Listening to the frequent discussions over controversial empirical claims, an unsophisticated reader could easily walk away with the view that only tradition and prejudice separate the sparring factions. Such a reader might think that most scientists cast a skeptical eye on paranormal phenomena, the claims for homeopathic dilution, the idea that the earth is relatively young, etc., merely because these scientists were taught opposing claims. As one poster's signature would have it, such critics merely engage in "school of thought bashing."
I think this view is wrong. I think it stems, in part, from an inadequate understanding of how to evaluate evidence. The evidential claims for many of these controversial notions exhibit common flaws. They are the kinds of flaws that scientists recognize from many, many past failures. It is this history of dead ends which seduced previous researchers with flawed evidence that informs the way scientists evaluate the evidential claims accompanying these controversial notions.
In this article, I will first list some of these evidential flaws and then discuss errors in relating evidence to theory. Of necessity, this is a short list that omits most such problems. It is largely biased by what I have seen in newsgroup discussions. (A true survey would require a book, of the order that David Fischer wrote for historians.) Finally, I will discuss when mere mistakes (which plague every research direction) turn into quackery.
In the foreground of such controversies are the various studies and experiments published in journals or elsewhere. Various professional posters in the science newsgroups often complain about readers who read all such studies and experiments as if they were the same. The problems listed below are a small sampling of the kinds of issues that the critical eye brings to the reading of these studies and experiments. (I purposely omit particular issues of experimental design and statistical analysis.)
There are unfortunately times when a study or experiment must rely on the measurement of very subjective experience: whether a patient feels better or worse, whether two drawings are similar, etc. This element of subjectivity is notorious for introducing unintended and subtle errors into the result. Studies that eliminate this element as much as possible put the result on firmer ground. Thus, it is better to measure the effect of a medicine through chemical or physical analysis or other objectively measured symptom than through patient report, it is better to compare discrete matches rather than drawings, and it is better to count light flashes with a photodetector than with one's eyes.
Studies and experiments that show a small difference between the test and the control when the test result falls within what well-established theory would predict are somewhat suspicious. This kind of result begs for different experimental design, tighter controls, or investigation of other possible causes.
If tightening the controls in an experiment turns a positive result into a negative one, this is virtually the death knell for the alleged phenomenon. Almost always, this shows that the positive results stemmed from a phenomenon other than the one the experiment is designed to detect. Future positive results are viewed suspiciously unless a good explanation for this history is forthcoming.
Negative results count more against a claim than positive results count for it. This is especially true if negative results continue over time as the alleged phenomenon is studied, even if they are few in number compared to the positive results. The reason is simple. If the phenomenon is real, those studying it should eventually reach the point where they can reliably demonstrate it and where they can teach others how to reliably demonstrate it.
It often takes a knowledge of the field of concern to evaluate these issues. The history of forward steps, set-backs, or stagnation set a context that underlies how a new study is received. This context usually is not explicit in the article or report on the study.
The flaws above concern a particular phenomenon that is alleged to occur and the experiments to evince it. The step from evinced phenomena to theory is also plagued by potential error.
Perhaps the most severe flaw of an empirical theory is that all evidence for it is very indirect. Sometimes this cannot be helped. For example, all historical theories suffer this flaw, since the past can only be observed through its effects on the present. (This makes the study of history particularly challenging.) But theories of current phenomena should admit fairly direct testing. For example, if the flow of qi energy through the body and the existence of molecular patterns from homeopathic dilution are true theories, those who study these things should be able to find experiments that fairly directly measure qi and these molecular patterns.
Similarly, theoretical knowledge should grow and become more detailed as experience increases. In the 1960s, molecular biologists could only mouth vague claims about DNA guiding the development of organisms. Now they can tell how this happens in more detail, and back this discussion by (tens of?) thousands of experiments that evince these details. Two centuries ago, Lavoisier described how oxygen combines with other elements to release energy. Our knowledge of chemical reaction has increased tremendously since then. But what has happened to the theoretical underpinnings of homeopathic dilution in two centuries? Why does it remain vague mouthings about "molecular patterns"?
As experimental and theoretical work progresses, more evidence and more sound evidence for the related phenomena should appear. If the phenomena predicted by a theory remain plagued by evidential flaws as research progresses, then the theory itself becomes very suspect.
Often the results claimed for a novel theory are potentially explained by well-founded theories. These alternative explanations need to be investigated, and such paths barred by better controls in future experiments.
A theory becomes especially suspicious when, in addition to suffering the above flaws, it directly conflicts with a theory that measures well by the same criteria. Using again the homeopathic theory of dilution as an example, if it is true, it will cause a revolution in chemistry and biology that makes cold fusion look like small potatoes. But its evidence remains far too indirect, too shallow, and too slippery to succeed at such a revolution, despite two centuries of research in it.
