In a closely watched decision, a California jury ruled last week that Monsanto owed former Bay Area school groundskeeper Dewayne Johnson $289 million in damages.  Though Monsanto maintains that its products are safe, pointing to clean bills of health from a wide array of national health agencies, Johnson's attorneys argued that the company covered up the risks of the weed-killers Roundup and Ranger Pro – which they say were responsible for their client's terminal cancer.

The lawsuit thus carefully avoided delving into the complex epidemiology of Roundup's active ingredient, glyphosate, arguing instead that the groundskeeper's illness arose from glyphosate's interactions with the weed-killer's other ingredients.  Yet with even less available evidence about the safety of Roundup's formulation than about glyphosate, the verdict has revealed far more about the challenges of evaluating science in the courtroom than about the herbicide's alleged risks.

The floodgates for the case were opened by a 2015 report from the International Agency for Research on Cancer (IARC), a semi-autonomous offshoot of the World Health Organization (WHO), which classified glyphosate as "probably" carcinogenic in humans.  IARC's evaluation, and the deluge of lawsuits it unleashed, has since set off a domino effect worldwide: in 2017, the European Parliament voted to ban glyphosate beginning in 2022, and several countries including France, the Netherlands, and Germany have either introduced their own restrictions on the herbicide or promised to do so.  Last week, a court in Brazil suspended the use of glyphosate until the government could carry out definitive tests on the product's toxicology.

Yet most experts have approached IARC's judgment with caution.  For one thing, the WHO has contradicted its own subsidiary by declaring glyphosate safe for humans, a position shared by other agencies, including the U.S. Environmental Protection Agency, the European Food Safety Authority, and the European Chemicals Agency.  Furthermore, IARC's verdict doesn't necessarily imply a major risk; after all, it applies the same Group 2a classification to a number of other substances and activities, such as working in a hairdressing salon and drinking hot coffee.

Meanwhile, over the course of the jury trial, further doubt was cast on the plaintiff's case with the testimony of Aaron Blair, former chair of IARC's working panel on glyphosate.  Blair was forced to admit to withholding key data from two massive studies: the North American Pooled Project, which gathered all case control data in the continent assessing the connection between pesticide exposure and cancer, and this year's update to the government-funded Agricultural Health Study, which examined more than 50,000 pesticide users between 1993 and 1997.  Blair insisted he did so because the data had not yet been released when the working group convened.  But he also revealed under oath that he had ignored data from a previous version of the study, which had been published when the panel convened – and admitted that had the data been included, the relative risk of developing cancer from using glyphosate would no longer have been considered statistically significant.

The plaintiff's team responded by accusing Monsanto of "bullying scientists" to suppress the risks of glyphosate and casting its own aspersions on the agencies that have deemed it safe.  Dr. Christopher Portier, an adviser to the IARC glyphosate review, claimed in court that the European food safety body had missed 15 different rodent tumors during testing and accused the EPA of ignoring a huge bank of key data.  The plaintiff's expert witnesses have also suggested the AHS data are unreliable because a large number of the study's original respondents dropped out and the researchers filled in the gaps using a scientific tool called imputation, which the plaintiff's team says distorted the eventual conclusion.

These divergent claims are blurred still further by the connections of the people making them.  Last year, for instance, it was established that Portier earned around $160,000 in consultancy fees from law firms suing Monsanto in the immediate aftermath of IARC's evaluation, leading to accusations that he was paid to influence the verdict.  Meanwhile, Johnson's lawyers questioned the expert testimony of California-based academic Kassim Al-Khatib, suggesting that his attack on the IARC findings is motivated by the two patents he holds for herbicides.

Amid all the claims and counterclaims, one thing is clear: it's seriously difficult to establish, much less prove, the risks of a given chemical in a court of law.  IARC's report provided only an "assessment" of glyphosate's carcinogenic properties, a far cry from irrefutable proof, and the challenge of bridging this gap is compounded by the skepticism with which expert witnesses are viewed by judges and lawyers.  Unlike regular witnesses, scientific experts are allowed to offer opinion on the stand, yet their credibility has been dented by a string of wrongful convictions guided by misguided forensic testimony as well as the fact that they receive monetary compensation.  The more unfamiliar the science, the more skepticism the scientist is likely to receive.

This is particularly true for epidemiology, the study of disease control, which provided a crucial contribution to the IARC verdict.  The field has long been difficult to evaluate in a court of law, and the credibility gap was laid bare by Judge Vince Chhabria in a recent hearing for a separate class-action suit alleging that glyphosate is carcinogenic, when he dismissed the opinions of one of the plaintiff's experts as "junk science."

Clearly, even experienced attorneys and judges have a difficult time getting a grip on such complex matters.  When the final decision is handed over to a group of laypeople, matters become even murkier – turning what should be a scientific evaluation into a set of objective opinions.  As Joe Stanley, the farming ambassador for the National Farmers' Union, put it, "[t]he decision of a lay jury in this poor chap's case doesn't alter the fact that all serious research proves glyphosate is safe."

This raises the question: is a courtroom really the right place to be making such weighty decisions in the first place?