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A Few Common Logical Fallacies That You Just Gotta Know


Flaw questions are one of the most common types of Logical Reasoning questions on the LSAT. That’s reason enough to study them thoroughly. Furthermore, the concepts that underlie flaw questions show up in myriad other questions in the LSAT.

What’s a Logical Fallacy?

Before we begin, the term “logical fallacy” sounds intimidating, but fallacies are nothing more than reasons an argument’s premises are inadequate to prove its conclusion. In general, when assessing an argument’s validity, you assume the premises are true. If you can explain why the conclusion doesn’t have to be true, even though the premises are true, congratulations, you’ve found the fallacy.

To make finding these flaws easier, it’s helpful to have a working knowledge of some of the most prevalent logical fallacies on the exam. First up, we have …

The Exclusivity Fallacy

A fallacy of exclusivity comes in two flavors. The first involves treating a list of options as if those options were the only options, when there might indeed be other options. The second involves treating the options as if they can’t be combined when maybe they can.

As an example of the first version, suppose an unnamed government official needs to travel from Washington to Philadelphia. He reasons, “I could take the train, but that would involve too much mingling with common folk for a high-ranking official like me. So I’ll have to charter a jet.” This ignores other options — say, if you have to charter something, just charter a town car.

For the second, imagine staring at a buffet. With so many delicious foods, how will you ever decide which one to eat? The fallacy here is that you, of course, can get a little bit of lots of different things. You don’t have to pick just one.

A hint that you might be dealing with an exclusivity fallacy is when the conclusion zeroes in on one option as the sole option. The word “only” comes up a lot. When you see that, ask whether there might be other options or whether the options might be combined.

The Causal Fallacy

A causal fallacy comes up whenever an argument attempts to conclude that there’s a causal relationship without enough evidence. Typically the jump from premises to conclusion is from correlation to causation. Let’s say a certain town has a nuclear power plant and has higher-than-average rates of cancer. The town’s residents blame the power plant for their elevated cancer rates.

“Blame” is a causal word, so the conclusion is that the nuclear power plant causes the high rate of cancer. Maybe it does. But the evidence given isn’t enough to prove it. There could be alternative causes. For example, maybe the town’s residents all work in an asbestos processing plant. Or maybe they have a really high rate of smoking.

Pretty much any time an argument’s conclusion posits a causal relationship, be on the lookout for a causal flaw. Cause and effect is very hard to prove. Pretty much the only valid way to prove a causal conclusion on the LSAT is with causal premises. So if there’s a causal conclusion without causal premises you have a causal flaw.

The Equivocation Fallacy

Equivocation fallacies also come in two versions. The first involves the ambiguous use of a word. If you claim to have taken the LSAT but really all you did was steal a book of prep tests from a bookstore, you’re guilty of equivocation. The verb “to take” has two meanings and you used one of them while acting like you meant the other.

The second kind of equivocation fallacy involves a shift between concepts. Say you choose an airline for its reputation for safety. Your flight is miserably uncomfortable and the service is awful. You think you were misled. However, while service and comfort matter, they’re not the same thing as safety. That you were uncomfortable doesn’t mean that you were unsafe.

The first kind of equivocation fallacy comes up occasionally in arguments on the LSAT, but it comes up far more often as a wrong answer. If you’re going to pick an answer such as “Inappropriately employs language that is vague” or “Illicitly allows a key term to shift in meaning,” you’d better be able to point to the key word that’s used with two different meanings. If you can’t, don’t pick that answer choice.

The second is far more common. If you feel like the conclusion isn’t quite talking about the same thing as the premises, or uses different words that are similar but not quite the same in meaning, there might be some equivocation going on.

The Comparison Fallacy

One more fallacy for today involves bad or incomplete comparisons. Arguments that commit this fallacy typically establish as a premise that two things are similar in some ways. The argument then concludes that these things are similar in some other respect.

Suppose I said that a Corvette has a V-8 engine and is a fast car that can take turns at a high rate of speed. Well, a heavy-duty pickup also has a V-8. So it, too, must be fast and able to take turns at a high rate of speed.

Um, no. The argument takes one similarity, the engine, as evidence of other similarities, and ignores relevant differences between the things being compared. Whenever an argument is premised on a similarity between two cases, look for a bad comparison.

There are more flaws to know, of course. There are many more fallacies that you’ll see often on the LSAT and should know well. But these four are common and a good start. If you’re good at spotting logical fallacies, it’ll help you in so many places on the LSAT. So get familiar with them.