All the problems above occur within conventional theoretical and experimental investigation. Whether and how they are resolved help determine which theories are accepted and which are rejected. Scientists live on the tension between two poles. Driving them to the exotic is their eagerness to discover new and revolutionary facts. Warning them away from quackery is a skeptical eye informed by knowledge of the myriad errors that have misled others in the past. Scientists looked at N-rays, slippery water, and cold fusion because of the exciting potential to discover something new. They turned away from these things because the evidence did not pan out. John A. Wheeler invited parapsychologists into the AAAS because he thought there was beginning to be some real science in what they did. Ten years later, he knew this had been a mistake.
The attraction of the new and exotic is very strong, and its lure is so bright that it sometimes causes people to lose their critical sense. And some people, unfortunately, never develop a critical sense. Those who have lost or never developed a critical sense create and join "schools" where quackery is born from weak theories and mistaken notions becoming institutionalized. These "schools" are full of the kinds of rationalizations that people use to justify their views when nothing else is available. There are far too many of these to list, but some of the more colorful signposts are listed below.
"Paradigm" is perhaps the most abused word in these discussions. Whenever a proponent of a controversial empirical claim counters criticisms of the evidence by reference to a "paradigm shift," it is time to put on one's hip-waders. To the extent that "paradigm" just means a new theoretical view, it prevails because of -- not in spite of -- sound evidence. The rise of quantum mechanics is frequently referenced as the paradigmatic example of paradigm shift. But the discovers of quantum mechanics did not have to philosophically argue their opponents into making a paradigm shift before quantum phenomena were accepted. The proponents merely presented ever increasing amounts of solid evidence.
To the extent that "paradigm shift" is used to describe something about the social and historical process of how research is done, it has little legitimate role in discussions of evidential quality. Most other uses are so vague that no significant meaning can be attached.
A quack will often reply that his ideas have evidence, just not the kind accepted by "science". The problem with this is that science is no more and no less than sum total of what we have learned about evaluating general empirical claims and their evidence. (Its application to modern research and the need for a new word such as "science" is merely because so much progress in this area has been made in the last three centuries.) With regard to general empirical claims, asserting that there is no scientific evidence is the same as asserting that there is no good evidence. Quacks want to find some room in between, but they cannot explain why we should accept the kind of evidence in their case that has proven so bad in other cases. In essence, they engage in a kind of special pleading that hangs on attaching some odd meaning to the word "science".
This phrase has almost no useful meaning. (Peter Kaminski take note!) If it is used by someone defending a controversial empirical claim, it is virtually guaranteed that the argument is bullshit.
Quack theorists often distort the rest of science is in order to make their favored notions seem more equal in comparison. Thus, "conventional" physics is sometimes accused of ignoring the observer. (Hah!) "Allopathic" medicine is sometimes described as based on non-holistic principles, as practicing the notion of "one symptom, one diagnosis, one cure," etc. ad nauseam . This is all bullshit.
Unless the writer is referring to physics or chemistry, the use of phrases such as quantum, the uncertainty principle, entropy, etc., are warning signs. If they are combined with other words in novel ways -- e.g.: "quantum psychology," "democratic entropy," etc. -- it is an almost sure sign of bullshit. (For Jeremy Rifkin, the rule is reversed. His writings about entropy are bullshit especially when he discusses physics and chemistry.)
Proponents of quack theories are full of excuses for why they have such meagre evidence of their beliefs. These range from "no one funds us" to "the conspiratorial and established institutions ignore us for political reasons." These excuses would not be needed if there were good evidence for the notions in question. The fact that these excuses are offered is almost an admission that the proponent believes despite a lack of good evidence. It it were otherwise, the proponent would focus on the evidence and argue for funding or institutional change because the evidence is so good, rather than excusing the lack of evidence because of these other factors.
This excuse usually comes in the defense of notions resurrected from older traditions, e.g., traditional Chinese medicine. In some sense, it falls under the "big lie" tradition. In a few minutes, someone with a modicum of historical knowledge should be able to think of several cases where millions of Chinese (or Amerindians or ancient Hellenes or ...) and millennia of experience were wrong. The fact is that we have learned a lot about how to perform and evaluate empirical research in the last three centuries and that this gives us a significant advantage over previous traditions. (One of the curious things about the resurrection of older traditions is that foreign traditions are more interesting that native ones. Thus, one hears arguments for qi and traditional Chinese remedies, but almost never for the four humour theory of disease and the frequent bloodletting and purges it prescribes.)
Once a "school" has developed around poor theories, it essentially halts all useful progress by its practitioners until the "school" is reintegrated with the larger scientific community. The institutionalization of theories in an uncritical atmosphere and away from the larger scientific community almost guarantees that there will be a continuing sequence of "positive" results, sometimes for centuries, even though the phenomena remain slippery, understanding remains vague, and discovery of new knowledge is left to the rest of science. In short, a duck is born. Quack, quack.