Genetic fallacy

A genetic fallacy is a logical fallacy that occurs when a claim is accepted or rejected based on the irrelevant source of the evidence, rather than on the quality or applicability of the evidence. It is also a line of reasoning in which a perceived defect in the origin of a claim or thing is taken to be evidence that discredits the claim or thing itself. The fallacy is committed when an idea is either accepted or rejected because of its source, rather than its merit.

Form
Most common:

Less common (except on the Internet, where it is more common):

"Second-option bias" form:

Claim-maker is smart/intellectual:

This obviously ignores the possibility that X is abusing his intellect to lie.

Bayesian
The genetic fallacy is a logical fallacy but is frequently not a Bayesian one: some sources are so consistently full of it that pointing this out reasonably constitutes a refutation for all practical purposes (e.g., Scopie's law). If something is bullshit 99.9% of the time, it may not be worth investigating &mdash; especially when there are more trustworthy sources or pressing issues available.

Example: If Andrew Wakefield were to write a paper on anything, it can be easily dismissed.

Extreme distortion of reality
You just can’t argue with those who think Nazis or apartheiders were left-wing progressives. They have no interest in discussion or debate. If you try anyway, they will kick you out of your own spaces and denounce you as a Cultural Marxist.

This exception mostly only can apply to Holocaust deniers and Stalin apologists. To deny the Holocaust or Stalin's purges requires dismissing overwhelming proof in favor of personal ideology, such that everything a Holocaust denier or tankie would say is probably filtered through a completely distorted view of reality. Someone believing in multiple conspiracy theories or a unified conspiracy theory is also likely to have a completely distorted view of reality.

Being "pro-life", an anti-vaxxer, member of PETA or global warming denialist, or even someone on the alt-right who isn't a Holocaust denier, is not grounds for dismissing that person out of hand for having an extremely distorted view on reality. It’s important to avoid doing so anyway, because fundamentalists use precisely the same reasoning to dismiss any argument by those outside their religion, since they believe the truth of their religion is self-evident and obvious to everyone. Overuse of this exception can lead one into the fallacy of opposition.

Miscellaneous

 * "I was brought up to believe in a god, my parents told me a god exists, and my parents would never lie to me, so this god must exist."
 * "Sure, the media claims that Senator Wales was watching pornography, but we all know about the media's credibility, don't we?"