On 10 August 2018, a jury assembled in California Superior Court in San Francisco found the agribusiness giant Monsanto (which is now part of the pharmaceutical company Bayer) liable for a 46-year-old groundskeeper’s development of Non-Hodgkin lymphoma. The groundskeeper, terminally ill Dewayne Johnson, alleged that his repeated exposure to Monsanto-marketed Roundup weedkiller products, whose active ingredient is glyphosate, caused his cancer. Monsanto was ordered to pay $289 million dollars in damages.

The carcinogenicity of glyphosate, a staple of global agriculture and by far the most commonly used herbicide in the world, has become an aggressively acrimonious topic whose narratives have often been framed, to the detriment of nuance and context, by warring tribes. Because Monsanto breeds genetically modified crops that are resistant to the herbicidal effects of glyphosate (“Roundup ready” crops), the equally polarizing topic of GMOs is part of this noisy debate as well.

For many who have fought for the recognition of glyphosate to be carcinogenic, the ruling has been touted as the conclusive and final evidence that the chemical causes cancer. Scientific truths, however, are not determined by juries, but through rigorously demonstrated consensus. The scientific consensus on glyphosate is far from a settled issue, and many scientists have taken significant issue with the verdict.

Who Decides If Something Causes Cancer?

No single body of experts or government officials is tasked with being the official voice of science when it comes to the cancer-causing potential of different chemicals. Several agencies both within the United States and internationally offer opinions on that subject, and they do not always reach the same conclusion, utilize the same methodological approaches, or even ask the same exact questions. Glyphosate is a prime example of multiple agencies coming to different conclusions on the safety of the same chemical.

The World Health Organization’s International Agency for Research on Cancer (IARC) is one group that offers determinations of a substance’s carcinogenicity, and their assessment is the most severe in its classification of glyphosate, is controversial, and is the scientific basis for most legal action against glyphosate.

In March 2015, using studies of humans exposed to glyphosate through agricultural work, as well as from laboratory experiments on animals, the IARC concluded that “there was limited evidence of carcinogenicity [for the herbicide glyphosate] in humans for non-Hodgkin lymphoma.” As such, they classified glyphosate as a Group 2A substance that is “probably carcinogenic to humans.”

In November 2015, the European Union’s Food Safety Authority (EFSA) weighed in on the carcinogenicity of glyphosate as well. Their review, which was concerned only with exposure to glyphosate through food, declined to classify glyphosate as a carcinogen, noting that “all the [European Union] Member State experts but one agreed that neither the epidemiological data (i.e. on humans) nor the evidence from animal studies demonstrated causality between exposure to glyphosate and the development of cancer in humans.”

In the United States, the National Toxicology Program (NTP), which comprises parts of several U.S. government agencies such as the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA), plays a role similar to the IARC. This group is tasked with maintaining a list of chemicals “known to be [a] human carcinogen” or “reasonably anticipated to be human carcinogens” which is updated on a semi-regular basis. Their latest update, from November 2016, does not list glyphosate as either a known carcinogen or as reasonably anticipated to be one.

In May 2016, a Joint Meeting of the World Health Organization and the United Nations’ Food and Agricultural Organization (FAO) was convened to discuss the threat glyphosate has on humans when it enters the body as a residue on a food. That body, like the EFSA, concluded “that glyphosate is unlikely to pose a carcinogenic risk to humans from exposure through the diet.”

Why Is It So Difficult To Determine If Something Causes Cancer?

Figuring out if something is a carcinogen is not an easy task for several reasons. First, an individual’s cancer risk is generally expressed in terms of chance percentages that a person will develop a given cancer within their lifetime and is affected by myriad genetic and environmental factors which make it hard to single out any one cause. Some chemicals can increase this risk very directly and clearly with minimal exposure, while others may pose a risk only at certain exposure levels, over certain time periods, or via certain routes. In the latter case, safe limits can be set by establishing acceptable intake levels.

