Logical Fallacies (mistaken reasoning/unlikely inferences)


Claims which use words like all, never, most, and seldom not as figures of speech but as ways of avoiding assessing or questioning the likeliness of an event which is part of the claim. Ex. Hondas never get better gas mileage than Volkswagens.

Red Herring
Statements intended to divert attention from the issue at hand to another, usually less significant, issue. Ex.You know, even though we disagree on whether to fly or drive, don't you think the new SUVs are less likely to roll over than they once were?

Statements which emphasize one part of the evidence (usually the favorable part) and ignore other parts, averages, and patterns. Ex. In three of the last six years we have had record snowfalls in the Cascade Mountains. Global warming is not a problem here.

Post Hoc
The implication that because something came before it was the cause of what came after. Ex. If I had not stopped to talk to Nancy, I would not have accidentally unplugged the computer when I leaned back in my chair.

The implication that because something is where it is, it has the character of the things around it. Ex. I saw John down by the bus depot yesterday talking to that school dropout, video freak Freddy. I didn't think John was like that.

A claim that contains a character in its premise that it contains in its conclusion. Ex. Students do not listen to my explanations because the students are stupid. (Stupidity means knowing better but choosing not to--it means not listening.)

A claim that bases its merit not on a direct look at the evidence but on the popular opinion of the evidence. Ex. If most of the news journalists are saying the same thing, then we had better accept their conclusions.

Experts and False Authority
A claim that bases its merit not on evidence or principle, but on the fame, notoriety, or political position of a person. This most difficult bias is only diminished by introducing people to formal experimental procedures and controlled sampling techniques, and , even then, facts and logic will not persuade many who have already found value in the falsehood. Ex. Republicans will not let the Bureau of the Census use controlled sampling techniques to get better estimates of the U.S. population because the currently undercounted poor are more likely to be Democrats than the overcounted rich.

Non sequitor
An argument where the conclusion does not follow from the premises. Ex. All girls have blue eyes; Jill is a girl; therefore, Jill is going to wear a blue dress to the ball.

Ad hominem
Much like slanting and association, a claim that the theory is flawed and the evidence is tainted because the experimenter or observer is flawed or tainted. Ex. Who are you going to believe that drunk Harry or the Police Officer.

Slippery Slope
The Tragedy of the Commons is a bit of a slippery slope. Is it true that if we give an inch they will take a mile? If we don't act now, will all hell break loose. Does Greenspan face all sorts of slippery slopes when comparing the current use of interest rates (to control inflation and soften recession) with other economic periods?


Student Contributions

Todd Browne 10/21/98

Rob, I got another example that I learned in my Economics class that

you could use for your list of logical fallacies(an error in reasoning

that usually leads to a incorrect conclusion). My example is a "fallacy

of composition" which may fall under your Bandwagon category, but here

it is anyway. A fallacy of compositon involves reasoning that what is

true of the part(or the individual) is necessarily true of the whole (or

the group). Ex--If I were to stand up at a baseball game, then I'll see

better. Therefore, if everyone were to stand up, then everyone would be

able to see better.