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  The Cochrane review also considered evidence that flossing may help reduce plaque, which is associated with cavities as well as other matters. On this, they concluded that

  Overall there is weak, very unreliable evidence which suggests that flossing plus tooth-brushing may be associated with a small reduction in plaque at 1 or 3 months. None of the included trials reported data for the outcomes of caries, calculus, clinical attachment loss, or quality of life.

  Here we can identify one source of difficulty and potential misunderstanding: a number of different questions are being conflated, including whether flossing improves your life. Let us concentrate on the two main issues, as reported on both by the Cochranes and the news media: plaque and gingivitis. Plaque matters because it can lead to dental caries, and gingivitis matters because it is the first stage of periodontal disease, which can lead to tooth loss later in life. More than 70% of Americans over 65 have some form of periodontitis, which is always preceded by gingivitis.163 If flossing reduces gingivitis, then it is likely that flossing reduces periodontal disease. Periodontal disease has been linked to serious illness, including increased risk of cancer and Alzheimer’s disease.164

  Dental floss defenders made this point. What the Cochranes concluded was not that flossing doesn’t help, but that we don’t have sufficient studies of high enough quality pursued over sufficiently long periods to demonstrate that it does help. The American Academy of Periodontology pointed out that “the current evidence fell short because researchers had not been able to include enough participants or ‘examine gum health over a significant amount of time.’ ” Dr. Philippe Hujoel, a professor of oral health sciences at the University of Washington, Seattle, called it “very surprising” that “we don’t have the … randomized clinical trials to show [flossing is] effective,” given how widespread the belief is that flossing does help.165

  But it is so surprising? Perhaps not. What we learned in 2016 was that we didn’t have the long-term, randomized clinical trials that would be necessary to prove the benefits of dental floss according to prevailing medical standards. It’s not that hard to understand why, in a world of cancer, heart disease, opioid abuse, and the continued use of tobacco products, such studies have not been done. It’s not egregious that researchers have focused their attention on matters that appear to be more serious. What is egregious is that in the absence of evidence that meets the “gold” standard of the randomized clinical trial, people have concluded that there is no evidence at all. That is both false and illogical.166

  Moreover, the gold standard of clinical trials is not just the randomized trial, but the double-blind randomized trial, and it is impossible to do a double-blind trial of dental floss. (This difficulty also plagues studies of nutrition, exercise, yoga, meditation, acupuncture, surgery, and any number of interventions of which the subject is necessarily aware.) Any study of floss usage will also require self-reporting, which, as we have seen, is disparaged. Moreover, if you believe that long-term flossing can prevent tooth loss in old age, it would be unethical to ask a control group to refrain from flossing for what would have to be the better part of their lives. The sort of study that would be required to convince those who subscribe to the “gold standard” is both impossible and arguably unethical to perform.167

  Donn interpreted his findings to say that existing studies show no long-term benefits even when floss is used properly; once again we are observing the fallacy of equating absence of evidence with evidence of absence.168 None of these studies was long enough to demonstrate long-term benefits. Dunn was also quoted as saying that there was “no good evidence.” Whether this is correct depends on your definition of “good,” but clearly there is evidence that flossing may have benefit.

  In the aftermath of the negative media coverage, dentists who support flossing appealed to clinical experience. Several articles quoted dentists, professors of dentistry, and deans of dental schools affirming that clinical practice reveals that those who floss have healthier teeth and gums than those who don’t. Some dentists went so far as to suggest that they can tell who among their patients is lying about their flossing habits simply by observing the conditions of their gums. (This reminds us of another reason why a good clinical trial would be hard to do: people lie about flossing. One study concluded that one in four Americans who claimed to floss regularly was fibbing.)169 And then there is the experience of patients—which is to say, all of us. Many of us have noticed that when we floss regularly our gums don’t bleed, and bleeding gums can be a sign of early periodontal disease. The dean of the dental school at the University of Detroit, Mercy, used this clinical and patient experience to suggest why high-quality trials had never been done: “They don’t do research on things that are common knowledge.”170

  How can we reconcile the experience of dentists and patients with the lack of high quality, long-term epidemiological evidence? We could dismiss these observations as correlation but not causation, but we could also view the experience of dentists and patients as a form of observation that confirms the hypothesis that flossing helps prevent gum disease. In other words, as in the case of the Pill, we can accept the experience of patients and clinicians as evidence, even if the explanation for that evidence is not fully clear. Put another way, we can reject the rejection of this evidence as “merely” anecdotal, and insist that these are case reports, and n is far greater than 1. Moreover, as with the Pill, we can consider mechanism.171 There is in fact good reason to think that dental floss is likely to be beneficial—that these correlations are in fact causally related—because it removes plaque and tartar that can contribute to gum disease, which, over time, can lead to tooth loss. Just as there is a known mechanism that links estrogen to serotonin and mood control, there is a known mechanism by which flossing is expected to prevent tooth loss.

