What is the Attack the Person Fallacy?

Start by reading this out loud to your kid and ask why the reasoning is flawed.

Bernadette the beaver has warned everyone in the pond that a storm is coming and the dam must be raised by tonight, or the pond will flood by morning. It should be noted, however, that Bernadette is a beaver and beavers love building dams above all else. She would happily reinforce that dam any time, storm or no storm. A call for more dam building is exactly what one would expect from her, and so her warning can be safely dismissed; there is no need for the other residents to spend their evening stacking logs.

This is the trickiest kind of Attack the Person, pointing out bias. It isn't silly to notice that Bernadette benefits from more dam-building. That's a fair reason to double-check her. The mistake was stopping there and ignoring her advice. Someone having a reason to say something is a reason to look harder at the evidence, not an excuse to ignore their comments altogether. It's possible that Bernadette had a bias and she was right.

In the 1840s, Ignaz Semmelweis, a physician at the Vienna General Hospital, noticed that the maternity ward run by doctors and medical students lost about one mother in ten to childbed fever, while the ward run by midwives lost far fewer. The doctors, unlike the midwives, often arrived at deliveries straight from performing autopsies. Semmelweis had them disinfect their hands in a chlorinated lime solution, and mortality in his ward dropped roughly tenfold. He reproduced the result after moving to a hospital in Budapest.

The evidence was strong and repeatable, and the profession largely rejected it. Some of the resistance was legitimate: there was no accepted theory of why it worked, since germ theory was still decades away. But much of it was Attack the Person. Accepting the finding meant conceding that doctors' own hands had been killing patients, and rather than engage that possibility, colleagues went after the man — abrasive, eccentric, foreign, unscientific. Those descriptions were largely accurate, and all of them were beside the point: the mortality figures did not depend on whether Semmelweis was easy to work with.

He was forced out, deteriorated, and died in an asylum in 1865, at forty-seven, of an infection. Germ theory arrived shortly after and confirmed he had been right. The case shows the fallacy's cost precisely. For years the profession answered the question "is Semmelweis insufferable?" — which was real, and irrelevant — while the question that mattered, "is he correct?", was answerable straight from his own mortality tables, and went unexamined.