Logical Fallacy

“The case of the ecologist who linked the cycles of the Canadian lynx and its prey, the snowshoe rabbit, with the sunspot cycle is instructive. The ecologist analyzed records of the Hudson Bay Company, which had been collecting pelts of the two species since 1735; he found that the two populations fluctuated up and down, displaying a periodicity of approximately ten years. Not surprisingly, the variations in the predatory lynx population tended to follow the ups and downs in the rabbit population with a time lag of a couple of years. Then the ecologist superimposed the two curves atop a similar graph representing the concurrent sunspot activity: voilà! The three cycles approximately coincided over a good portion of their range. The ecologist leaped to the conclusion that the annual fluctuations of the lynx and rabbit populations were controlled by the eleven-year sunspot cycle….” (57)

This is an excerpt from Lawrence E. Jerome’s Objections to Astrology. The logical fallacy here is post hoc ergo propter hoc, or faulty cause-effect. The ecologist jumped to the conclusion that the correlation between the populations and the sunspot cycle meant that one caused the other. Correlation is not causation. One did not necessarily cause the other; rather, the two showed similarities that may or may not have been due to coincidence. Just because the populations changed after the sunspot cycle doesn’t mean the cycle caused the changes.