The Most Common Logical Fallacies Found on the LSAT
The LSAT is very much a test of critical reasoning. It wants to know if you just accept what you’re told, or if you’re able to look at facts given and still argue against conclusions.
In order to be able to argue against a conclusion, however, you first need to know what’s wrong with the argument. Since the LSAT doesn’t care if the premises are true (we’re expected to just accept them as such), we instead have to attack the assumptions.
It might seem like every argument on the LSAT is flawed in its own special way. However, there are some broad categories that flaws fall into, and noticing these underlying flaws can help you spot the error before heading to the answer choices.
The following list isn’t comprehensive (and there is some overlap between the categories!), but it’s a solid starting point.
Percent vs. Amount
Description: The argument jumps from facts about percentages to conclusions about amounts, or vice versa.
Why this is a flaw: If the percent of your income that you spend on rent just went down, what might have happened? Maybe your rent went down. In that case, the decreased percent also reflects a decreased amount. But maybe you got a promotion and make twice as much money. In this case, the percentage of your salary going to rent will go down, but the total amount you pay for rent will stay the same.
Think about this if the argument: Has premises/conclusions about percents, rates, or frequencies.
Unproven vs. Untrue
Description: An argument concludes that an opinion is false because there isn’t enough evidence to prove it.
Why this is a flaw: Scientists have failed to prove Bigfoot doesn’t exist. This doesn’t mean that he does exist. Conspiracy theorists have failed to prove that Bigfoot does exist. This doesn’t mean that he doesn’t exist.
Think about this if the argument: Presents as its premise the failure of someone to prove something, a lack of evidence for something being the case, or a flaw in a study that reached a particular conclusion. “The study that reached conclusion X was flawed, therefore conclusion X is false” is the most common manifestation of this flaw.
Ad Hominem
Description: Instead of addressing someone else’s argument, the author insults them or attempts to breed distrust in them, attacking the person rather than the argument the person made.
Why this is a flaw: “The study concluded that smoking was safe, but the study was undertaken by Marlboro. Clearly the results of the study are false.” If this were real, would we trust the results of this study? Absolutely not. But does this accusation address the actual substance of the study? Not at all. An Ad Hominem argument might give you a good reason to doubt the speaker, but it doesn’t actually address the substance of the speaker’s argument.
Think about this if the argument: Accuses someone of hypocrisy or of having a stated interest in the outcome of their argument.
Causation Flaw
Description: The argument concludes that because two things are associated, or happen in sequence, that one of them must have caused the other.
Why this is a flaw: Ice cream sales and drowning deaths are strongly correlated. This doesn’t mean eating ice cream makes swimming less safe—in this instance, it just happens to be summer (which causes both the increase in ice cream sales and drowning deaths).
Think about this if the argument: Has a conclusion that brings up a causal relationship. This is among the most common flaws on the test, and it’s particularly prevalent in Strengthen and Weaken questions.
Circular Reasoning
Description: The argument’s conclusion is functionally the same as its premise. Or, less frequently, in order for the premise to be true you must assume the conclusion to be true as well.
Why this is a flaw: It’s meaningless to form an argument where your premise is the same as your conclusion. “I’m the best because I’m the best” doesn’t really make for compelling reasoning.
Think about this if the argument: Has two sentences that say literally the same thing.
Comparison Flaw
Description: The argument compares two things without considering all relevant factors.
Why this is a flaw: Just because a Honda Civic is more comfortable than a Lamborghini doesn’t mean it’s a better car.
Think about this if the argument: Compares two things or uses an analogy.
Conditional Logic Flaw
Description: The argument treats a conditional statement like it can be reserved or negated.
Why this is a flaw: All puppies are cute. Does that mean all things that are cute are puppies? Nope. Does that mean that if something isn’t a puppy, it isn’t cute? No again.
Think about this if the argument: Includes any conditional statements.
Equivocation
Description: The argument switches between two definitions of the same word.
Why this is a flaw: I like my steak rare. But I don’t care for aardvark steak, even though it’s the rarest steak in the world.
Think about this if the argument: Uses the same word multiple times but in different contexts.
False Choice
Description: The argument ignores other possible solutions/explanations; or it ignores the middle ground between two opposites.
Why this is a flaw: If I can’t go to the dance with Maria, that doesn’t guarantee I’m going to the dance with Megan. Maybe I’m going to go with Marsha. And if the dinner I cook for my date doesn’t come out hot, that doesn’t mean it came out cold. Maybe it was Goldilocks-style just right. Or maybe it was…tepid.
Think about this if the argument: Makes a recommendation in the conclusion; or talks about extreme ends of a spectrum.
Opinion vs. Fact
Description: An argument treats people’s opinions as established fact.
Why this is a flaw: We are NOT LIVING IN A POST-TRUTH WORLD!
Think about this if the argument: Brings up what people think/believe/opine.
Part vs. Whole
Description: The argument ascribes characteristics of a group to each member of that group, or states that because each part of something has a characteristic, the whole must have that characteristic.
Why this is a flaw: Just because each Lego brick in a model is lightweight doesn’t mean a scale model of the Empire State Building made of Lego bricks is lightweight. And just because America touches the Atlantic and Pacific oceans doesn’t mean that Ohio touches the Atlantic and Pacific oceans.
Think about this if the argument: Talks about groups and individuals.
Possible vs. Certain
Description: The argument establishes that something is possible or probable, and then concludes that something is definitely going to happen.
Why this is a flaw: Just because I might be able to make it to the party, doesn’t mean I’ll definitely be there.
Think about this if the argument: The premise of the argument deals with a possibility.
Relative vs. Absolute
Description: The argument jumps between comparative terms and absolute ones.
Why this is a flaw: Just because Shaq is shorter than Dikembe Mutombo, doesn’t mean he’s short.
Think about this if the argument: Has a relative premise and an absolute conclusion, or vice versa.
Straw Man
Description: A speaker argues against an argument the other side didn’t quite make.
Why this is a flaw: It’s easy to win an argument when you incorrectly rephrase the other position to make it more absurd. Cigarette companies did this for a while—science argued that smoking caused cancer, and they trotted out people who smoked for decades without health issues because they reframed the argument as “Smoking always causes cancer.”
Think about this if the argument: Has a speaker that tells you what the other position’s argument is.
Temporal Flaw
Description: The argument assumes what is true in the past will continue to be true, or past odds influence future chances.
Why this is a flaw: For the former, it used to be true that most people believed the Earth was flat, but it’s not true anymore (for now). For the latter, a coin flip coming up heads 20 times in a row doesn’t change the odds of the next flip (hint: they’re still 50-50).
Think about this if the argument: Talks about trends or different time periods.
Unrepresentative Sample
Description: A survey/study reaches a conclusion about a group based on data from a sample that doesn’t really represent the group.
Why this is a flaw: If you asked a bunch of frat guys how much they drank in a week, that wouldn’t accurately predict how much the average person drinks in a week.
Think about this if the argument: Has a conclusion that generalizes about a group, or the argument itself includes a survey/study/poll. ?
Matt Shinners is a Manhattan Prep instructor based in New York City. After receiving a science degree from Boston College, Matt scored a 180 on his LSAT and enrolled in Harvard Law School. There’s nothing that makes him happier than seeing his students receive the scores they want to get into the schools of their choice. Check out Matt’s upcoming LSAT courses here!