I don’t floss. This makes me, as my dentists always seem to imply, a naughty boy, a disgusting human willing to walk around with bits of food stuck between my teeth. Of all the dreaded parts of any dentist visit, the worst is receiving a condescending lecture on the merits of flossing.
Then I found a secret weapon: a 2019 research review by Cochrane, an independent network of scientists widely considered to produce the most golden of gold-standard research. They pulled together studies that investigated the impact of flossing or using other devices that clean between teeth on dental health: 35 randomized-controlled trials, 3,929 participants total.
The results were dismal. Flossing “may” reduce gingivitis, the meta-analysis found, but the effects were uncertain and barely significant statistically. None of the studies investigated whether flossing prevents cavities.
At my next appointment, when the dentist innocently inquired about my flossing habits, I dropped my newfound knowledge bomb. She replied that it’s good to floss and we left it at that. Hours later, I received a phone call from another dentist at the same practice. “My colleague told me what you said about flossing,” he said, exasperated. He went on to say something like, “Look, I know about the evidence, but if I had to choose whether someone only brushed or only flossed, I would choose floss.”
I thought I was born into the age of science and reason. But what my trip down the flossing rabbit hole taught me is that this is nowhere nearly as true as I hoped it to be. It wouldn’t matter much if it was just dentistry, which has long been the Wild West of medicine. But the more I learn about science, the more I discover basic mysteries that I assumed were solved long ago. Perhaps we’ve exited the Dark Ages, but our own age still seems rather dim.
How do you make sense of a world where the scientific sands are always shifting and where so much remains unknown? It’s taken me a long time, but I’ve made my peace with it by learning to hold everything loosely, to remain ever humble about what we do know and ever optimistic about what we will.
It was hard to get there, because I had to learn over and over again that extreme conviction requires extraordinary evidence, and the evidence we have is usually far from extraordinary. For instance, our frontline anti-depression drugs are supposed to work by changing serotonin levels in the brain, but a review published last year found that there’s no consistent evidence that serotonin has much to do with depression at all. (Maybe that’s why antidepressants don’t seem to work that well, especially in the long term.) It seems obvious that sunscreen should protect you from skin cancer, but a 2018 meta-analysis could not confirm that this is true and concluded that more studies were needed. A popular ingredient used in many over-the-counter cold medicines probably doesn’t do anything either.
Even in basic science, mysteries abound. Physicists still aren’t certain whether cold water freezes faster than hot water. Astronomers hypothesize that invisible “dark” matter and energy fill the universe, but they don’t really know what it is. Nobody even knows where butts come from.
When I was younger, I met every scientific revelation and reversal with righteous credulity. For instance, if I found evidence that sunscreen didn’t prevent skin cancer, then it doesn’t, and thank goodness we finally know the truth! On beach trips, I would dramatically refuse any proffered bottle of sunscreen, lecture my family and friends about its overblown promises and occasionally curse the name of the Hollywood director and sunscreen shill Baz Luhrmann.
As I got older and finished up my Ph.D., my credulity gave way to despair. Good science is simply too hard! If you really wanted to know even something as simple as whether flossing works, you needed thousands of people randomized to floss or not, and then you needed them to actually floss (or not), and then you needed to follow up with them years later. Anyone willing to undergo such an ordeal for science surely isn’t representative of humanity as a whole, and how could you know that they’re flossing properly, anyway? Best just to give up.
What sustains me now is neither certainty nor hopelessness, but a determined, humble optimism. The right answers are often simply unknown, and I might die without getting to know the truth. And yet the truth will be known one day. Just as we solved many of the mysteries that befuddled our ancestors, our descendants will solve many of the mysteries that befuddle us. Our ignorance is profound, forgivable and temporary. There are only two true errors: One is believing that we have no errors left to make, and the other is believing that those errors are permanent and irreversible.
It’s hard to maintain this attitude when you realize just how much we have left to figure out. And we often have to cultivate this attitude for ourselves, because our educators and experts often fail to give it to us. When I was in high school, my science teachers implied that the universe was a collection of dull, ancient facts, and my job was to memorize and repeat them. Nobody mentioned all the mysteries left unsolved or suggested that it might be great fun to solve them.
Experts, for their part, are perfectly comfortable issuing blanket recommendations with scant evidence and then shaming people for not following them. In one galling example from February 2020, then-Surgeon General Jerome Adams issued a scathing tweet: “Seriously people STOP BUYING MASKS! They are NOT effective in preventing general public from catching #Coronavirus, but if health care providers can’t get them to care for sick patients, it puts them and our communities at risk!” There was no scientific basis for such a strong claim; even three years later, a recent meta-analysis — again from our friends at Cochrane — concluded, “There is uncertainty about the effects of face masks.”
Our teachers and officials may think the public simply can’t handle uncertainty, and maybe that’s why they project so much confidence, even when the science is faulty. But the best way to cultivate informed citizens is to give them the evidence that we have, not the evidence we wish we had.
My dentists could have been matter-of-fact with me about flossing: “Look, nobody’s ever run a good study on this, but it makes theoretical sense why flossing would work, and in my experience it helps patients, so I encourage you to do it.” That would have made me feel respected and even curious — maybe I would have aspired to be the person who runs the conclusive flossing study. At the very least, I might have picked up some floss. For now, I remain skeptical that scraping the sides of my teeth with a piece of nylon is a good use of my time, but I also remain hopeful that someday we’ll know for sure.
Adam Mastroianni is an experimental psychologist and the author of the science blog Experimental History.
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