Logical Fallacy #8: Appeal to Common Practice
Appeal to Common Practice: "All the kids are doing it!"
In this fallacy, "common practice" (the fact that large numbers of people think or act a certain way) is taken as evidence that the practice is justified or reasonable or morally correct. It sneaks into people's world view in some insidious ways.Have you ever thought, said, or heard someone else say, "Most of the world believes in some kind of a god, so God must exist."
The problem with this kind of reasoning is that it performs an end run around the need for compelling argument or evidence supporting the existence of a god.
Reality is not something we vote on. Either a thing is true or it is not, regardless of how the polling breaks. And since science and empirical testing are the best tools we have for determining the facts about the world around us, we must look to the preponderance of evidence for clues about reality, not to the trend of public opinion.
I recall how years ago, the columnist Marilyn Vos Savant (once listed in the Guinness Book of World Records for the highest measured IQ) wrote in her weekly Parade column a piece that included this puzzle, known as the "Monty Hall Problem:
"Suppose you're on a game show, and you're given the choice of three doors. Behind one door is a car, the others, goats. You pick a door, say #1, and the host, who knows what's behind the doors, opens another door, say #3, which has a goat. He says to you: 'Do you want to [change your selection and] pick door #2?' Is it to your advantage to switch your choice of doors?"
Vos Savant answered (correctly) that it is always statistically more advantageous to switch. The public's reaction was loud and swift: thousands of letters poured in "explaining" to Vos Savant the error of her ways. Among these were letters from literally hundreds of mathematicians and other academics, all of whom scolded her soundly for failing to see the answer to such a simple problem.
In this case, large numbers of people thought a certain way, including some serious academicians—people who ought to know. They seemed to have logic and common sense on their side. Everyone thought so. Even people who really didn't know one way or another felt that if that many people—including that many professors and mathematicians—thought Vos Savant was wrong, then she probably was indeed wrong. This is an appeal to common practice (which we may also recognize as a sub-species of the appeal to authority).
The only trouble was, she was right. It's always better to switch.
(And if you are unfamiliar with the "Monty Hall Problem," and you are one of those who think the "smartest woman in the world" got it wrong, I can address this is another post if you like.)
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