion in limine question, and how I decide which of those fights to have is described in how I decide when to object.

Attorney Joe Serpa has practiced Massachusetts criminal defense since 1995 and tries cases in the District Courts and Superior Courts of Greater Boston. Serpa Law Office can be reached at 617.936.0201.