Certain logical fallacies are common on the LSAT. The more comfortable you are with them, the easier it will be for you to recognize and manipulate flawed arguments on the LSAT test. That means more points on test day.
This article is the second in a series of lessons that cover common logical fallacies.
The series includes the following articles:
Common LSAT Flaws
The following logical flaws are among the most common you will find on the LSAT test. Remember, it’s not important to remember the name of the flaw. You won’t be tested on fallacy names. But it is important to recognize the style of flaw so that you can use it to your advantage on test day.
Over-generalization
The author argues that because certain instances are true, all instances must be true.
The students at Yale are a bunch of thieves. I was on a ski trip with some Yale students and my wallet was stolen.
Red Flag: Just because a part of a group behaves in a certain way, doesn’t mean the whole group does. If one person on a ski trip is a thief, it doesn’t mean that every student at Yale is.
Bad Sample
The author makes an argument based on a small or unrepresentative sample.
What do you mean your wedding cake was bad? I gave you samples to try before you even ordered it?
Red Flag: Think about how the sample is different from what is actually in question. Can’t a cake be bad even if another cake sample months before was good?
Fallacy of Composition
The author argues since the parts have a certain characteristic, the whole must also share that characteristic.
This new car features the finest engine, axle, electrical system, and chassis. It must be the best car on the market.
Red Flag: Ask yourself if the parts gathered together are necessarily the same as they were separately. Just because a car uses great parts, couldn’t it be put together all wrong?
Fallacy of Division
The author argues that since the whole has a certain characteristic, the parts must share that characteristic.
Beth must be an excellent chef. She graduated from Le Cordon Bleu.
Red Flag: Ask if there can be an exception in a part that doesn’t exist in the whole. While Le Cordon Bleu has a reputation for graduating fine chefs, are every single one of their chefs “excellent.”
Fallacy of Negative Proof
The author argues that because something has never been proven false, it must be true. Or, if something cannot be proven true, it must be false. This is a type of false consequences argument, since it assumes that all things must be proven true or false.
No “rational explanation” has ever been offered to explain away the results of Voodoo. Many eyewitnesses recount first hand experience with trances, zombies, and curses. The effects of Voodoo must be real.
Red Flag: Ask yourself if the opposite of the conclusion could be true. Could a rational explanation for Voodoo exist, even though we’ve never discovered it?
Fallacy of False Consequences
The author gives you a limited number of choices, even though many other choices exist. A false choice usually uses the word “or.”
Vote for Smith or pay high taxes.
Red Flag: Think of other options that are within the scope of the argument. Do the other candidates actually support high taxes? That seems unlikely.
Complex Question
The author combines two or more unrelated points into one statement, trying to force you to accept or reject them all at once.
Did you stop beating your wife?
Red Flag: Identify the separate points made in the argument: first, do you beat your wife, and second, did you stop?
Slippery Slope (Chain Reaction)
The author presents a long chain of improbable events, making huge leaps in logic at each step. This argument often uses “if/then” statements.
We cannot legalize marijuana. Marijuana is known as a “gateway” drug, which means marijuana users will eventually use harder drugs like heroin or crack cocaine. If all of our youth start smoking crack cocaine, our entire society will crumble.
Red Flag: Ask yourself if the consequences of each statement would necessarily follow. Would all marijuana users turn to crack cocaine? Even if they did, would “all of our youth” fall down that slippery slope? Even then, if an entire generation smoked crack cocaine, would the entire society fail?
Coincidence (post hoc ergo prompter hoc)
The author argues that since X preceded Y chronologically, it must have caused Y.
Television causes obesity. Americans are twice as likely to be fat today as they were in 1945.
Red Flag: Just because two things happen in succession doesn’t mean they have a causal relationship. How do we know that television causes obesity, when more people eat fast food instead of healthy meals, and fewer people walk to work?
Shared Cause
The author argues that X causes Y, when in fact, something else causes both X and Y.
Your fever is causing you to break out in a rash.
Red Flag: Could there be a shared cause of both? Could an unmentioned illness be the cause of both the fever and the rash?
Insignificant Cause
The author points out a real, but small, cause that is insignificant compared to a larger cause.
Using hairspray is causing a hole in the ozone layer.
Red Flag: Does one small cause seem reasonable enough? Sure, certain types of hairspray contribute to greenhouse gasses, but compared to car exhaust it seems insignificant.
Multiple Causation
The author argues for a single cause, when in fact there were many contributing causes.
His heart attack was caused by eating too much red meat.
Red Flag: This is similar to the insignificant cause. Is this one cause enough? Red meat contributed to his heart attack, but so did a sedentary lifestyle and a high-pressure job.
Reversed Causation
The author confuses the cause for the effect, and vice-versa.
I drank more because I was losing my job.
Red Flag: Think about the reverse. Isn’t it more likely that the author lost her job because she drank?
In the next article, we’ll take a look at some of the more sneaky ways the LSAT corrupts reasoning with Secret LSAT Logic Flaws.