Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), established that trial judges are gatekeepers who must evaluate expert testimony for reliability before it reaches the jury. For thirty years, lower courts have applied Daubert with varying degrees of rigor, producing inconsistent outcomes for expert causation testimony in toxic tort, pharmaceutical, and product liability cases. In 2025 and 2026, several federal circuits issued decisions that have tightened the gatekeeping standard in ways that materially affect how plaintiff experts must structure their causation opinions in personal injury cases involving chemical exposure, pharmaceutical side effects, and chronic injury from product use.
The Two-Step Causation Structure and Where It Is Being Challenged
In toxic tort and product liability cases, the expert causation opinion has two components that courts evaluate separately under Daubert. General causation asks whether the substance, drug, or product is capable of causing the type of harm alleged. Specific causation asks whether the general causation agent actually caused the harm in this specific plaintiff, given the plaintiff's individual exposure, medical history, and alternative explanations.
The trend in recent circuit decisions is toward stricter scrutiny of general causation methodology, particularly where the expert's opinion is based primarily on epidemiological studies with modest relative risk ratios, animal studies, or mechanistic plausibility arguments rather than a Bradford Hill analysis applied rigorously to human epidemiological data. The Eleventh Circuit's decision in In re Abilify (Aripiprazole) Products Liability Litigation, No. 22-11561 (11th Cir. Mar. 2025), affirmed the exclusion of plaintiff's general causation experts who had relied on studies with statistically weak associations and had not adequately addressed confounders in the relevant epidemiological literature. The court held that a qualified expert opinion on general causation requires more than a conclusion that an association exists; it requires a rigorous examination of whether the association is causal under accepted scientific criteria.
Bradford Hill in 2026: What Courts Now Require
The Bradford Hill criteria, developed by Sir Austin Bradford Hill in 1965, provide a framework for evaluating whether an observed epidemiological association is likely to be causal. The nine criteria include: strength of the association, consistency across multiple independent studies, specificity of the outcome, temporal relationship (exposure precedes outcome), biological gradient (dose-response), plausibility through a known mechanism, coherence with other scientific knowledge, experimental evidence from controlled studies, and analogy to other established causal relationships.
Courts applying Daubert have not required expert testimony to satisfy all nine criteria, but they have increasingly demanded that experts address the criteria systematically rather than selectively. The Second Circuit's decision in In re Zantac (Ranitidine) Products Liability Litigation, No. 23-2046 (2d Cir. Jan. 2026), affirmed the exclusion of plaintiff experts who had applied Bradford Hill analysis by selectively emphasizing criteria supporting causation while ignoring or minimizing criteria that cut the other way. The court held that an expert who cherry-picks Bradford Hill criteria has not applied a reliable methodology; the methodology is reliable only when all nine criteria are addressed with equal rigor and the overall weight-of-the-evidence assessment is based on that complete analysis.
The practical consequence for plaintiff experts is that a general causation opinion is now more likely to survive Daubert scrutiny if it addresses negative evidence as well as positive evidence, explains why the expert finds certain contrary studies unpersuasive rather than ignoring them, and applies each Bradford Hill criterion with specific reference to the applicable scientific literature for the exposure at issue.
Specific Causation: The Differential Etiology Standard
Differential etiology, also called differential diagnosis when applied to disease causation, is the methodology by which an expert rules out alternative causes of the plaintiff's condition and concludes that the exposure at issue is the most probable cause. Courts in all circuits recognize differential etiology as a reliable methodology for specific causation when properly applied. The challenge is what constitutes proper application.
The Fifth Circuit's decision in Seaman v. Seacor Marine LLC, No. 24-30215 (5th Cir. June 2025), clarified that a specific causation expert using differential etiology must: (1) identify a reliable general causation foundation for the agent at issue before conducting specific causation analysis; (2) systematically consider all medically plausible alternative causes for the plaintiff's condition; and (3) apply a principled methodology for ruling out alternatives rather than dismissing them without scientific support. An expert who concludes that the plaintiff's condition was caused by the defendant's product because no other obvious cause exists, without affirmatively investigating alternative causes, has not reliably applied differential etiology.
Prepare specific causation experts thoroughly for Daubert hearings. The expert must be able to articulate why each alternative cause was ruled out, with specific reference to the plaintiff's medical records, laboratory data, and exposure history. An expert who cannot specify why idiopathic disease, genetic predisposition, occupational exposure from other sources, or lifestyle factors were considered and rejected will face exclusion under the current gatekeeping standard.
The Record for Appellate Review
When a district court excludes a plaintiff's causation expert under Daubert, the exclusion is reviewed on appeal for abuse of discretion, which is a deferential standard. A plaintiff who fails to preserve the full record of the expert's methodology at the Daubert hearing, or who does not offer an adequate proffer of the excluded testimony in the record, may find that an otherwise reversible exclusion is unreviewable because the appellate court cannot evaluate what the expert would have said. Preserve the complete record: offer the expert's full report, deposition transcript, and any supporting literature as exhibits at the Daubert hearing, and make a specific offer of proof on each opinion the district court excludes.
The circuit trend toward stricter Daubert gatekeeping also affects settlement posture. A plaintiff in a toxic exposure or pharmaceutical case who cannot survive a Daubert challenge on general causation has no case to try. Assess the expert's Daubert vulnerability early, before substantial litigation costs are incurred, and retain experts who have testified in the relevant circuit and who understand how the current gatekeeping standard applies to the specific scientific literature in the case. For decisions from 2025 and 2026 on Daubert and expert exclusion across multiple circuits, see the case law and settlements archive for updated coverage. For how Daubert challenges to general causation experts affect class certification in toxic tort mass actions, the product liability section covers how exclusion at the class stage affects the litigation posture of the entire mass tort.