Bad Evidence – Fallacies and Poor Appeals

Overview: One of the best ways to understand how to find good evidence is to recognize bad evidence. Similarly, if you’re searching for delicious morel mushrooms in the woods, it’s good to know what the poisonous toadstools look like so you don’t make a pasta with them and kill your whole family. Perhaps this is a bad analogy, but I don’t think so.

1. Citing False Evidence – This isn’t really a fallacy, but it’s the most common way we misuse evidence, and it’s worth pointing out. Some people just make up evidence to suit their needs. They are dirty liars.

Example: I’m the best frisbee player in the world. Last year, The New York Times dubbed me, “Frisbee Thrower of the Millenium.” Don’t bother looking it up. They don’t have a searchable archive.

2. Appeal to Questionable Authority – Citing a person as an expert who is not actually an expert or citing a weak source. This happens most often when we appeal to someone who is generally smart and maybe even an expert in one field, but nevertheless lacks the extra-special expertise of a true authority on the subject.

Example 1: My dad told me that Santa exists, and he’s the best dentist in Idaho! How dare you question him!

Example 2: I read that the only way to keep ghosts away is by sending money to It says so right on the website!

3. Appeal to Popularity – Citing popular opinion as evidence of truth. This assumes that a widely held belief must be true or that something everybody wants must be desirable. Belief doesn’t make a claim true; truth makes it true.

Example 1: Every culture tells ghosts stories. Therefore, ghosts must exist.

Example 2: 50,000 Elvis fans believe that he’s still alive! That many people can’t be wrong!

Example 3: Fried butter is the best-selling food at the fair! It must be really tasty, and maybe even good for you!

4. Appeal to Abstraction – An attempt to pass a claim by associating it with popular (but vague) values. For example, freedom to me may be the ability to vote and publish controversial ideas. Freedom to my brother might be the right to pet any and every dog he darn well wants, even if it’s inside a stranger’s house. A mayor who promises to fight for freedom may get both of our votes, but for very different reasons. (Also called Glittering Generality.)

Example: Vote for me! I’m all about America and stuff! All the stuff you guys like! That’s me!

Example 2: The right to pet all dogs is really important. It’s about liberty, freedom, and the equality of dogs everywhere. Don’t you like equality? (See Equivocation and Obfuscation)

5. Bad Analogy – Comparing one situation to another to draw a conclusion, but the situations do not share enough common elements to support your conclusion.

Example: Grapes are bad for dogs, so they must be bad for people too. Get those grapes away from me!

Example 2: Everyone loved it when I made fart sounds in first grade. Everyone laughed. I’m sure it will work during Thanksgiving dinner! This is going to be great!

Example 3: The American Revolution profoundly changed American society. Therefore, it’s likely that the war in Afghanistan will do the same thing.

6. Confirmation Bias – Only siting evidence that supports your claim, while ignoring evidence that supports rival claims.

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