Correlation and Causality

Several decades ago, even in socialist schools, it was the custom to teach children the phrase "post hoc, ergo propter hoc,'' which means "after this, therefore on account of this.'' This tiny Latin lesson illustrated the fallacy in concluding that a thing which follows another thing is therefore caused by it. See Oxford English Dictionary VII p 1164 (1933-1970). The practice of incorrectly reasoning in this way is termed "post hoc'' reasoning.

Examples of post hoc reasoning are boundless and have been the source of much humor. Mark Twain often used post hoc humor such as in his story about the woman who died after neglecting to adopt the vices of smoking and drinking. Mark Twain's audiences in post Civil War America were very literate in the logic of common sense learned in a free enterprise society. If he were with us today, Mark Twain might find it prudent to have a helper planted in his audiences to give cues about the appropriate times to laugh.

Americans today are still partially immunized against post hoc arguments by this legacy from our intellectual past. Some mutant forms of post hoc logic do afflict us, however, such as the notion that a man cannot stand on his own two feet in society without tax subsidies and preferences because his ancestors over a century ago were enslaved or the fallacy that, since many Americans lead productive and decent lives after attending socialist schools, the socialist schools are to be credited with their accomplishments.

(Selective observation is usually a part of such arguments. Socialist schools which encourage intellectual dishonesty and moral degeneration are not credited with the subsequent moral decay of adults.) Today, far more people fall victim to a close relative of the post hoc virus than to the original virus itself. This new affliction is the fallacy that when two things are statistically correlated they are, therefore, cause and effect. Paraphrasing the original fallacy, people have come to believe "statistically together with this, therefore on account of this.'' (Fear of scholars who read Access to Energy prevents me from attempting the Latin.) Examples of the logical error that correlation proves causality appear almost daily in newspapers, magazines, and other information sources. They also appear regularly in scientific publications. "Scientists'' who should know better are even using this logic in situations where one of the two correlated phenomena is only postulated or even known not to exist.

In the article "Fried Frogs' Eggs'' (Access to Energy 21-9 p 4), we reported the claim by scientists that world-wide amphibian populations are decreasing because of increased UV light levels caused by depletion of the ozone layer. UV light is capable of disrupting living systems as illustrated by its use in water purification and the sterilization of medical instruments or its known role in the induction of skin cancer. It was no surprise, therefore, that these people found that frogs eggs had a higher hatch rate when protected from UV light.