Admissibility ExpertsJudge Vince Chhabria recently rendered a ruling on the admissibility of expert witnesses in the products liability cases against Monsanto, the makers of Roundup®, which the plaintiffs claim can cause non-Hodgkin’s lymphoma. The lawsuits allege that glyphosate, the herbicide in Roundup, can cause non-Hodgkin’s lymphoma, and that Monsanto failed to warn consumers or regulators about the alleged risk.

Glyphosate is the active ingredient in Roundup, an herbicide manufactured by Monsanto. Roundup became commercially available in 1974, and glyphosate-based herbicides are now widely used across the U.S. and throughout the world, on large-scale farms and in residential backyards. The EPA doesn’t currently consider glyphosate likely to cause cancer.

The question before Judge Chhabria at this initial phase of the proceedings was that of general causation—whether a reasonable jury could conclude that glyphosate, a commonly used herbicide, can cause non-Hodgkin’s lymphoma at exposure levels people realistically may have experienced. If yes, the action can proceed to the next phase, whether each particular plaintiff’s Non-Hodgkin’s Lymphoma was caused by the glyphosate in the Monsanto herbicide. If the answer is no, the plaintiffs’ cases cannot go forward. Further, the answer must be no unless the plaintiffs can present at least one reliable expert opinion in support of their contention.

The plaintiffs retained six experts they contended would provide opinions that satisfy their burden at the general causation phase. Each of these experts reviewed the available scientific evidence and concluded that glyphosate is capable of causing NHL in humans. Monsanto moved to exclude the plaintiffs’ experts and has put forward its seven retained experts, each of whom provides a contrary view of the science. Before ruling on these motions, the judge held seven days of hearings to assess the testimony of many of these experts.

Judge Chhabria said there were two “significant” problems with the plaintiffs’ presentation, which combine to make this a “very close question.” The plaintiffs and some of their experts rely heavily on the decision by the International Agency for Research on Cancer (“IARC”) to classify glyphosate as “probably carcinogenic to humans.” This classification is not as helpful to the plaintiffs as it might initially seem, the judge noted. To render a verdict for a plaintiff in a civil trial, a jury must conclude, applying the “preponderance of the evidence” standard, that the plaintiff’s NHL was more likely than not caused by exposure to glyphosate. And at this general causation phase, the question is whether a reasonable jury could conclude by a preponderance of the evidence that glyphosate can cause NHL at exposure levels people realistically could have experienced. Judge Chhabria said that the IARC inquiry is different in that it’s a public health assessment, not a civil trial. Public health assessments generally involve an effort to identify hazards and an evaluation of the risk that the hazard poses at particular exposure levels.

The IARC’s decision that a substance is “probably carcinogenic to humans” is a hazard assessment, which is “merely the first step in determining whether the substance currently presents a meaningful risk to human health. The IARC leaves the second step, risk assessment, to other public health entities, the judge explained. As a result, the public health inquiry doesn’t “map nicely onto the inquiry required by civil litigation.”

Judge Chhabria also found that the evidence of a causal link between glyphosate exposure and non-Hodgkin’s lymphoma in the human population seems “rather weak,” and it called into question the credibility of some of the plaintiffs’ experts, who confidently identified a causal link.

However, the judge wrote that the question at this phase wasn’t whether the plaintiffs’ experts are right, but rather whether they have offered opinions that would be admissible at a jury trial. Ninth Circuit case law, the judge held, emphasizes that a trial judge should not exclude an expert opinion merely because he thinks it’s shaky, or because he thinks the jury will have cause to question the expert’s credibility. Provided an expert opinion is premised on reliable scientific principles, it shouldn’t be excluded by the trial judge. The weaknesses in an unpersuasive expert opinion can instead be exposed at trial via cross-examination or testimony by opposing experts.

The judge concluded that “a reasonable jury could conclude” that the herbicide in Roundup can cause a form of cancer, and that the claims against the company could proceed. Judge Chhabria held that “…the plaintiffs have presented evidence from which a reasonable jury could conclude that glyphosate can cause [non-Hodgkin’s lymphoma] at human-relevant doses.”

The judge found that while the opinions of the plaintiffs were “shaky,” there were admissible.

Monsanto’s motion for summary judgment was denied.