LSAT Flaw Questions: How to Name the Error and Pick the Answer

Flaw in the Reasoning is the most common question type in LSAT Logical Reasoning, and it beats students in a signature way. You read the argument and can tell something is off. Then you read the five choices. Every one of them describes a real logical error in respectable-sounding language, and the thing you noticed is suddenly hard to find.

That gap between seeing the problem and naming it is the whole question type. Here is how to close it.

5 Jump to the practice questions Five Flaw questions, level 1 to 5, answered and explained right on this page. Practice now

What a Flaw question actually asks

Every Flaw question starts with a short argument that does not hold up. Then it asks what is wrong with it. The wording varies a little:

• Which one of the following most accurately describes a flaw in the argument's reasoning?
• The argument is most vulnerable to criticism on the grounds that it…
• The reasoning in the argument is flawed because the argument…
• Which one of the following most accurately expresses a questionable technique employed by the argument?

All four are the same question. Notice what they are not asking. Not whether you agree with the conclusion. Not what fact would hurt the argument; that is a Weaken question. Not what the author is assuming; that is an Assumption question. They ask you to describe the move the author made, and the right answer is the one description that fits this argument and no other.

Why they are harder than they look

On most Logical Reasoning types, the wrong answers turn obviously wrong once you understand the argument. Flaw questions are different:

On a Flaw question, all four wrong answers usually describe real logical errors, written in the same formal language as the right one.

A choice reading "it presumes the truth of the very conclusion it sets out to establish" is a fine description of circular reasoning. It is simply not what this argument did. So "is this a real flaw?" is a useless question, because the answer is almost always yes. The question that works is "is this this argument's flaw?"

This is also why memorizing fallacy lists backfires. Go to the choices hunting for a familiar label and you will find five of them. What separates the credited answer from the decoys is the description you formed yourself, before the choices had a chance to sound reasonable.

The method: four steps

This is the same analyze, predict, eliminate approach that runs all of Logical Reasoning, tuned for Flaw. In order:

1. Find the conclusion. The flaw always sits between the evidence and the conclusion, so first pin down where the argument lands.
2. Find the support. What evidence is actually on the table?
3. Say the gap in plain language. No jargon: "they've got evidence about X and a conclusion about Y." This is the step everyone skips and the one that decides the question.
4. Translate, then match. Turn your plain-language gap into the LSAT's wording and find the choice that says it.

Step 3 is the whole method. Steps 1 and 2 are careful reading, and step 4 is mechanical once you have a prediction in hand. Watch it run.

A worked example

Steps 1 and 2. The conclusion is the last sentence: Mara has overcome her fear. The support is a general rule (anyone who has overcome the fear feels calm) plus a fact about Mara (she feels calm).

Step 3, the gap. The rule says beating the fear gets you calm. It doesn't say calm only comes from beating the fear. Plenty of people are calm for other reasons. The rule is being run backwards. No jargon, and nothing missing. Say that much before you look at the choices and the question is already over.

Step 4, translate and match. Calm is a necessary consequence of having beaten the fear, and the argument uses it as though it were sufficient to prove the fear was beaten. Now the choices:

1. It presumes the truth of the very claim about Mara that the argument sets out to establish. Circularity. The conclusion about Mara is inferred from her calm, not assumed. Cut.
2. It relies on Mara's report of her own feelings, which may not correspond to the emotional state she is actually in. The tempting one. Cut, for a reason worth a paragraph of its own.
3. It treats feeling calm, a necessary result of overcoming the fear, as though it showed the fear was overcome. This is the prediction, nearly verbatim. Keep.
4. It draws a sweeping conclusion about nervous speakers in general from the single case of one individual. The conclusion is about Mara only, not about speakers generally. Cut.
5. It supports its conclusion by appealing to the authority of psychologists rather than to evidence about Mara. No authority is cited; the psychologist reasons from a stated rule. Cut.

The answer is C. The missed points on this question go to B, and it is worth seeing why.

