The Committee Notes to the newly implemented amendments to Federal Rule of Evidence 702 make clear that the “[j]udicial gatekeeping” of expert evidence is “essential.” Federal courts in New York have played an important role in pioneering and developing this concept. Indeed, the idea of courts as gatekeepers in the expert context finds its roots in the Eastern District of New York, with the late Chief Judge Weinstein coining the term in a 1985 opinion in In re Agent Orange Product Liability Litigation. Three decades later, the Southern District of New York offered one of the most thorough illustrations of careful judicial gatekeeping in In re Mirena IUS Levonorgestrel-Related Prod. Liab. Litig. (No. II). Now, New York can also call itself home to the first MDL-wide decision to exclude experts under Rule 702’s new formulation.
The plaintiffs in In re Acetaminophen – ASD-ADHD Products Liability Litigation, — F. Supp. 3d —, 2023 WL 8711617 (S.D.N.Y. Dec. 18, 2023), alleged that they or their children developed autism spectrum disorder (“ASD”) and/or attention-deficit/hyperactivity disorder (“ADHD”) from in utero exposure to acetaminophen. To support this alleged link, the plaintiffs proffered five expert witnesses – an epidemiologist, a toxicologist, a teratologist and geneticist, a pharmacologist, and a psychiatrist – and the defendants moved to exclude each of them under Rule 702. The court, in a 148-page opinion, painstakingly explored the opinions of plaintiffs’ experts and found they didn’t pass muster.
Although the court cited the new language of Rule 702, as well as the Committee Note’s reaffirmation that “[j]udicial gatekeeping is essential,” the opinion’s introduction hints at a more fact-driven reason for the court’s impressive attention to detail – public health. As the court noted, untreated pain and fever in pregnancy are associated with poor outcomes for mother and child. Acetaminophen is considered the only pain reliever and fever reducer indicated for use during pregnancy because other analgesics, like NSAIDS, are associated with increased risks of miscarriage or birth defects. Acetaminophen thus fills a critical niche in the market, with an estimated 60% of pregnant women in the U.S. using the product. The public health and drug safety issues raised by the litigation ostensibly motivated the court to proceed with caution, the court openly stating its opinion that “[i]t matters to get this right.”
As the court explained, research into the possible connection between acetaminophen and ASD/ADHD has been ongoing for more than a decade. Yet numerous professional societies around the world, as well as the FDA (which submitted its current view on the evidence regarding causation at the court’s invitation), have concluded that the evidence is insufficient to support a determination of causality. As the court put it, “[t]he epidemiological evidence is highly heterogeneous, and major medical organizations and regulators have cautioned against drawing causal inferences from the existing body of scientific literature. Nevertheless, three of plaintiffs’ experts draw such an inference.”
Against that backdrop, it should come as no surprise that the court took a “hard look” at the experts’ methodology, fulfilling its gatekeeping duties as articulated by Second Circuit precedent in Mirena II (which we discussed here) and the newly amended Rule 702. The court’s primary concern was with the experts’ uses of “transdiagnostic” approaches to evaluate the scientific literature. Under this approach, the experts didn’t separately determine whether in utero exposure to acetaminophen can cause ASD or ADHD. Rather, the experts combined literature addressing ASD and ADHD into a single analysis. The experts attempted to justify this approach by arguing that the underlying symptoms of the disorders “transcend diagnostic boundaries” and that ASD and ADHD are grouped together as neurodevelopmental disorders in the Diagnostic and Statistical Manual of Mental Disorders (“DSM”). But, as the court noted, the diagnostic criteria for the two conditions are “undeniably distinct,” and the experts’ analyses didn’t focus on other neurodevelopmental disorders categorized in the same section of the DSM. The court concluded that examining the “potpourri of evidence” merely “obscured limitations in the scientific literature.” For example, only two studies had ever examined the connection between prenatal acetaminophen exposure and ASD, one of which found no association at all and the other of which found an association only when ASD co-occurred with hyperkinetic disorder. The court also took issue with the experts’ inconsistent explanations of which studies were included in that “potpourri,” as the experts notably omitted studies suggesting no link between acetaminophen and ASD or ADHD – including one that was co-authored by one of the plaintiffs’ experts. Highlighting a “list of inconsistencies [that] could go on and on,” the court excluded the experts’ transdiagnostic assessments of the literature.
But that wasn’t the end of the troubles that plaintiffs’ experts faced, as the court systematically pointed to inconsistencies or inadequate explanations undermining the methodology of each of their other opinions as well. For example, in excluding the plaintiffs’ teratology expert, the court found that the expert had cherry-picked isolated findings, ignored inconsistent findings, and disregarded limitations expressed by studies’ authors in reaching his conclusions. The court raised similar concerns with respect to plaintiffs’ neuroscience and pharmacology experts, who – in addition to ignoring inconsistencies and limitations in the data – each failed to link the data with the ultimate conclusions, rendering their conclusions mere ipse dixit. While the nuance of the court’s opinion cannot be adequately summarized in a blog post, the through line is clear: the court expected each of plaintiffs’ experts to “engage meaningfully” with all of the relevant data, whether that data be for or against plaintiffs’ position in the litigation. The experts failed to do so and were excluded.
The new amendments to Rule 702 are an exciting development that will hopefully embolden courts to discharge their gatekeeping duties carefully. However, In re Acetaminophen highlights a related and important practice point: careful gatekeeping is a hard job that can take considerable effort. This opinion was 148 pages, and last year’s similarly impressive opus in In re Zantac (which we discussed here) was more than twice as long. But when it comes to the analysis of dubious science, the result is worth the effort. To quote the In re Acetaminophen court – “[i]t matters to get this right.”
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