Genetic fallacy

“You got that from a TV show”

Dana: We should put the things we use every week on the front shelves of the storage room and move the holiday boxes to the back.

Morgan: You got that from one of those decluttering shows. That’s not a serious source.

Where Dana heard the idea says nothing about whether it works. The plan makes a checkable prediction: fewer boxes moved to reach everyday things. Morgan could test that in an afternoon, and the TV show is no reason not to.

“It came to me in a dream”

Theo presents a new layout for the community garden plots. “It came to me in a dream, fully formed. Ideas that arrive like that are meant to be.”

This is the positive direction. The dream explains why Theo has the idea, not whether each plot gets enough sun or the paths leave room for a wheelbarrow. Dreaming isn’t a process known to produce good garden layouts, so the origin adds nothing in either direction. The layout might still be excellent, but that has to be checked by looking at the plan itself.

“You only think that because it was your dad’s”

Sam says their fifteen-year-old car is still reliable. A friend replies: “You only think that because it was your dad’s car and you’re sentimental about it. So it’s probably a lemon.”

The first sentence makes a fair point: affection isn’t a good way to judge a car, so Sam’s confidence deserves less weight than it would if it came from a mechanic’s inspection. The second sentence is where the reasoning fails. A belief held for a poor reason isn’t false for that reason; it’s just not yet supported. Whether the car is reliable depends on its repair record, not on why Sam likes it.

The fallacious pattern, in either direction:

• The claim came from origin O.
• O is disreputable (or admirable).
• Therefore the claim is false (or true).

The philosopher Kevin Klement sets out forms of reasoning from origins that are not fallacious, because the origin is connected to the truth of the claim:

The difference is the second premise. A process’s track record for producing true beliefs bears on the belief; how we feel about the source doesn’t.

Klement also points out an asymmetry. A reliable origin can make a belief likely to be true, but an unreliable one usually can’t make it likely to be false: guessing, wishful thinking or a magic eight-ball is just as likely to land on a true answer as a false one. To show a belief is probably false from its origin alone, you’d need a source that is reliably wrong, which is rare. That’s why, Klement argues, reasoning from origins is far more often legitimate when it supports a belief than when it discredits one.

“Genetic fallacy” works as an umbrella name. The philosopher Abraham Kaplan defined it as “the fallacy of judging the truth of an assertion on the basis of its source rather than by the evidence or argument available for it.” Several narrower errors make that move, each focused on a different kind of origin:

• The person who says it. Rejecting a claim because of who makes it is an ad hominem; accepting it because of who makes it, when their say-so isn’t good evidence on the question, is an appeal to authority. The two are mirror images: the same reliance on a source, with the judgment reversed.
• When the idea arose. Chronological snobbery rejects an idea for being old, the appeal to tradition accepts it for being old, and the appeal to novelty accepts it for being new. Each treats a date as if it were evidence.
• How someone came to believe it. “You only believe that because of how you were raised” or “because you want it to be true” explains a belief’s causes and treats the explanation as a refutation, as in the car example above.

Logic textbooks don’t agree on how these fit together. Klement notes that some define the genetic fallacy as covering both positive and negative judgments from origins, while others count only attempts to discredit. He also reports that some textbooks, Irving Copi’s Introduction to Logic (1953) among them, file the genetic fallacy under ad hominem rather than the other way round, while others treat the two as separate fallacies of relevance. The shared flaw is the same in every version: a fact about where a claim came from is standing in for evidence about the claim.

The term also has an older, narrower sense. According to Klement, it was probably introduced in the 1920s or 1930s for a mistake about scientific theories: confusing an account of how a theory was discovered with an account of whether it is justified. The philosopher Wesley Salmon put it this way in 1963: “the error of treating items in the context of discovery as if they belonged to the context of justification is called the ‘genetic fallacy’.”

An origin is relevant whenever it tells you something about how likely the claim is to be true.

• The origin is a process with a known track record. A reading from a calibrated instrument, a measurement from a survey, or what someone saw clearly in good light comes from a process that produces true beliefs far more often than false ones. Trusting a claim because of that origin is reasonable.
• You’re relying on the source because you can’t check the claim. If a claim reaches you only through one person or document, its reliability is the main evidence you have. This is the testimony case described under Ad hominem.
• The origin is a reason to look harder, not a verdict. A study funded by a company that sells the product is a fair reason to examine its methods more carefully. Andrew Ward, writing about research funded by drug companies, argues that pointing out such an origin is appropriate because it calls for closer scrutiny, though it doesn’t show the results are wrong.
• The claim is about the origin. “This recipe came from my grandmother” or “this rule started as a misprint” is answered by the history, because the history is the subject.
• You’re answering an argument that itself appeals to origins. If someone argues that a belief must be true because that’s the best explanation of why people hold it, offering a better explanation of how the belief arose is a direct rebuttal (see the look-alike below).

The test: does the origin tell you how reliable the source of the claim is, and are you using it to set your confidence, or to skip checking the claim itself?

The trail sign

Two hikers disagree about how far it is to the lake. One says: “The park service measured this trail with GPS, and their sign says 3.2 miles. The guy at the trailhead was guessing. I’ll go with the sign.”

This picks one claim over another because of where each came from, but the origins differ in exactly the way that matters: a GPS survey is a measuring process with a good track record, and a passing guess isn’t. Nothing about liking or disliking the sources is involved. This is the known track record condition above.

The vending machine rumor

Student: Everybody at school says that if you press B4 twice, the vending machine drops two snacks. That many people wouldn’t believe it unless it worked.

Friend: The rumor started as an April Fools’ post on the student forum two years ago.

The friend is explaining where a belief came from, which looks like the car example. But the student’s argument was about origins: it claimed that the best explanation for the rumor is that it’s true. A better explanation of how the rumor spread answers that argument directly. It doesn’t prove the machine never drops two snacks (pressing B4 twice would settle that), and the friend doesn’t claim it does. This is the answering an argument about origins condition above.

Judging claims by their sources is usually a sensible shortcut. Nobody can check most of what they believe, and where a claim came from is often the best evidence available. The error is carrying that shortcut into cases where the claim can be checked directly, or where the source has no particular record for or against the truth.

An explanation of why someone believes something can also feel like a refutation. Once you can tell a story about how a belief arose that doesn’t mention its being true (they heard it on TV, they wanted it to be so), the belief seems debunked. But as Klement points out, a story like that shows at most that the belief wasn’t well supported, not that it’s false.

Feelings about a source also spill over onto its ideas. The same kind of spillover that makes one good trait color judgments of a person’s other traits, the Halo effect, makes ideas from admired sources seem stronger and ideas from disliked sources seem weaker.

• Separate the two questions. “How did this idea come about?” and “Is it true?” have different answers. Ask what evidence would settle the second.
• Ask what the origin shows about reliability. Is the source a process with a track record on this kind of question, or just one you happen to like or dislike?
• If the origin is poor, lower your confidence instead of reversing it. A claim from a doubtful source is unsupported until checked. That isn’t the same as false.