An ad hoc hypothesis is one created to explain away facts that seem to refute one’s theory.
Ad hoc hypotheses are common in paranormal research and in the work of pseudoscientists. But they are not limited to pseudoscientists. Another type of ad hoc hypothesis occurs in science when a new scientific theory is proposed which conflicts with an established theory and which lacks an essential explanatory mechanism. An ad hoc hypothesis is proposed to explain what the new theory cannot explain.
Finally, rejecting explanations that require belief in occult, supernatural or paranormal forces in favor of simpler and more plausible explanations is called applying Occam's razor. It is not the same as ad hoc hypothesizing. For example, let's say I catch you stealing a watch from a shop. You say you did not steal it. I ask you to empty your pockets. You agree and pull out a watch. I say, "Aha!, I was right. You stole the watch." You reply that you did not steal the watch, but you admit that it was not in your pocket when we went into the store. I ask you to explain how the watch got into your pocket and you say that you used telekinesis: you used your thoughts to transport the watch out of a glass case into your pocket. I ask you to repeat the act with another watch and you say "ok." Try as you will, however, you cannot make a watch magically appear in your pocket. You say that there is too much pressure on you to perform or that there are too many bad vibes in the air for you to work your powers. You have offered an ad hoc hypothesis to explain away what looks like a good refutation of your claim. My hypothesis that the watch is in your pocket because you stole it, is not an ad hoc hypothesis. I have chosen to believe a plausible explanation rather than an implausible one. Likewise, given the choice between believing that my headache went away of its own accord or that it went away because some nurse waved her hands over my hand while chanting a mantra, I will opt for the former every time.
It is always more reasonable to apply Occam's razor than to offer speculative ad hoc hypotheses just to maintain the possibility of something supernatural or paranormal.
ESP researchers have been known to blame the hostile thoughts of onlookers for unconsciously influencing pointer readings on sensitive instruments. The hostile vibes, they say, made it impossible for them to duplicate a positive ESP experiment. Being able to duplicate an experiment is essential to confirming its validity. Of course, if this objection is taken seriously, then no experiment on ESP can ever fail. Whatever the results, one can always say they were caused by paranormal psychic forces, either the ones being tested or others not being tested.
Martin Gardner reports on this type of ad hoc hypothesizing reaching a ludicrous peak with paraphysicist Helmut Schmidt who put cockroaches in a box where they could give themselves electric shocks. One would assume that cockroaches do not like to be shocked and would give themselves shocks at a chance rate or less, if cockroaches can learn from experience. The cockroaches gave themselves more electric shocks than predicted by chance. Schmidt concluded that "because he hated cockroaches, maybe it was his pk that influenced the randomizer!"
Ad hoc hypotheses are common in defense of the pseudoscientific theory known as biorhythm theory. For example, there are very many people who do not fit the predicted patterns of biorhythm theory. Rather than accept this fact as refuting evidence of the theory, a new category of people is created: the arhythmic. In short, whenever the theory does not seem to work, the contrary evidence is systematically discounted. Advocates of biorhythm theory claimed that the theory could be used to accurately predict the sex of unborn children. However, W.S. Bainbridge, a professor of sociology at the University of Washington, demonstrated that the chance of predicting the sex of an unborn child using biorhythms was 50/50, the same as flipping a coin. An expert in biorhythms tried unsuccessfully to predict accurately the sexes of the children in Bainbridge's study based on Bainbridge's data. The expert's spouse suggested to Bainbridge an interesting ad hoc hypothesis, namely, that the cases where the theory was wrong probably included many homosexuals with indeterminate sex identities!
Astrologers are often fond of using statistical data and analysis to impress us with the scientific nature of astrology. Of course, a scientific analysis of the statistical data does not always pan out for the astrologer. In those cases, the astrologer can make the data fit the astrological paradigm by the ad hoc hypothesis that those who do not fit the mold have other, unknown influences that counteract the influence of the dominant planets.
When Wegener proposed his theory of continental drift he could not explain how continents move. It was suggested that gravity was the force behind the movement of continents, though there was no scientific evidence for this notion. In fact, scientists could and did show that gravity was too weak a force to account for the movement of continents. Alexis du Toit, a defender of Wegener's theory, argued for radioactive melting of the ocean floor at continental borders as the mechanism by which continents might move. Stephen Jay Gould noted that "this ad hoc hypothesis added no increment of plausibility to Wegener's speculation."