Expert witnesses appear in civil and criminal trials because they have in-depth knowledge of complex topics relevant in the case. With this knowledge, the right expert can explain processes, summarize the state of knowledge in their field, or provide clarity on specialized scientific, technical, or similar issues.
Ideally, expert testimony helps the judge or jury understand key facts and reach better conclusions. In some instances, however, an expert witness’s own conclusions may cross the line from focusing on the facts to implying or stating a legal conclusion.
Expert witnesses who stray into “legal conclusion” territory are outside their lane. Defining the applicable law is the judge’s responsibility, and judges will not tolerate expert witnesses who cross that boundary, whether or not they do so intentionally.
Keeping an expert witness focused on the facts is essential to winning a case. Here’s how to do it.
Expert Witness Testimony: How Far is Too Far?
Consider the example of Krys v. Aaron, a 2015 case before the U.S. District Court for the District of New Jersey. Central to the case was the question: Did the defendants know that a hedge fund’s excess cash was intermingled with their other holdings?
The plaintiffs offered a professor of law as an expert witness. The professor clarified certain points as to industry practices and to various regulations that applied to the case – areas that any expert witness might have been expected to cover in the same position.
However, the expert witness in Krys went further, offering two opinions: First, that applicable laws entitled the plaintiffs to segregation of the excess cash, and second, that the “defendants’ ‘many’ egregious violations, ‘especially when viewed in their totality in the circumstances of this case constitute a knowing fraud, breach of fiduciary duty and trust, and conversion.”
The district court found that these opinions constituted impermissible legal conclusions, saying that the expert had “render[ed] a legal opinion concerning whether various agents of [the defendants] complied with their obligations under federal securities law.”
Rather than simply strike the expert’s report entirely, however, the district court attempted to clarify the line between permissible expert opinions and impermissible legal conclusions.
Understanding the Rules Governing Expert Testimony
Expert witnesses are allowed to offer testimony that “embraces an ultimate issue to be decided by the trier of fact” under Federal Rule of Evidence 704(a). This allowance creates space for an expert witness to complete their line of reasoning on a case, instead of leaving a judge or jury to draw its own conclusions.
While an expert witness may discuss ultimate issues, however, the expert may not tell the fact-finder what result to reach. The example given in the notes on FRE 704(a) is of a will: While an expert witness can discuss whether they believe the will’s author had the mental capacity to “know the nature and extent of his property and the natural objects of his bounty and to formulate a rational scheme of distribution,” the expert cannot go one step further and say whether the person “had capacity to make a will.”
The Krys court found that, in this context, the law professor’s expert testimony was admissible, except where it “reache[d] the specific conclusion that any of the defendants acted in compliance with and/or in violation of applicable legal duties or segregation requirements.”
How to Help Experts Testify When an Ultimate Issue is at Stake
When an expert’s testimony “embraces an ultimate issue to be decided by the trier of fact,” the risk of the expert facing objections to that testimony increases. Attorneys can help their expert witnesses avoid accusations of rendering an impermissible legal opinion in several ways.
First, help expert witnesses focus on the abstract, not the particular. The Krys court stated that while an expert “may provide an opinion on whether a party’s conduct or actions meet the underlying bases for an ultimate issue in the case,” the expert may not “merely instruct the jury on the result to reach based upon a party’s specific conduct or actions.”
For example, consider the example given in FRE 704. The ultimate issue to be decided by the jury is “Did the testator have the capacity to make a will?” This is the question your expert witness must stop short of answering directly.
Instead, assist the expert to spend time focusing on the elements that constitute capacity. What does it mean to “know the nature and extent of your property,” to “know the natural objects of one’s bounty,” and “to formulate a rational scheme of distribution?”’ What evidence would indicate, generally, that each of those criteria has been met?
It may even be permissible for an expert witness to go so far as to say whether or not the evidence in this case supports each of those criteria. Generally speaking, however, it will not be permissible for the expert to take the final step: “And therefore, this testator had/lacked capacity.” That step should be left for the trier of fact.
By focusing on questions like “what do we mean by this phrase?”, “what types of evidence would tend to fulfill the requirements listed here?”, and “do we see that evidence in this case and if so, what does it look like?”, attorneys and expert witnesses can build a clear picture of the applicable criteria for the trier of fact without explicitly stating which conclusion the fact-finder must draw.