Argumentum ad hominem – logical fallacies

This article will focus on the informal logical fallacy of argumentum ad hominem or ad hominem argument (see note 1). Although the ad hominem argument is used frequently in science discussions, many individuals misuse the term. Our job here is to describe it accurately. Or as accurately as we can.

Description of an ad hominem argument

The ad hominem argument applies to any argument that centers on emotional (specifically irrelevant emotions) rather than rational or logical appeal. Generally, an ad hominem argument occurs when an individual attempts to refute a claim by attacking the maker of the claim rather than focusing on a refutation of the claim itself.

Simply, the fallacy passes the eye test when one observes that the arguer is attack the source of the claim rather than debunking it with evidence with counter arguments.

The argument is a subset of the genetic fallacy, which attempts to misdirect the line of reasoning by invoking the origins of a claim, rather than the quality of evidence. The appeal to false authority is another type of logical fallacy that related, as it focuses on the maker of the claim rather than the assertion itself.

What isn’t an ad hominem argument?

Some people misuse this argument, by accusing individuals of this logical fallacy in discussions. An insult is not a generally considered a form of this argument, because it is not used to rebut a contention.

Many types of insults are considered poisoning the well, that is, attempting to associate negative emotions about a person making an argument to distract from the actual evidence in the argument. Anti-vaccine activists use the Big Pharma Shill gambit, wherein the attempt to dismiss the evidence by proclaiming them to be on the payroll of some company.

Of course, if the gambit is used to dismiss the authority and attempts to use additional evidence either to support their arguments or debunk the other side, it’s not an ad hominem argument. It is poisoning the well, of course.

Criticizing or critiquing someone’s qualifications to make an argument is not an ad hominem. If someone is a false authority, or lacks any credentials, then it is useful to point out the lack of qualifications as part of the discussion, as long as it is in context of debunking the claims of the false authority.

In general, an ad hominem argument occurs when one attacks the person making an argument rather than the argument itself. And criticism is not an ad hominem argument if a person’s credentials are actually the topic of the argument.

It is subtle, and that is why it is frequently misused.

Example of an ad hominem argument

“Your evidence about the safety of vaccines is irrelevant, because you eat a quart of  ice cream alone every night. All of that high fructose corn syrup has damaged your brain, and you cannot possibly know anything about vaccines.”

Example of not an ad hominem argument

“You have completely ignored the overwhelming evidence that vaccines do not cause autism. You have provided no valid and robust evidence that contradicts the science. All I can say is that you are an uninformed and ignorant cretin about vaccines, and you should refrain from participating in these arguments.”


  1. I generally italicize latin words. However, according to both the Associate Press Stylebook and various scientific stylebooks, italicization of latin words has been deprecated, especially if they are used commonly. There are a few exceptions, like scientific words like in vivo, in vitro, or in situ because the two words together having meaning. We learn something new every day.