The twelve flaw families

The LSAT reuses a small set of bad moves, and these twelve are the ones worth knowing cold. Use them to speed up the translation in step 4, not to replace step 3. Your own description still comes first.

Learn the first two cold before the rest. Correlation-to-causation and necessary-vs-sufficient account for a large share of the Flaw questions on any given test.

The four traps

Almost every missed Flaw question is one of these four. Recognizing the shape of a trap while you are in it is usually enough to get out.

1. A real flaw, but not this argument's

The default decoy: it describes a textbook error accurately, and the argument did not commit it. Reading the choice harder will not help, because there is nothing wrong with the choice. The only thing that beats it is a prediction it fails to match.

2. The mirror image

The same distinction as the credited answer, run in the wrong direction: necessary and sufficient trade places, or cause and effect. At speed the two look identical. When two choices are built from the same pair of terms, reread the stimulus sentence that contains them. That is where the question is decided.

3. A true observation that is not a flaw

The choice says something accurate about the argument without identifying anything wrong, or raises a doubt the argument survives, the way B did above. The grant-it-everything test handles both.

4. The conclusion swap

The choice describes a flaw that would be real if the author had concluded something slightly different. Choice D above is a mild version: it describes a conclusion about nervous speakers in general, when the psychologist concluded something only about Mara. Usually the swap is subtler. A choice might trade will for might, or the trend for this year's figure. This is why finding the conclusion is its own step: approximately right is not right.

Practice: five Flaw questions

Work these the way you would on the test: say the gap in your own words before you look at the choices. Levels 4 and 5 are genuinely hard. If you are new to the type, getting the first three is a good result.

Interactive practice set

Five Flaw questions, hardest last

One per difficulty level, each from a different flaw family. Pick an answer to see the full explanation. Keys A–E select; arrow keys move between questions.

This set is interactive with JavaScript enabled. Without it, all five questions are below with their answers behind a reveal.

Forge180 practice question · Logical Reasoning · Flaw · Level 1

Environmental columnist: The regions of the country that generate the largest share of their electricity from wind power also have the lowest rates of childhood asthma. This shows that generating electricity from wind power reduces the rate of childhood asthma.

Which one of the following most accurately describes a flaw in the argument's reasoning?

1. It draws a conclusion about an entire population from a sample too small to be representative of it.
2. It assumes that what is true of each region individually must also be true of the country as a whole.
3. It treats a condition that is sufficient for an outcome as though it were also necessary for that outcome.
4. It relies on the testimony of an authority whose expertise lies in an unrelated field.
5. It concludes that one thing causes another from the mere fact that the two are correlated.

Show the answer

E is correct. The columnist observes that regions high in wind power tend to have low childhood-asthma rates and concludes that wind power reduces asthma. That is a textbook correlation-to-causation error: a mere association between two things is treated as proof that one causes the other, when an unmentioned common factor could explain the pattern. E names this flaw directly. The other choices describe errors (small sample, misplaced authority, part-to-whole, sufficient-versus-necessary) that this argument simply does not commit.

A. The argument is not based on a small sample; it cites a pattern across regions. Sample size is never at issue, so this mischaracterizes the reasoning.

B. The argument makes no inference from individual regions to the whole country; it draws a causal conclusion. This describes a part-to-whole error the argument does not make.

C. The argument involves no claim about a condition being sufficient or necessary; it confuses correlation with causation, not sufficiency with necessity.

D. The columnist appeals to no authority at all, let alone one speaking outside its field. The argument rests on a correlation, not on anyone's testimony.

E. The only evidence is that high wind-power regions and low childhood-asthma rates go together. From this correlation the columnist leaps to the causal claim that wind power reduces asthma, ignoring that some other factor (such as those regions being less industrialized) could account for both. Inferring causation from correlation alone is exactly the flaw.

Forge180 practice question · Logical Reasoning · Flaw · Level 2

Columnist: The astronomer Dr. Vance claims that the proposed orbiting telescope would yield little valuable science for its enormous cost. But Dr. Vance has spent her whole career working on ground-based telescopes, which would compete with the new telescope for the same limited research funding. So her claim that the orbiting telescope is not worth its cost should simply be dismissed.

