Canceling Hypotheses (Conspiracy Theory)


The argument defends one hypothesis by proposing a second hypothesis to explain the lack of evidence in support of the first hypothesis. That is, the second hypothesis cancels or undermines the predictions made by the first hypothesis.



Conspiracy theories are a particularly important example of this fallacy. The conspiracy generally involves a conscious effort to destroy or cover up the evidence that would otherwise have supported a given hypothesis. However, the term "conspiracy theory" is generally used only when there has been a conscious effort to hide evidence, and not all examples of Canceling Hypotheses involve such conscious effort.



"The earth was created just six thousand years ago. The reason atomic dating and erosion patterns suggest a much older earth is that God created the earth in an 'aged' condition."


"Based on a theory that brain size would have some correlation with personality, Paul Broca predicted that criminals would have smaller average brain size than non-criminals. Weighing of brains failed to support this hypothesis, but Broca noted that the criminals in his study had died quickly of violent deaths, while the non-criminals had died of long illnesses, during which their brains had probably diminished in size."


"Strange lights and unusual military activity around Roswell, New Mexico, in 1947 were due to a crash landing (or even possibly the shooting down) of an alien spacecraft. The government denies that this is so, and is covering up the evidence so the public will not panic."



Sometimes, of course, there are factors that hide evidence. Much evidence about ancient civilizations has been destroyed by grave-robbers, for example. The fossil record is notoriously incomplete, due in part to erosion and other geological forces. Sometimes there even are intentional cover-ups, as when Nixon's staff intentionally hid evidence of the Watergate break-in. However, we discover what factors are responsible for hiding evidence by looking at other evidence. We expect that evidence will eventually come to light - even in the case of an intentional cover-up - provided that we continue looking for evidence.

Like many fallacies, the fallacy of Canceling Hypotheses attempts to shut down further inquiry, rather than keep the investigation going. It is an attempt to defend the original hypothesis against lack of evidence; it is not an invitation to look for evidence supporting the second (canceling) hypothesis. Indeed, in a really elaborate conspiracy theory, it may be necessary to propose a third hypothesis to explain why we can find no evidence of a cover-up! By shutting down the investigation, it effectively makes the hypothesis untestable.

The fallacy of Canceling Hypotheses roughly corresponds to the Deductive fallacy of Vicious Circle. It is the Retroductive version of using two mutually supporting arguments, neither of which would be acceptable without the support of the other.


Source: I first became aware that this was an important fallacy in scientific reasoning from reading Stephen Jay Gould's The Mismeasure of Man, New York: Norton, 1981. I coined the ugly term "canceling hypotheses." The term "conspiracy theory" is catchier, and has greater popular currency (see, for example, the movie of that title starring Mel Gibson); but, it must be remembered that not all instances of this fallacy involve a conspiracy.


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