StATS: What is critical thinking? (created 2005-05-17).

A lot of people use the term "critical thinking" in an offhand way. Usually the writer who uses this term is trying to imply that anyone who disagrees with the writer's theory or belief is naive. Critical thinking is also produced as an excuse to attack a particular theory or to promote "equal time" for a competing belief.

Critical thinking, however, is much more complex than this. It involves more than just fault finding. After all, no theory is perfect and unflawed. Rather it looks at the evidence supporting a theory in addition to the critical evidence. It evaluates the quality of these arguments rather than the quantity. Nit pickers will always find hundreds of little things to complain about, but a critical thinker will brush aside trivial concerns about a theory if other supporting evidence for that theory is strong.

A perfect example of critical thinking versus nit picking is the developing recognition in the 1950s and 1960s that cigarette smoking causes cancer. This body of research is summarized well in Gail 1996. The research studies that identified a link between smoking and cancer were not perfect. You couldn't use randomized trials and you couldn't use blinding and you couldn't use a host of other research methods that would make these studies more rigorous. There were animal studies, but the nitpickers pointed out that you can't always extrapolate from animals to humans. There were far higher rates of smoking in cancer patients than among controls, but the nitpickers pointed out that these were retrospective studies and people didn't always remember things accurately or report them truthfully.  Prospective studies were harder to do, but when these also showed a link between smoking and cancer, the nitpickers had another counterargument. They pointed out that people who smoke also tend to have other vices and bad habits that might account for the differences seen.

A critical thinker would look past the individual nitpicks, though. A critical thinker subjects the objections to a theory or belief to the same level of scrutiny as the theory or belief itself. The problem with the nitpickers, of course, is that each time a different type of study came out that showed a link between smoking and cancer, they had to come up with yet another explanation. And all of these alternate explanations had to be true simultaneously in order to negate the growing body of evidence. It becomes a house of cards that eventually collapses under its own weight.

One of the more fascinating episodes in this history was an alternate theory proposed by the nitpickers. The theory was that it wasn't the smoking that caused cancer, but rather the cancer that caused smoking. This seems like a bizarre claim, since the smoking often precedes the cancer by several decades. The explanation offered was the certain people have a genetic makeup that makes them more vulnerable to cancer. The same genes that make a person cancer prone might also make a person more susceptible to nicotine addiction.

A critical thinker would have to carefully examine this claim. It's true that in many situations, you can't easily identify whether A causes B or B causes A. For example, there is a fair amount of research that seems to indicate that an optimistic and hopeful attitude is associated with better outcomes when facing a serious illness or disease. Does this positive outlook cause these people to have a stronger immune system that allows them to better battle the disease? Or is it possible that the disease itself is less virulent in certain people, possibly in ways that are difficult to measure directly. Perhaps these people with the less virulent disease know something that the doctors don't and it manifests itself as a more positive attitude. So maybe it is prognosis of the disease itself that causes the differing attitudes and outlooks rather than the attitudes and outlooks affecting the prognosis of the disease.

With cancer and smoking, however, a critical thinker can easily identify which is the cause. If it were true that a genetic predisposition to cancer were associated with a genetic predisposition to addictability, then these cancer genes would operate the same way in countries where cigarettes were easily available as in countries where cigarettes were hard to come by. That's not really the case, though. In China, cigarettes came into widespread use much later than in Europe and America. The late surge in lung cancer cases in China that followed the wide distribution of cigarettes contradicts the hypothesis that it is the genes rather than the cigarettes.

Keep in mind that contrasting the rates of smoking and cancer in different countries is, by itself, a rather weak form of evidence. There are many obvious and subtle differences between China and America. The value of the data on smoking and cancer in China, however, is that it is grossly inconsistent with the belief that cancer causes smoking. To maintain this belief, you would have to postulate that a mutation occurred in the genes of most Chinese people right around the time that cigarettes were widely adopted.

In summary, it's not so much that the belief that smoking causes cancer is based on perfect and unflawed data, but rather that the flaws are trivial compared to the flaws in the arguments of those who believe in alternative explanations.

was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Critical appraisal.