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The ad hominem fallacy attacks the person making an argument rather than the argument itself: 'You can't trust his climate data — he drives an SUV.' The genetic fallacy dismisses a claim because of its origin rather than its merits: 'That idea came from a pharmaceutical company, so it must be biased.' Both fallacies redirect attention from argument content to irrelevant features of the source. Note, however, that source credibility is sometimes legitimately relevant — not every appeal to character is fallacious.
Distinguish the fallacious from the legitimate: attacking a witness's credibility in court is not ad hominem when their honesty is genuinely at issue. Practice by generating cases where source attacks are relevant vs. irrelevant.
From your study of informal fallacies you know that fallacies are errors in reasoning — moves that appear to support a conclusion but don't actually provide good reasons for it. The ad hominem and genetic fallacy are both failures of relevance: they redirect attention from the argument to something that doesn't bear on whether the conclusion is true. The core move in an ad hominem is "don't believe P because of something about the person asserting P." The core move in the genetic fallacy is "don't believe P because of where P came from." In both cases, the question "but is P actually true?" gets sidestepped.
The ad hominem comes in several varieties. The abusive form attacks character directly: "You can't trust her analysis of immigration policy — she's a radical." The circumstantial form claims someone's interests or affiliations bias their view: "He only says vaccines are safe because he works for a pharmaceutical company." The tu quoque ("you too") form dismisses a claim by pointing out that the speaker violates it: "You say I shouldn't smoke, but you smoke too." Each redirects from the argument's content to something about the source. But notice what all three share: even if the personal attack is entirely accurate, it still doesn't establish that the conclusion is wrong. A hypocrite can be correct. A person with a financial interest can still report accurate data.
The genetic fallacy applies the same misdirection to origins rather than persons. "That theory was developed by Nazis" is a genetic fallacy if used to dismiss the theory's truth (though it might be relevant to our ethical relationship with the idea's proponents). "Your belief in God came from your upbringing, so it's just cultural conditioning" commits the genetic fallacy if it's meant to show the belief is false. Origins explain how you came to hold a belief; they don't determine whether the belief is true. A belief arrived at by bad means can still be correct; a belief arrived at by rigorous means can still be wrong.
The crucial nuance — one that separates careful reasoners from rote fallacy-spotters — is that source is sometimes legitimately relevant. In a court of law, impeaching a witness's credibility is not an ad hominem if their honesty or perceptual reliability is genuinely at issue. If someone is testifying about what they saw, and you show they were not in the room, that's relevant. If a scientist is publishing data, and you show they falsified previous data in similar studies, that is a legitimate reason to scrutinize their claims more carefully. The fallacy arises when source attacks substitute for engagement with the argument — when "this person has bad character" is treated as a complete rebuttal rather than one consideration among others. The test: ask whether the personal or origin fact you're citing actually bears on the truth of the claim or the quality of the evidence. If yes, it may be legitimate. If it's simply a distraction from engaging with the argument, it's fallacious.
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