Specific causation causes some courts to become costive, and sometimes, courts overuse so-called differential etiology as a laxative. The phrase “differential etiology” is an analogy to differential diagnosis, the reasoning process by which clinicians assess the identity of a disease or disorder. Differential etiology, like laxatives, can be overused and misused.
Last month, the Ninth Circuit affirmed a district court’s summary judgment in a glyphosate case. Engilis v. Monsanto Co., No. 23-4201, D.C. No. 3:19-cv-07859-VC (9th Cir. August 12, 2025). The trial court found that plaintiff’s expert witness’s differential etiology was unreliable because the putative expert witness acknowledged that obesity could be a cause of plaintiff’s disease, but then failed reliably to rule out obesity as a differential etiology. Instead, the excluded expert witness glibly inferred that glyphosate was a cause of plaintiff’s cancer. The trial and appellate courts were faced with a great example of invalid, motivated reasoning, or the lack of reasoning.
The Ninth Circuit’s affirmance was significant because it clearly acknowledged that there was no presumption of admissibility, and that the district court was well within its discretion to find that the proffered expert witness opinion had failed to meet the requirements of Rule 702.[1]
The decision in Engilis was simple and straightforward; it was based upon specific or individual causation or its absence. In cases involving diseases with multiple potential causes, none of which is necessary for the outcome, an exposure or lifestyle factor may be capable of causing a particular disease, but that factor may not have played a causal role in everyone who experienced the exposure or lifestyle factor and who developed the disease. (Not everyone who smoked cigarettes develops lung cancer, and not all lung cancer patients smoked.) Courts and litigants are thus left with the puzzle of individual causation.
In a case such as Engilis, courts can basically assume, arguendo, that glyphosate can cause the claimed outcome (Non-Hodgkin’s Lymphoma or NHL), but then insist that there is competent and sufficient evidence to show that the claimant’s specific case of NHL was caused by the claimed exposure.
Some courts and commentators have suggested that a process of “differential etiology, by analogy to differential diagnosis, can get a claimant to the finish line. This attempted solution assumes arguendo that glyphosate can cause NHL, but then it still must resolve whether this specific case of NHL (or whatever claimed) was caused by the claimed exposure.
As suggested above, differential etiology is something like constipation, which is resolved by the process of elimination. Formally, the reasoning process is an “iterative disjunctive syllogism.” We start with an exhaustive listing of the possible established general causes of the claimed disease:
A or B or C (exhausting the possible general causes of the claimed disease).
Because the diseases may multifactorial, the set of disjuncts may be more complex:
A or B or C or A*B or B*C or A*C or A*B*C.
But if the claimant had never been exposed to A, we can deduce:
B or C or B*C.
And if the claimant had never been exposed to B, we can infer that:
C.
And if C is the tortogen under investigation, for which general causation was established, the claimant would have an unequivocal submissible case to the jury.
Of course many diseases have unknown causes, so-called idiopathic or sporadic cases. In such instances, any proper differential etiology must include a disjunct D, for idiopathic cause. We can see that the iterative disjunctive syllogism in such cases leaves us with uneliminated D in some of the remaining disjuncts, and the claimant cannot reach an unequivocal conclusion in support of his claim.
There may perhaps be a solution to this problem that turns on the effect size, and the probability of attribution associated with each uneliminated disjunct, but that is a story for another day.
[1] See Paul Driessen, “Nation’s most liberal court rejects plaintiff expert’s claims that glyphosate caused couple’s cancer,” Eurasia Review (Sept. 23, 2025).
