Cross-examining a Forensic Expert

Cross-examining a Forensic Expert

We have all been there—facing an opposing expert witness supported by graphs and computer analysis; opinions shrouded in mystery, veiled with glib rhetoric. How do you take on such a witness? The savvy litigator would do well to consider challenging the expert’s adherence to the “scientific method.”

What is the Scientific Method?

The Oxford English Dictionary defines the scientific method as “a procedure that has characterized natural science since the 17th century, consisting in systematic observation, measurement and experiment, and the formulation, testing and modification of hypotheses.” Among the principles of the scientific method is the devising and testing of reasonable alternative hypotheses.

Thinking in terms of the scientific method may help you confront and challenge an expert witness’s analysis in court litigation.

Challenging An Expert Based Upon Other Potential Rational Hypotheses

A failed retaining wall is a problem, but how do we uncover the cause of the problem? We do so by looking for testable hypotheses that explain the problem and the observable “anomalies” that may be associated with a problem. As we do so, it is important to keep working back from the manifestation to the trigger event. What makes a scientific investigation interesting and complex is that each step back may reveal “the cause” or just another link in the causation chain.

In doing so, one must push back against the proclivity to jump to a desired conclusion. Resist stopping your search when you think you have a winner. Keep the dynamic nature of the scientific method in mind and force yourself to categorize and link even fringe facts, since they may broaden your understanding of the anomalies and other observations.

Look for and identify each hypothesis that may be able to explain the observed anomalies and the ultimate problem at hand. The key is to not stop with the obvious. Intelligent conjecture will aid in a creative and potentially powerful cross-examination of an opposing expert.

Cross-Examination of Opposing Expert Based on a Failure to Test Other Rational Hypotheses

A common flaw in a forensic investigation is the rush of the expert to prove up a theory that supports or rebuts a particular theory of liability. A strong scientific investigation will attempt to systematically eliminate rational alternative hypotheses until all reasonable challenges have been answered. This is the heart of a properly conducted scientific investigation and fertile ground for cross-examination. Consider the following testimony:

A. “I found that the cracks experienced at the plaintiff’s home were caused by shallow soil creep.”

Q. “Did you investigate the possibility that the damage was caused by a deep seated slide?”

A. “I did not see any evidence that would lead to such a conclusion.”

Q. “Did you investigate such a possibility?”

A. “I considered it to be very unlikely and that such an investigation would be disproportionally expensive.”

Q. “Therefore, you elected not to pursue the possibility that the cause of the damage was deep seated movement?”

A. “Yes, some possible explanations are sufficiently unlikely as to not merit the time and cost of an expensive investigation.”

Q. “So you chose the least expensive investigation that presented the least likelihood of liability to your client?”

A. “Deep seated slides are very rare.”

Q. “Do you contend that a hypothesis that the damage was caused by deep seated movement is irrational?”

A. “No, unlikely, but not irrational.”

Q. “So you chose to not investigate a rational hypothesis that if true would have supported my clients theory of the cause of damage to his home.”

A. “Yes, because I considered it highly unlikely.”

The Legal Basis for Excluding Unsupported Expert Testimony

In addition to undermining the expert’s analysis, highlighting the failure to rule out alternative hypotheses may give rise to grounds to exclude the expert’s testimony.

In Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747, the California Supreme Court echoed a series of decisions issued by the federal courts in the early 1990s and laid out a roadmap for practitioners to challenge expert testimony, admonishing trial judges to act as a “gatekeeper” for expert testimony.

In fact, the California Supreme Court encouraged trial courts to examine the expert’s use of foundational materials to see whether the expert’s conclusions are logically supported by the materials used, and to preclude any expert’s testimony where it is speculative or otherwise improper.1 This includes an inquiry into not only the type of material on which an expert relies, but also the expert’s reasoning and whether the data actually supports the expert’s reasoning.2

If your cross-examination elicits testimony along the lines of the example above—an admission of a failure to disprove an alternative rational theory—the expert has arguably not met the foundational requirements and the expert’s opinion may be subject to exclusion.

Under Evidence Code sections 801 and 802, a trial court may exclude expert opinion testimony that is: (1) based on matter of a type on which an expert may not reasonably rely, and (2) based on reasons unsupported by the material on which the expert relies, or speculative.3

The Sargon decision, combined with Section 803 of the California Evidence Code, provides a strong basis for a motion to strike an expert’s testimony where it can be shown that the expert failed to adhere to the basic principles of a scientifically valid investigation, including ruling out plausible alternative rational hypotheses.

If you bear in mind this and other principles and processes of the scientific method, your examination of forensic experts, both on direct and on cross—and your motions to disqualify or strike—will be pointed and powerful.


[1] Sargon, supra, 55 Cal.4th at 770-771.
[2] Id. at 771.
[3] Id. at 771-772; see also Cal. Evid. Code § 803, providing “[t]he court may, and upon objection shall, exclude testimony in the form of an opinion that is based in whole or in significant part on a matter that is not a proper basis for such an opinion.”

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