Association Fallacy

The association fallacy assumes or asserts that because a thing/argument/position shares some characteristics with another thing/argument/position, then it must share all or most of them. It also assumes or asserts the merit of a thing/argument/position based on the virtue of those who (at least allegedly) held it.

For example, "Dr. Leonard McCoy is a Starfleet medical doctor, and a man. Dr. Beverly Crusher is also a Starfleet medical officer; therefore, Dr. Crusher is also a man." (She is most certainly not.) Or, "I am allergic to lemons, which are yellow. Bananas are yellow; therefore, I am allergic to them." Or, "Paul is loud, obnoxious, and rude, and wears fedoras. Terry wears fedoras; therefore, Terry is loud, obnoxious, and rude."

Or, Brad tells his mother that he plans to switch to a vegetarian diet. Brad's mother, who thinks vegetarianism is a load of hogwash and hooey, says to Brad: "You do know that Hitler was vegetarian, do you?" - the implication being that vegetarianism is somehow intrinsically linked to Hitler's genocidal ideals, or that the fact that Hitler was vegetarian should have any bearing on whether Brad should eat a vegetarian diet or not. It's not, and the fact that Hitler preferred to eat this way has no bearing on whether one should choose a vegetarian diet. Hitler was also fond of dogs, and you'd probably find few people who'd argue against liking dogs on these grounds.

On the flip side of the coin, Suzette tells her friend Dennis that she plans to switch to a vegetarian diet, and tries to encourage Dennis do to the same. Dennis has reservations, so Suzette informs him that Anne Hathaway, one of his favorite actresses, is vegetarian. While this may be true, it doesn't have any relevance to whether Dennis should adopt a vegetarian diet or not. It isn't as if celebrities are always right, nor will becoming vegetarian magically make Dennis into an A-list actor, nor is it likely that Anne Hathaway will come and give him pats on the head for it.