The exclusion of this testimony, which was the sole evidence of causation, led to summary judgment for the manufacturer.
On August 12, 2025, the United States Court of Appeals for the Ninth Circuit affirmed a district court’s exclusion of expert testimony purporting to link glyphosate-based herbicide exposure to the development of the plaintiff’s blood cancer and upheld summary judgment in favor of the product manufacturer. The decision is significant for toxic tort litigation and particularly for matters venued in district courts within the Ninth Circuit, which confirmed there is no presumption in favor of admission of expert testimony. Rather, the proponent of expert testimony must always establish the admissibility criteria of Federal Rule of Evidence 702 by a preponderance of the evidence.
Background
Engilis v. Monsanto arose from multidistrict litigation involving claims that exposure to a glyphosate-based herbicide caused the plaintiff’s chronic lymphocytic leukemia, a form of non-Hodgkin’s lymphoma. The plaintiff relied on the testimony of a board-certified oncologist who conducted a differential etiology to establish specific causation. The expert “ruled in” various potential causes of the plaintiff’s chronic lymphocytic leukemia, including the herbicide, and “ruled out” others, notably obesity, as contributing factors.
The district court excluded the expert’s opinion under Federal Rule of Evidence 702, finding the methodology unreliable because the expert failed to reliably rule out obesity as a potential cause of the plaintiff’s cancer. The exclusion of this testimony, which was the sole evidence of causation, led to summary judgment for the manufacturer.
Ruling
The Ninth Circuit affirmed, holding that the district court (1) did not abuse its discretion by granting Monsanto’s motion to exclude the expert’s testimony and (2) because the excluded opinion was the sole evidence upon which the plaintiff relied on to establish causation, summary judgment was properly granted in favor of Monsanto.
Key findings included:
Insufficient Factual Basis for Opinion
The expert’s report relied on a “Plaintiff Fact Sheet” to assert that the plaintiff was not obese, but did not reference medical records, body mass index or other objective data. At the Daubert hearing, the expert conceded he could not determine whether the plaintiff was obese and had not examined the plaintiff or reviewed relevant metrics.
Information Not Disclosed in Expert Report Excluded
The expert’s attempt to discount body mass index as a measure of obesity was not disclosed in his report and thus was inadmissible under Federal Rules of Civil Procedure 26(a)(2)(B) and 37(c)(1).
Deficient Differential Etiology
The expert did not provide a reasoned basis for ruling out obesity as a risk factor for non-Hodgkin’s lymphoma, either through reference to scientific literature or detailed clinical experience. The court emphasized that, while clinical experience may inform expert opinions, it cannot substitute for a reasoned, scientifically supported explanation, nor may the expert set the literature aside without consideration or explanation. When ruling out potential causes, an expert must provide reasons for rejecting alternative hypotheses using scientific methods and procedures and must rely on more than subjective beliefs or unsupported speculation.
No Presumption of Admissibility
The panel clarified that, under both the pre- and post-2023 amendments to Rule 702, the proponent of expert testimony bears the burden of establishing admissibility by a preponderance of the evidence and there is no presumption in favor of admission. Recognizing that, while several Ninth Circuit cases have stated that Rule 702 should be applied with a “liberal thrust” favoring admission, the caselaw “should not be understood to suggest a presumption of admission.”
Broader Effect
The Ninth Circuit’s decision reinforces the district court’s essential gatekeeping role under Rule 702, requiring that expert opinions be grounded in sufficient facts and reliable methodology to ensure the testimony is relevant and reliable. Regarding causation opinions based on differential etiology, the ruling underscores that experts must provide scientifically sound reasons for ruling out alternative causes and that conclusory assertions or reliance solely on clinical experience without supporting data or literature are insufficient. Finally, for parties in toxic torts and products liability litigation, the decision provides clear guidance on the evidentiary standards to address the junk science opinions Rule 702 was meant to exclude.
For More Information
If you have any questions about this Alert, please contact Michael L. Fox, Sean Patterson, any of the attorneys in our Products Liability and Toxic Torts Group or the attorney in the firm with whom you are regularly in contact.
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