  This was explained by Dr. Sebastian G. Ciancio, the chairman of the department of periodontology at the University at Buffalo: “Gum inflammation progresses to periodontitis, which is bone loss, so the logic is if we can reduce gingivitis, we’ll reduce the progression to bone loss.” But severe periodontal disease may take five to twenty years to develop, so this effect cannot be demonstrated in a clinical trial that lasts only weeks or months. Dr. Wayne Aldredge, president of the American Academy of Periodontology put it this way: “It’s a very insidious, slow, bone-melting disease.… You don’t know if you’ll develop periodontal disease, and you can find out too late.”172 In short, the “gold standard” of the randomized clinical trial is unable to reveal the benefits that periodontists predict. The clinical trials that have been undertaken were not the right tools for addressing that question.

  The term “gold standard” should remind us that there are silver and bronze standards, too—or at least there should be. As Nancy Cartwright and Jeremy Hardie have argued, the ideal of a uniform gold standard is misguided: No one would use gold for household pipes; it is too expensive. Nor would we use gold for cooking knives: it is too soft.173 The best tool depends on the job, and that applies to intellectual jobs as well as industrial and household ones.

  What would be the right tool to investigate dental floss? One might be a different sort of clinical trial. The American Dental Association notes that disappointing results might be the result of poor flossing, which, they noted, is a “technique sensitive intervention.”174 The New York Times concluded: “So maybe perfect flossing is effective. But scientists would be hard put to find anyone to test that theory.”175 With due respect, that is an ill-informed remark, because scientists have tested that theory. The clinical trials reviewed by the Cochranes did not examine the impact of flossing technique, but a review of six trials in which professionals flossed the teeth of children on school days for almost two years, saw a 40% reduction in the risk of cavities.176

  That is a huge effect. So consider this alternative headline: “A New Job Opportunity: Science Shows the Need for Professional Flossers.” Imagine the social change that might have ensued and the
employment opportunities created. On our way to work, instead of stopping at Peets or Starbucks for a quick latte or a Drybar for a blow-out, we could stop at a flossing bar for a five-minute professional floss.

  What Does It Take to Produce Reliable Knowledge?

  There are many ways in which scientists can fail to live up to their own standards, as well as ways in which the standards they set can be unhelpful, incomplete, inadequate, or inappropriate to a particular situation. Still, I believe there are some themes that we may glean from these diverse cases. They are: (1) consensus, (2) method, (3) evidence, (4) values, and (5) humility.

  Consensus

  In chapter 1, we saw that historians, philosophers, and sociologists have come to focus on scientific consensus because there is no independent measure of what scientific knowledge is. We cannot identify science by any unique method. We can only identify claims as being scientific based on their provenance, that is to say, based on the way they were established and by whom. Scientific facts are claims about which scientists have come to agreement.

  Some skeptics have used this argument to try to discredit contemporary science, claiming that there was a consensus supporting eugenics or rejecting continental drift.177 This, they argue, proves that scientific consensus is an insufficient basis to command our trust, a faulty foundation for decision-making. But these claims are misplaced: scientists did not have a consensus about eugenics or continental drift. Social scientists, socialist geneticists, and some mainstream geneticists critiqued eugenics; the rejection of continental drift was a distinctly American affair. (Europeans for the most part withheld judgment, which is a different thing.) Nor was there a consensus over the Limited Energy Theory, the Pill, or dental floss. Gynecologists liked the Pill for its efficacy; psychiatrists were concerned about its psychological health side effects. Short-term epidemiological studies fail to find strong evidence for beneficial effects of flossing, but nearly all clinicians observe benefits. And leading women physicians pointed out the obvious flaws in the Limited Energy Theory.

  A key finding from historical inquiry into these episodes, then, is that in all of these cases there was significant, important, and empirically informed dissent within the scientific community. When we see disputes within scientific communities across geographic, disciplinary, or other gaps, this should command our attention. Debates may arise between different types of scientific experts examining a common topic—psychiatrists and gynecologists—or between different types of people—male doctors and female ones—or between scientists in the same field bringing different background assumptions and values. These debates occur because different groups of scientists are emphasizing different bodies of evidence, highlighting different aspects of those bodies of evidence, or bringing different values and background assumptions into the interpretation of evidence.

  Scientific consensus is hard to come by. This is an underappreciated fact. Therefore, in any debate, it is crucially important that we evaluate whether an expert consensus prevails or not. In 2004 I wrote a paper asking: Is there a scientific consensus on anthropogenic climate change? I had discovered that no one had analyzed the scientific literature with this question in mind and it seemed to me that any discussion of a mooted question should begin with an analysis of this sort.

