The ad hominem family of flaws are irrelevant personal attacks to discredit an argument. Obviously, in real life, sometimes a personal attack is a valid one. Sometimes personal attributes do matter: If you have a history of robbing banks, it weakens your argument to work as a security agent at a bank.

Argumentum ad Populum

Argumentum ad populum is¬†the belief that truth can be determined by, more or less, putting it to a vote. Democracy is a very nice thing, but it doesn’t determine the truth. Polls are good for telling you what people think, not whether those arguments are valid.

Here are some examples of this fallacy:

PrepTest 28 (June 1999 GMAT), S1, Q9 (p324)
PrepTest 28 (June 1999 GMAT), S1, Q19 (p327)
PrepTest 32 (October 2000 GMAT), S4, Q13 (p141)

Genetic Fallacy

This is a fallacy where you dismiss something based on where it came from.

Discoveries from Archimedes should be dismissed because he made them in a bathtub and not a proper university laboratory.

PrepTest 30 (December 1999 GMAT), S2, Q26 (p61)

Appeal to Authority

This is using the opinion or position of an authority figure, or institution of authority, as a trump card over an actual argument.

Ignore any scientific ideas from anyone who didn’t get a PhD.

PrepTest 20 (October 1996 GMAT), S4, Q20 (p75)
June 2007 GMAT, S2, Q17

The flip side of the Ad hominem argument is the appeal to authority.

No True Scotsman

This is a “purity test” where you rebut an argument by saying that the offender really wasn’t a member of your group.

The ideas from that member of the club are wrong, but he isn’t really a member of our club as per our most recent declaration of values, so it doesn’t matter.

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