Evan M. Tager, Featured Expert Contributor, Judicial Gatekeeping of Expert Evidence

Evan M. Tager, a Partner in the Washington, DC office of Mayer Brown LLP, with Jonathan S. Klein, an Associate with the firm.

May a court exclude an expert opinion reliant on scientific studies that offer no meaningful support for the expert’s conclusions? Most federal decisions (including this S.D.N.Y. decision that we recently wrote about here) correctly say “yes.” But in many states, important questions remain about the extent to which a state court can scrutinize the studies that underpin an expert opinion. In Pennsylvania, which continues to apply the Frye general‑acceptance test that preceded Daubert, the Pennsylvania Supreme Court will soon decide how closely trial courts in Pennsylvania can scrutinize the scientific evidence that purportedly underlies the expert opinion.

In Walsh v. BASF, the estate of a long‑time golf‑course groundskeeper, Thomas Walsh, sued nearly three dozen pesticide manufacturers after Walsh died from acute myelogenous leukemia. Unlike in the typical toxic‑tort action, Walsh “kept a diary of the chemicals used on the courses and the dates of their applications.” 191 A. 3d 838, 840 (Pa. Super. Ct. 2018). After years of litigation, Monsanto, Bayer, and several other defendants moved to exclude the opinions of the estate’s two experts on causation. The defendants argued that the studies cited by the  experts failed to support their conclusions and that, at bottom, the experts attempted to pass off as science what amounted to pseudo‑scientific speculation.

Although both experts ostensibly applied the Bradford-Hill criteria (a set of nine factors regularly used to determine causation), the trial court agreed that the experts’ cited evidence failed to support their conclusions. 2016 WL 11432920 (Pa. Ct. of Common Pleas Oct. 5, 2016). For example, most of the studies cited by the estate’s general‑causation expert concluded only that some chemicals increase the risk of a “chromosomal aberration” (rather than that the chemicals cause cancer). Other studies involved non‑human subjects or in vitro testing. And few of the studies involved the pesticides sprayed by Walsh; instead, the studies typically examined chemicals in a similar “class.” Because none of the studies confidently concluded that the pesticides sprayed by Walsh can cause cancer in a human—much less the particular type of leukemia to which Walsh succumbed—the trial court concluded that the expert’s reliance on the cited literature “is not in accordance with generally acceptable scientific methodology.” Id. at *3.

The trial court also concluded that the opinion of the specific‑causation expert suffered from similar defects. For example, one study cited by the specific‑causation expert specifically cautioned about “the non‑applicability of human studies to livestock studies,” but the expert nevertheless relied on the study to conclude that pesticide exposure caused Walsh’s leukemia. Id. at *5. Another article cited by the expert concluded that some pesticides cause “DNA damage,” but the author cautioned that he lacked sufficient data to draw conclusions about cancer. Id. at *6. Ultimately, the trial court concluded that the two experts’ reliance on “animal studies, test tube studies, and studies that include significant limiting language” deviated from “generally acceptable scientific methodology.” Id.  As a result, the trial court excluded both opinions.

Pennsylvania’s intermediate appellate court reversed the exclusion of the expert testimony and rebuked the trial court for acting as a “gatekeeper.” 191 A.3d at 844–49. According to the appellate court, the trial court role’s under Frye is to ensure “that the expert relied upon and conventionally applied a scientific method generally accepted in the relevant scientific community.” Id. at 844. As far as the appellate court was concerned, the trial court’s role ended after the trial court determined that the estate’s experts ostensibly applied the Bradford-Hill criteria, a well‑recognized methodology. Id. at 844–45. By scrutinizing whether the evidence cited by the estate’s experts supported the experts’ conclusions, the trial court “impermissibly set itself up as a super expert in the field of medicine.” Id. at 846 (internal quotation omitted).

In dissent, Judge Bender explained that “such screening [of the experts’ conclusions] is necessary to prevent experts from evading a reasoned Frye inquiry merely by making references to accepted methods in the abstract.” Id. at 849.

The Pennsylvania Supreme Court granted review to address three questions: first, whether the Superior Court erred in forbidding trial courts from acting as “gatekeepers,” second, whether the Superior Court erred in holding that trial courts may not examine the propriety of an expert’s “extrapolating from a broad class of products and injuries to a specific product and injury,” and finally, whether the Superior Court erred in failing to explain how the trial court abused its discretion in excluding the opinions of the estate’s experts. The defendants’ opening briefs are due on May 15, and briefing will likely conclude by August.

As the U.S. Supreme Court explained nearly three decades ago in Daubert, the courts play an essential role in ensuring that a jury decides a case based on sound science. Expert testimony that sounds plausible but actually depends on unsupported leaps of logic—such as an expert opinion that cites exclusively to studies about different species or different chemicals—risks impressing on the jury an erroneous belief that valid scientific evidence supports the expert’s testimony. Unless the trial court can scrutinize the expert’s application of a scientific study to the particular facts of the case, the expert can smuggle “junk” science into the courtroom by simply referencing the study, no matter how little support it offers for the expert’s conclusion.