The Fight Against Junk Science Heats Up Under Rule 702

Introduction

This December marks the two-year anniversary of the Federal Rules Committee adopting a heightened standard for expert evidence and strengthening judges’ gatekeeping responsibilities. Since the implementation of the amended Rule 702, judges across the country have begun to rise to the challenge.

ATRF has long been a staunch supporter of applying rigorous scientific standards before evidence is admitted in court and has consistently warned against the questionable science underpinning much of today’s mass tort litigation.

In 2025, several circuit courts fully embraced the heightened standards, reinforcing the integrity of expert testimony and keeping junk science out of their courtrooms. Yet, despite this encouraging progress, some courts have resisted reform — continuing to allow unreliable expert evidence to fuel abusive and profit-driven litigation. Junk science remains a central tactic in the trial lawyer playbook, driving the mass tort machine. Judges must remain vigilant in enforcing Rule 702 and upholding their critical role in curbing mass tort abuse.

All Eyes on Circuit Courts

Over the past two years, district courts have grappled with the amended Rule 702 and, in many cases, have been hesitant to fully embrace its heightened standards without clear direction from the circuit courts. In 2025, a handful of circuits provided that much-needed guidance — clarifying and reinforcing the judiciary’s critical gatekeeping role. Others are poised to issue rulings in 2026.

Positive Developments

Earlier this year, the Federal Circuit made clear that “the gatekeeping function of the court” requires judges “to ensure that there are sufficient facts or data for [the expert’s] testimony.” Where that factual basis is lacking, the Court held, the opinions are “unreliable and therefore inadmissible under Rule 702.”

Before 2025, the Eighth Circuit had long adhered to the misconception that “[o]nly if an expert’s opinion is so fundamentally unsupported that it can offer no assistance to the jury must such testimony be excluded.” That changed this year in Sprafka v. Medical Device Business Services Inc., where the Court held that expert opinions “lack reliability” and must be excluded when they are not grounded in an adequate factual basis.

The Fifth Circuit also reinforced Rule 702’s heightened requirements in multiple cases this year, holding that “expert testimony may not be admitted unless the proponent demonstrates to the court that it is more likely than not that the proffered testimony meets the admissibility requirements set forth in the rule.” The Court further emphasized that expert evidence must rest on sufficient facts and data and that judges must act as gatekeepers to ensure that expert opinions “reflect a reliable application of principles and methods to the facts of the case.”

Finally, the Ninth Circuit issued a mixed bag of decisions in 2025. Historically, the Court had instructed that Rule 702 should be applied liberally and that doubts about admissibility should favor admission. But in Engilis v. Monsanto Co., the Ninth Circuit abandoned that approach, declaring that “‘[s]haky’ expert testimony, like any expert testimony, must still be ‘admissible,’” and that this requires the trial court to determine whether it satisfies Rule 702’s threshold requirements. The Court further cautioned that a trial court “cannot abdicate its role as gatekeeper” or “delegate that role to the jury.”

Following the Engilis decision, just one month later, the Court took a step back in Thomas v. Smith’s  Food & Drug Centers. The Court entirely overlooked the 2023 amendment and instead observed that “The Ninth Circuit has placed great emphasis on Daubert’s admonition that a district court should conduct this analysis ‘with a liberal thrust favoring admission.’” The Court took a surprisingly narrow view of the gate-keeping role that judges are only “supposed to screen the jury from unreliable nonsense opinions, but not exclude opinions merely because they are impeachable.”

Courts’ Failure to Embrace Gatekeeping Role

FOURTH CIRCUIT

Despite significant progress in other jurisdictions, the Fourth Circuit took a disappointing approach in Sommerville v. Union Carbide Corp. (Aug. 2025). In overturning the district court, the appellate court incorrectly held that “questions regarding the factual underpinnings of the [expert witness’s] opinion affect the weight and credibility of the witness’s assessment, not its admissibility.” This reasoning undermines the gatekeeping function established by Rule 702 and improperly shifts the burden to juries to sift through unreliable expert testimony — effectively inviting junk science into the courtroom.

ELEVENTH CIRCUIT

The Eleventh Circuit is reviewing a problematic lower court’s decision in Lang v. Sig Sauer, Inc. Here, in addressing a post-trial motion, the court reiterated its prior opinion that arguments asserting that experts’ causation opinions lacked sufficient factual basis merely “go to the weight of that expert’s testimony, not its admissibility,” and did not consider whether the requirements of Rule 702(b) were met. Instead, according to the court, “the proper remedy to Defendant’s critiques of Plaintiff’s experts’ testimony was to allow Defendant to vigorously cross-examine Plaintiff’s experts and for the Court to carefully instruct the jury on Plaintiff’s burden of proof, as was done at trial.”