Second, designing studies to determine exposure limits or the carcinogenic potential of any chemical are hampered by the fact that one cannot ethically design studies that intentionally expose humans to various concentrations of a chemical of unknown safety. That means scientists are left with the less precise options of including large-scale studies of populations incidentally exposed to a chemical, studies performed on laboratory animals, or studies on cell lines. All of these approaches are subject to disagreements about what kind of data one should allow or discard in making such determinations.

The nature of the studies included in the review of glyphosate’s carcinogenic potential is one of the reasons for the differing conclusions reached by different agencies and involve incredibly complex arguments that span multiple scientific disciplines. One major difference between the “probable carcinogen” IARC conclusion and the EFSA conclusion concerns what substance, specifically, was considered. Here the EFSA explained one difference in approach:

The International Agency for Research on Cancer (IARC) said earlier this year that glyphosate was genotoxic and would “probably” cause cancer in humans. However, the IARC report looked at both glyphosate — an active substance — and glyphosate-based formulations, grouping all formulations regardless of their composition. The EU assessment, on the other hand, considered only glyphosate.

Another controversy in the debate revolves around which animal studies should be included in a given review. A fundamental difference between the IARC and EFSA decisions involves the different animal studies under consideration, according EFSA:

The EU peer review concluded that no significant increase in tumour incidence could be observed in any of the treated groups of animals in the nine long term rat studies considered. IARC, on the other hand, interpreted two studies as showing statistically significant carcinogenic effects. Similarly, with the mice studies, IARC identified positive carcinogenic trends in two studies that the EU peer reviewers assessed as insignificant.

Furthermore, these scientific debates have been complicated by allegations of improper influence from Monsanto toward the scientists conducting the reviews, who in some cases had served as consultants for Monsanto or received money from Monsanto. Similarly, Monsanto has been accused of interfering in the peer review process behind some research papers concerning glyphosate.

The Johnson v. Monsanto case alleged that it was the specific combination of chemicals in the Roundup product Johnson used (Roundup Pro and Ranger Pro), rather than glyphosate alone, that caused his cancer. In addition to absorbing the basics of herbicide use, oncology, epidemiology, and toxicology, a jury of non-specialists had to navigate these competing facts, biases, and circumstances to render a verdict on a topic of considerable controversy in the scientific community. The case was decided after three days of jury deliberation.

Does a Legal Verdict Always Uphold A Scientific Truth?

Court cases like Johnson v. Monsanto, in which a jury is asked to decide the likely cause of an illness, are often challenging because they sometimes have the effect of asking for certainty when such certainty does not exist even within the expert community, as discussed in Popular Science:

Cases … which try to prove that a particular chemical or product is the cause of an illness like cancer, are complicated: they rely largely on juries, made up of members of the public, combing through and coming to a conclusion about the evidence produced through scientific and medical studies. But there’s a problem with this: Within the medical community, much of this evidence is messy and still up for debate. What the legal system considers enough evidence to establish that exposures causes illness is different from the standards of science—and trying to fit the two together can be hazardous.

Edward Cheng, a professor at Vanderbilt Law School, specializes in the legal processes that allow for expert testimony or scientific evidence to be presented in court. He explained to us via email that causation, a connection scientists are often wary of making, is explicitly or implicitly what juries are asked to rule on in a tort case such as Johnson v. Monsanto:

In order to recover for a tort, the plaintiff has to prove that the defendant (or the defendant’s product) caused the harm. So even if it is not explicitly challenged, the causation inquiry is implicitly there. And there is good reason for causation to be the key battleground. To the extent that products liability tends to shade toward strict liability (meaning liability without fault), causation becomes the linchpin. And causation, while easy to assert, is notoriously difficult to prove.

