General Causation & Daubert in MDLs

The Analytical Gap: General Causation Under Amended Rule 702

The gate is Federal Rule of Evidence 702, and the December 2023 amendment tightened it. A qualified expert may testify only "if the proponent demonstrates to the court that it is more likely than not that" the testimony "is based on sufficient facts or data," "is the product of reliable principles and methods," and — the language the amendment gave teeth — "the expert’s opinion reflects a reliable application of the principles and methods to the facts of the case" Fed. R. Evid. 702. The Advisory Committee was explicit that this was a correction: the rule was "amended to clarify and emphasize 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," and Rule 702(d) was "amended to emphasize that each expert opinion must stay within the bounds of what can be concluded from a reliable application of the expert’s basis and methodology." That architecture sits on top of Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), which made the trial judge a gatekeeper who "must ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable" — scrutiny trained "solely on principles and methodology, not on the conclusions that they generate," and satisfied only where an "inference or assertion" is "derived by the scientific method" and "supported by appropriate validation ... based on what is known."

In mass-tort MDLs the amendment bites hardest at general causation, the first step of a two-part inquiry. As the Camp Lejeune court framed it, "[t]oxic exposure torts generally proceed in two steps—an expert demonstrates that a particular harm can be caused by the exposure to a degree of scientific certainty (general causation) and an expert opines that this plaintiff’s exposure was a cause in fact of his or her harm (specific causation)" Camp Lejeune Water Litigation v. United States, No. 7:23-cv-00897 (E.D.N.C. Jul 22, 2025). General causation is the chokepoint, and the marquee application of the amended rule came in the acetaminophen MDL, where Judge Cote excluded all five of the plaintiffs’ general-causation experts. In re: Acetaminophen - ASD-ADHD Products Liability Litigation, No. 1:22-mc-03043 (S.D.N.Y. Dec 18, 2023) held that "[s]ound scientific methodology in assessing general causation requires an expert to evaluate all of the scientific evidence when making causation determinations," that "an expert must not cherry-pick from the scientific landscape and present the Court with what he believes the final picture looks like," and that in applying the Bradford Hill criteria "an expert must explain 1) how conclusions are drawn for each Bradford Hill criterion and 2) how the criteria are weighed relative to one another." The experts’ "transdiagnostic" approach flunked that test — it "obscured instead of informing the inquiry on causation," leaving "too great an analytical gap between the data and the opinion proffered," an opinion "connected to existing data only by the ipse dixit of the expert."

The Roundup court drew the same line in the glyphosate litigation. To be admissible, a general-causation expert must offer "independent and relatively comprehensive opinions that the epidemiological and other evidence demonstrates glyphosate causes NHL in some people who are exposed to it," and that "requires a serious engagement with the relevant literature: general causation experts must be able to assess whether a study is credible, to explain why they relied on one study more than another, and to articulate how they reached their conclusion in the face of conflicting evidence" In re Roundup Products Liability Litigation, 737 F. Supp. 3d 893 (N.D. Cal. 2024). An expert who leaned on a meta-analysis headline was excluded because "[a] reliable general causation expert cannot substitute an uncritical reliance on" a "meta-risk ratio for a critical engagement with the epidemiological evidence," and the stakes are dispositive: "[a]n admissible opinion on general causation is necessary for a plaintiff in this MDL to survive summary judgment." The same pressure now runs through the talc MDL, where the court, granting a show-cause order, reiterated that "specific causation must be established with evidence admissible under Rule 702, which requires ... that an expert’s opinion be the product of reliable principles and methods and reflects a reliable application of the principles and methods to the facts of the case" In re Johnson & Johnson Talcum Powder Products, No. 3:16-md-02738 (D.N.J. Jul 22, 2026).

For the litigator, the amended rule is a checklist, not a slogan. If you are proffering a general-causation expert, build the report to survive a preponderance showing on every Rule 702 element: engage the whole body of literature rather than the favorable slice, read the underlying studies instead of secondary summaries or a meta-analysis’s headline ratio, and make each inferential step — especially every Bradford Hill judgment and the weight assigned to it — explicit, so the opinion never outruns the methodology. If you are challenging one, target the analytical gap: surface the alternative explanations and conflicting studies the expert waved past, press the "how did you weigh this" question the amendment now demands an answer to, and remember that in these MDLs a failed general-causation opinion is not a trial problem but a summary-judgment one.