Say that one in a thousand people has disease x. There’s a test for x, but five percent of the time it says someone has x when he doesn’t. You, the test says, have x. How likely is it that you do?

A little math goes a long way here. If one in a thousand people is infected but five in a hundred people falsely test positive, then out of every ten thousand people tested, ten will have the disease and test positive while five hundred will test positive without having the disease. The question, then, is whether your positive result is one of the ten out of 510 that’s accurate. There is only a 1.96 percent chance (10/510) that you’re infected.

Now consider this. In 2013 a few dozen doctors and medical students were asked this question, and around three-quarters of them got the answer wrong. In fact, almost half of them said there’s a ninety-five percent chance you have the disease.

Statistical thinking does not come easily to us. Our primate brains are not designed to grasp it, or even to reach for it, naturally or intuitively. Reason bows to passion. Instinct substitutes for evidence.

Much of what passes for “common sense” is nonsense. Take our aversion to ambiguity. Would you rather wager on drawing a red ball from a bowl with an equal number of red and white balls, or from a bowl with a random distribution of such balls? If you’re like most people, you’d much prefer to bet on a draw from the first bowl. The risk hasn’t changed; only your perception of it.

We dislike uncertainty, too, and will do strange things to evade it. An optical illusion causes a light in a dark room to appear to move. Ask individuals how far the light moved, and they will offer different estimates. No surprise there—the light didn’t move. Ask a small group the same question, however, and they will coalesce around a common answer. More than that, each member of the group will, if taken aside, confirm that he believes that answer is correct.

When certainty is unavailable, people will accept certitude in its stead. Using words to describe emotions (“That’s not fair!”) is not the same thing as making an argument. In a group setting, however, this fact does not count for much. A person can pull a group in his preferred direction merely by expressing his position with great feeling and confidence.

It’s not just certainty that people want from the group. They also crave solidarity. They want to belong. If the tribe accepts you, you live. If it rejects you, you die. So says your primate brain. Divide children into two randomly assigned groups, and they will quickly form a preference for their groupmates. They will even assume that the other group is comprised of worse behaved children.

“A desire for fraternity and we-ness,” biologist Mark W. Moffett writes, can eclipse “the goal to achieve sound solutions.”

Here is one especially promising way to stoke tribal enthusiasm. It is a ritual. Start by putting twelve men and women in an impressive room. Have them swear a solemn oath. Then have them watch a grand speech contest among smart and serious people wearing robes and other ceremonial attire. (A person who has just completed the ritual will often speak in euphoric terms about the camaraderie it produces among the twelve participants.)

Some of the speakers should appeal frequently to an us-versus-them mindset. This attitude has driven the primate brain to action for generations without number—since our distant forebears were living in trees. The world may be excruciatingly complex, but the stories told in the ritual should be delightfully simple. Vilification is the key. In one of the stories, everything “they” do causes harm. That, indeed, is why “they” do it! “They” are greedy, black-hearted fiends. “They” must be punished. “They” must be brought low.

Ensure that much of the ritual revolves around disgust. The great weight of the evidence suggests that we benefit immensely from nuclear power, genetically modified food, and vaccines. Yet around half of Americans oppose the use of nuclear power, around half think genetically modified food is unsafe or unhealthy, and a vocal minority assert that vaccines are dangerous. Some of the underlying concerns about these things might ultimately be justified—openness to further inquiry and new evidence is what makes science science. But the dissenters generally do not base their views on careful investigation or cost-benefit analysis. They base them, rather, on a vague conviction that the items in question are icky. The primate brain recoils from anything it perceives to be defiled, polluted, or unnatural.

Once your band of twelve is tightly knit, angry, and disgusted, set before it a genuinely sympathetic victim. Let someone explain that this humble man of flesh and blood is one of “us,” and that he has been hurt by the distant corporate “them.” Let the side of “us” tell a real tale of human struggle, and leave the side of “them” to respond with technical—better yet, statistical!—arguments about cohort studies and risk ratios.

The chances are decent that by ritual’s end, twelve private citizens will be ready to act as one avenging hammer.

Which brings us, at last, to the recent jury verdicts in favor of plaintiffs who contend that Bayer AG’s Roundup herbicide gave them cancer.

To show that Roundup caused his cancer, a plaintiff must first show that Roundup can cause cancer. Now it is true that if you douse cells in a Petri dish in massive quantities of glyphosate, Roundup’s active ingredient, the cells’ genetic material will be damaged. The question, however, is whether real live humans who experience real-life exposure to glyphosate in real-world quantities are more likely to develop cancer. And the largest study to grapple with that question found, after tracking more than fifty-thousand people for more than twenty years, that there is no link between glyphosate and non-Hodgkin’s lymphoma, the cancer at issue in the recent court trials.

Even if he can establish causation in principle, a plaintiff must show that Roundup is what caused cancer in him. Most cases of non-Hodgkin’s lymphoma are idiopathic—the reason the patient developed the disease is unknown. A federal trial judge in San Francisco noted that this fact alone might have been “the end of the line for the plaintiffs” in much of the country. But the court of appeals above him, the U.S. Court of Appeals for the Ninth Circuit, has declared that “medicine partakes of art as well as science,” and the judge believed himself required to let the plaintiffs rely on experts whose opinions “lean strongly toward the ‘art’ side of the spectrum.” Even then it was “a close question,” the judge wrote, whether the plaintiffs should be allowed to present their case to a jury, but in his view they had “barely inched over the line.”

Come trial, of course, subtlety of this sort disappears. During one Roundup trial’s closing arguments, the plaintiff’s lawyer concocted an image of the defendant’s executives sitting in a boardroom, waiting for the phone to ring. “Behind them is a bunch of champagne on ice,” he said, and if they hear of a low damages award, “champagne corks will pop” and “‘Attaboys’” will be “everywhere.” In another of the trials the plaintiffs’ lawyer was reprimanded for violating no less than four pre-trial evidentiary rulings during her opening statement. These violations, the judge concluded, “were premeditated.”

The lawyers paid no real price for these tactics. The attorney who argued in bad faith during her opening was fined $500. No mistrials were declared.

One last thing about the primate brain: if you excite some people to fury and then have them discuss their fury with each other, they will usually rile themselves up and become more furious still. A 2000 study found that when jurors head into deliberation wanting to impose punitive damages, the jury’s ultimate award is typically higher than what the median juror would have awarded before the deliberation started. Often it is higher than what any individual juror would have awarded.

The jury awards against Bayer have been undamped effusions of rage. In August 2018 a former groundskeeper won $289 million. In March a man won more than $80 million. And in May a couple that had used Roundup outside their home won more than $2 billion. The trial courts have reduced each of these awards.

There are now more than 18,000 plaintiffs suing Bayer over Roundup. A mediator is trying to help resolve these cases. The voice of rumor whispers that Bayer might settle them for around $8 billion.

We proudly use juries to give the people a role in the execution of the laws. Sometimes their main contribution is a dose of popular prejudice. The arrangement has the weakness of its strength.

Also published by on WLF’s contributor page.