Juries have, in several cases, held companies liable for causing health problems which later scientific inquiry conclusively found to be unrelated. In one particularly egregious example from the 1980s, the Supreme Court of Rhode Island upheld a jury verdict that posited a connection between a man’s head injuries from a car accident and his death from lung cancer a year later. There was no plausible scientific argument to be made that would support such a contention, but the motorist who caused the car accident was required to pay the family of the man who died of lung cancer $110,000.

In the 1980s and 90s, the silicone producer Dow Corning was sued over allegations that their implants caused breast cancer and a host of other diseases, and they ended up paying $3.2 billion to settle such claims. In 1999, the NIH’s Institute of Medicine was tasked with investigating the relationship between breast implants and cancer, “Examining more than 1,200 references, the group addressed the major questions about implants,” the New York Times reported at the time, “In every case, the committee concluded, there was no convincing evidence that implants were at fault.”

While later research demonstrated a potential link to an extremely rare form of cancer (anaplastic large-cell lymphoma, or ALCL) that may form in scar tissue associated with breast implant surgery, this was not, generally speaking, the disease addressed by most of the legal action against Dow Corning.

Presently, there are similar debates occurring in courtrooms across America regarding the role Johnson & Johnson Baby Powder has had in cases of ovarian cancer and mesothelioma. We asked Cheng how frequently juries may get issues like these wrong, and he told us that such a question is hard to answer confidently:

There are a couple of reasons why this is a hard question to answer. First, discussions about jury mistakes frequently revisit the same historical missteps. It seems as if the same 1980s or 1990s cases are brought up every time someone wants to suggest juries don’t know what they are doing. But one high-profile goof every decade doesn’t mean that the courts are running amok … Second, most cases settle, so the cases that go to verdict are not necessarily a representative sample. Finally, often times there is an active controversy in the scientific community — in those cases, how does one conclude that the jury was “flawed”? Indeed, what if the jury was “right” (accurate in gauging the scientific climate) at the time, but the science changes?

This question is one Cheng is actively researching. “What we need is a more systematic review,” he told us.

What Does The Ruling Mean for Me?

Since the verdict in Johnson v. Monsanto, the topic of glyphosate cancer risk has become prominent in media reports. Following the ruling, the non-profit Environmental Working Group (EWG) published a report alleging high levels of glyphosate found in cereals commonly used by children, such as Quaker Old Fashioned Oats or Cheerios.

Combined with the news of the verdict, this made for a powerful one-two punch of glyphosate fear, with outlets such as CBS reporting several food products showed “[glyphosate] levels higher than what some scientists consider safe for children.” While “some scientists” may consider the levels presented by EWG to be dangerous for children, EWG’s methods are controversial and their findings largely unrelated to the ruling in Johnson v. Monsanto.

The jury in the Johnson v. Monsanto case had specific questions to answer, and none of them involved determining carcinogenicity from glyphosate exposure in food. In point of technical fact, the jury was not considering glyphosate solely, but the formulations contained in Roundup Pro and Ranger Pro to which he was exposed in high levels as part of his job — which involve much higher levels than would be found in food as residue, and which involves different routes of exposure than food.

Defenders of glyphosate would argue that the chemical has been used safely for over 40 years, is required for most global agriculture, and lacks an alternative that is clearly safer. After the trial, Scott Partridge, the vice-president of Monsanto, maintained that the “verdict doesn’t change the four-plus decades of safe use and science behind the product”. The company has announced they intend to appeal the case.

Supporters of the jury’s decision argue that this is a landmark case against the much-maligned Monsanto corporation, demonstrating both the company’s negligence and their willingness to hide evidence of harm in favor of profit. They argue the juries decision affirms their contention that glyphosate is unequivocally carcinogenic. According to Johnson’s lawyers, Monsanto is facing more than 4,000 similar cases across the US.

While a jury may have ruled with certainty that Roundup Pro and Roundup Ranger caused Dewanye Johnson’s cancer, scientists — unfettered by the legal rules of evidence or the pressure to render timely verdicts — continue to have their doubts.