Argumentum ad nauseam

Also known as Argument from repetition or argumentum ad infinitum.

A claim is repeatedly made until everyone is tired of resisting the arguer(s), at which point the arguer(s) consider it proven true.

This fallacy is a more extreme variant of Proof by Assertion.

Examples

Of course you're a Communist, because we experts on the House Un-American Activities Committee have let it be known that you are. Just admit it.

Of course you're gay, because we rednecks in the area have let it be known that you are, behind your back. Just admit it. It doesn't matter if you're not attracted to the same sex!

You've heard it said a thousand times that god exists, so where's the confusion? Clearly he exists. Just accept it.

Of course JFK was killed by a lone gunman, we always hear it on NPR, ABC, CBS, NBC, CNN, and so on. Just admit it's true. Don't talk to me about laws of physics and evidence!

People have always said that 9/11 was done by Muslim extremists, you hear it everywhere from everyone, every year, so it must be true.

Weaknesses

If a person or group merely repeats a claim, that does not make it true. It doesn't matter how often or how widely they repeat it. It doesn't matter if they propagate the claim in respected sources or in back alleys. If they do not prove it is true, then it remains unproven.

You can always point out the extreme effort that is expended to repeat a claim, which may be proportional to the degree to which a claim appears to any common person to be nonsense. Think of the example of the hysterical backstabber who calls 25 people to spread some slander.

You can question the conduct of people who eagerly believe claims merely because they are repeated ad nauseum. Are they courageous in failing to question these claims, or sheepish? Are they leaders or do they prefer to be led? Do they have authoritarian personalities?

Problems

Propagandists, unlike everyday bullshitters, have resources to create and deploy campaigns in the mass media that ensure a lie is repeated frequently and widely. They may pay for false news stories to be written and broadcast (called VNRs on television) pretending to be legitimate news. They may be able to ensure that a lie makes it into textbooks and scholarly journals. Domination in this way does not make a lie any truer.

• Social Media Detectives: Is That Viral Video For Real?
• Study: 'Video News Releases' Common in Local TV

One can observe the effort by some to deliberately wear down people who hold specific views by engaging them in petty squabbles. This is related to Argumentum ad nauseum insofar as it is an attempt at domination by sapping disputers of energy to continue. On the Internet these people are often called trolls but in fact there have been public announcements by governments of the intention to hire such people to undermine political opponents.