(personal attack, poisoning the well, Guilt By Association, Damning the Source, a type of genetic fallacy)
Latin for “to the man,” this fallacy occurs when, in an attempt to undermine or dismiss your opponent’s argument, you attack their personal character instead the validity of the evidence. This is a fallacy because the validity of an argument has nothing to do with the character of the person presenting it. Ad Hominem fallacies don’t address the actual points of the argument; they just dismiss them out of hand. This can be done in overtly (by a direct attack on them) or more subtly (casting doubt on someone’s credibility), and are very common in political campaigns. These attacks are often used to discredit an argument without actually having to debate it and are often used when there is little that can be said to counter the argument itself.
The truth is that bad people often make valid claims, and good people often make invalid claims. Just because you like the person isn’t a good enough reason to agree with them on everything, and just because you dislike them doesn’t mean you should dismiss their arguments entirely.
It is important to note, however, that not all criticisms of character are Ad Hominem attacks. It largely depends on the evidence presented. It’s one thing to disagree with someone because you don’t like them as a person; it’s another to disagree with them because their position is based on faulty evidence, and their propensity to push faulty claims leads you to dislike them.
Examples:
“He’s so evil that you can’t believe anything he says.”
“After Sally presents an eloquent and compelling case for a more equitable taxation system, Sam asks the audience whether we should believe anything from a woman who isn’t married, was once arrested, and smells a bit weird.”
“Saddam can’t have WMD’s because George Bush said he does, and he’s a liar.”
“Saddam must have WMD’s because the UN can’t find them.”
“Who cares if the French oppose invading Iraq? They haven’t won a war in centuries.”
“There is no reason to listen to the arguments of those who oppose school prayer, for they are the arguments of atheists!”
“The American Trial Lawyers Association favors of this piece of legislation, so you know it has to be bad for ordinary citizens.”
Principal Smith: I think your child should be held back a grade. He is younger than his classmates, and the material seems to be a bit too advanced for him.
Parent: Screw you, you fascist jerk!
Pete: Have you read the new book by Amanda Hugginkiss?
Rick: Are you kidding? She’s dumber than a box of wet hammers.
“I can’t believe you’re going to read that book about Jesus; it was written by a Muslim!”
“I know that the administrator said that filling out all these forms was crucial to obtaining financial support for the university. But these forms take a lot of time. Of course an administrator wants there to be more paperwork. Without more forms to process, they’d all be out of work.”
“You can’t trust doctors; if no one was sick, they’d make no money. They want to keep us all sick!”
“Senator Kaholic had argued persuasively in favor of federal subsidies for dairy products. But the Senator comes from Wisconsin, which has a huge dairy industry. So of course she would argue for that position. Therefore, it seems reasonable to vote the subsidies down.”