  In a recent issue of the Hedgehog Review, the editors wrote that “when we hear conflicting scientific pronouncements being issued on almost any subject (climate change, diet, vaccination) … it is not hard to see why science, and particularly scientific authority, has become the target of heated contestation and debate.”178 This claim is wrong on two counts. First, it has cause and effect backward. These issues are contested because various groups—the tobacco and fossil fuels industries, advocates of deregulation, parents of autistic children who feel inadequately supported, some evangelical Christians—are unhappy with scientific authority. Some of them want science to be devalued.

  Because science has challenged their interests or beliefs, they challenge science. Contestation is the outcome of a conflict about authority. Second, these are not conflicting scientific pronouncements. On most of the scientific issues that are highly contested in American culture—evolution, vaccine safety, climate change—there is a scientific consensus. What is lacking is cultural acceptance by parties who have found a way to challenge the science. This is the source of the contestation, not conflicting positions within the scientific community. Political and cultural debate is by no means illegitimate, but political debate masquerading as science is dishonest. It has led to the sort of confusion displayed by the editors of Hedgehog Review and many others.

  Consensus analysis of peer-reviewed literature (as I have done) is a means to determine whether scientists agree. If they do, then we can take the next step to identify who is contesting their findings, and why. In our book, Merchants of Doubt, Erik Conway and I were able to show that climate science was being contested by the fossil fuel industry, whose economic interests were threatened, and by Libertarian think tanks and conservative scientists whose political beliefs were challenged. Rather than admit this, they challenged the science as a means to protect their economic interests and political commitments.

  If there is informed dissent within the scientific community, more (scientific) research may well be needed. If, however, the dissent is emanating from outside the relevant expert scientific community, then we have a different issue at stake. In the latter case, more scientific research is unlikely to settle the matter, because non-scientific objections are not driven by scientific considerations and therefore will not be resolved by more scientific information.

  This is not to say that non-scientific objections are invalid, but only that they should not be confused with “scientific pronouncements.” There can be important moral objections to social programs based on science, even if the underlying science is legitimate. And, as the contraceptive pill case illustrates, relevant information can emerge from outside specialist communities. My intent in presenting the Pill case was not to say that patients were necessarily correct, but rather that they had relevant information that should not have been disparaged simply because it came in the form of self-reporting.

  How do we judge if non-experts have relevant, useful, and accurate information? This is not an easy question to answer. We have clear markers of scientific training and expertise: higher education, membership in scientific and learned societies, records of publication and research grants, H-indices, awards and prizes, and the like. Scientists know who their scientific colleagues are and what their track records look like. Scientists (for the most part) know which journals have rigorous peer review and which do not.

  Judging information from outside the expert world, however, is a different and trickier matter.

  Scholars have identified several categories worthy of attention. One is other professionals who have relevant information. This could include nurses and midwives, for example, who have direct contact with patients and may differ from physicians on questions such as pain management.179 A second category is people who may not have professional training, but whose daily experiences may lead them to relevant knowledge and understandings, such as farmers and fishermen.180 We might say that these people have daily “on the ground” experience, and therefore may see things that scientific experts, for whatever reason, have missed. (Earth scientists call this “ground truth,” in this case referring to what geologists on the ground see and therefore know about, as compared with evidence, for example, from satellite remote sensing.) As Brian Wynne has stressed, the non-expert world is not “epistemically vacuous.”181

  A third category is what Marjorie Garber has called “amateur professionals.”182 These are people—perhaps independent scholars or scholars from other fields—who have educated themselves on a particular subject. Developing expertise outside of conventional avenues of credentialism is certainly possible (although if a scholar from one field moves into another, they can establish credentials by publishing). A
fourth category is citizen scientists: people who earn their living in other ways, but participate in science out of love or interest. In some domains—astronomy, entomology, ornithology, and the search for extraterrestrial life—citizen scientists have played significant roles in observing things that professionals do not have the time, money, or human resources to track.

  People in all these categories may have knowledge relevant to a particular scientific question. They have a known relation to their object of study and basis for claiming a role in scientific conversations bearing on that study. Where their experience and expertise overlap with scientific expertise, we should pay attention and not automatically discount what they have to say, nor assume that their claims are necessarily in conflict with those of scientific experts.183 Often expert and lay perspectives can be reconciled or seen as complementary. But, again to draw on Wynne, we should not misunderstand a claim for recognition of these knowledge categories as a claim for their intellectual superiority or equivalence.184 Just because someone is close to an issue does not mean he or she understands it; conventional notions of objectivity assume distance for just this reason. Parents of autistic children will have detailed knowledge of their children’s conditions, but this does not mean that they are in a position to judge what caused it.185

  Respecting professional diversity and lay expertise is also a different matter from heeding “dissent” from people with no credible claim to expertise—celebrities, K-Street lobbyists, or the op-ed writers of the Wall Street Journal or the New York Times. When people without relevant expertise criticize science, we should consider the possibility that something fishy may be going on. If people are attacking science, there is something at stake, but it is not necessarily something scientific. Indeed, it is probably not.