How Will This Impact Mass Tort Litigation?

Regardless of the product involved, the trial bar’s playbook remains the same. Plaintiffs’ lawyers pour millions of dollars into advertising campaigns in plaintiff-friendly Judicial Hellholes® to recruit potential claimants. They also rely heavily on both traditional and social media to amplify their narratives — often spreading inaccurate, speculative, or scientifically unsupported claims through sympathetic outlets.

To further legitimize these claims, they frequently enlist so-called experts to present misleading or litigation-driven science, both in the courtroom and in the court of public opinion. When credible scientific evidence fails to support their causation theories, they manufacture “new” science to fill the gap and then use it to influence juries and shape public perception. This troubling pattern perpetuates misinformation and distorts the fairness of the civil justice system.

Adopting and enforcing a standard for expert evidence to be more likely reliable than not under Rule 702 is essential to disrupting this cycle. Strong gatekeeping will help ensure that litigation is grounded in sound science — not speculation or assertions that are more likely to be wrong because they do not meet the preponderance of the evidence standard — and prevent mass torts from being fueled by baseless accusations and junk science.

Tylenol Multi-District Litigation

Never is rigorous judicial gatekeeping more critical than now, as the Second Circuit considers a highly con-sequential case involving allegations that prenatal use of Tylenol can cause autism in children.

In re: Acetaminophen — ASD-ADHD Products Liability Litigation is a multidistrict proceeding examining claims that acetaminophen use during pregnancy leads to autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) in children. In December 2023, Judge Denise Cote of

the U.S. District Court for the Southern District of New York exemplified the gatekeeping role required under Rule 702 by excluding testimony from the plaintiffs’ experts and dismissing the litigation.

Judge Cote recognized the “great public health significance” of the litigation, noting its potential implications for families and communities nationwide. In dismissing the case, she found that the plaintiffs’ five expert witnesses relied on existing studies but failed to use that literature to form reliable, discrete opinions. Instead, they applied a broad “transdiagnostic” approach that conflated ASD, ADHD, and other neurodevelopmental disorders—without demonstrating that their methodology was generally accepted in the scientific community.

Addressing one of the plaintiffs’ principal experts, Dr. Andrea Baccarelli, Judge Cote wrote that “despite the identified risk of genetic con-founding, Dr. Baccarelli gives short shrift to the issue. The discussion in his reports is incomplete, unbalanced, and at times misleading.”

Judge Cote’s decision in the Tylenol litigation is a model of what Rule 702 was designed to achieve: ensuring that expert testimony admitted in federal court is based on sound science, not speculation. She rightly concluded that the plaintiffs’ experts “failed to show their methodology is generally accepted” and “downplay[ed] studies undercutting their thesis while emphasize[d] those that align with it.”

SECOND CIRCUIT APPEAL

The case is now on appeal before the Second Circuit. Oral arguments were initially scheduled for early October but were delayed until late November 2025 after a new Mount Sinai study was conveniently published just in time for the appellate briefing — and prominently promoted by Health and Human Services Secretary Robert F. Kennedy Jr. at the White House.

In the wake of the court being inundated with “new” research and information following the White House and HHS announcements, oral arguments were postponed until November.

Dr. Diddier Prada led the Mount Sinai study which was co-authored by several regulars from the plaintiffs’ expert bench, including Harvard’s Dr. Andrea Baccarelli. While plaintiffs and federal agencies point to the study as new evidence, it suffers the same flaws Judge Cote already identified in the multidistrict litigation. Dr. Baccarelli’s “navigation guide” methodology was rejected by the court as cherry-picked and unreliable.

The study’s other two authors are Ann Z. Bauer, a consultant for plaintiffs’ lawyers who has vocally promoted her theories on social media throughout the litigation, and Dr. Beate Ritz, who served as a plaintiffs’ expert for lawsuits related to Paraquat, baby food and Roundup.

Notably, the Mount Sinai paper initially claimed causation between Tylenol use in pregnancy and autism, but amid peer review, softened claims instead to “association.” While offering no new scientific consensus, this litigation theory woven into academia has, however, successfully armed plaintiffs’ lawyers with a “fresh” citation for their latest arguments.

The onus now falls on the Second Circuit to follow the lead of other circuits and safeguard the integrity of expert testimony by strictly enforcing amended Rule 702. The Court should affirm Judge Cote’s well-reasoned decision and reject the plaintiffs’ attorneys’ reliance on unsupported “junk science.”