Which one of the following most accurately describes a flaw in the argument's reasoning?

1. It draws a conclusion about astronomers in general from facts about one particular astronomer.
2. It assumes that whatever holds true for ground-based telescopes must also hold true for orbiting telescopes.
3. It treats the mere absence of proof that the telescope is worthwhile as establishing that the telescope is not worthwhile at all.
4. It dismisses a claim by pointing to the interests of the person who made it rather than addressing the claim's truth.
5. It relies on the testimony of a person whose expertise on the matter at hand has not been established.

Show the answer

D is correct. The columnist rejects Dr. Vance's claim not by showing it false but by noting that she has a career interest in the funding that the orbiting telescope would draw away. That is an attack on the person's circumstances rather than on the claim itself, the ad hominem flaw described in D. C misreads the argument as an appeal to ignorance about the telescope's worth, and E, A, and B each name errors (dubious authority, hasty generalization, false analogy) the argument does not commit.

A. The columnist draws no general conclusion about astronomers as a group; the reasoning is entirely about Dr. Vance and her one claim.

B. The argument makes no inference from properties of ground-based telescopes to properties of orbiting ones; the mention of ground-based work is only used to impute a motive.

C. The mildly tempting choice. It is on-topic because it mentions the telescope's worth, but the columnist never argues from a lack of proof that the telescope is worthwhile. The conclusion is only that Dr. Vance's claim should be dismissed, not that the telescope has been shown to be worthwhile or worthless.

D. The columnist gives no reason to think Dr. Vance's claim is false. Instead the claim is set aside solely because Dr. Vance has a professional stake in ground-based telescopes. Attacking the arguer's interests rather than the argument is exactly this flaw.

E. The argument does not rest on the word of an unqualified authority; if anything it works against an astronomer's testimony. No appeal to questionable expertise occurs.

Forge180 practice question · Logical Reasoning · Flaw · Level 3

Ferry-line spokesperson: Residents along the strait complain that the wake thrown by our new high-speed vessels is eating away at their shoreline. In the four years our vessels have run, not one study has shown that our wake causes any erosion whatever. The residents are therefore mistaken: our vessels are not eroding the shoreline.

Which one of the following most accurately describes a flaw in the argument's reasoning?

1. It assumes that the shoreline along the strait has not in fact eroded over the past four years.
2. It concludes that a claim is false merely because no study has yet shown it to be true.
3. It rejects the residents' claim on the ground that the residents want the ferry service curtailed.
4. It treats a condition that would be enough to produce erosion as though it were also required for erosion.
5. It overlooks the possibility that four years is too short a span for erosion of this kind to register.

Show the answer

B is correct. Not having found something is not the same as having found that it is absent. The spokesperson moves from the fact that no study has established the wake as a cause to the conclusion that the wake is not a cause. That move would be legitimate only if a study capable of detecting the effect had been run and had come back negative, and the stimulus reports nothing of the kind. Silence in the literature is treated as testimony against the claim.

A. The spokesperson denies that the vessels are causing the erosion, not that erosion is occurring. The argument is compatible with severe erosion from some other source.

B. The only evidence offered is the absence of a study establishing the causal claim. From that absence the spokesperson infers the claim's falsity. Absence of a demonstration is not a demonstration of absence, particularly where no one is said to have looked.

C. The residents' motives never come up. The spokesperson responds to the substance of their complaint, however badly.

D. No sufficient or necessary condition for erosion is stated anywhere in the argument. The reasoning turns on what the studies did and did not find.

E. This is the near-miss, and it points at something real. If erosion of this kind takes decades to register, the four-year silence is exactly what one would expect whether or not the wake is at fault. But this names one particular reason the evidence is uninformative, and the argument would be flawed even if four years were ample. The defect is the inference from no evidence to falsity, which this choice does not state.

Forge180 practice question · Logical Reasoning · Flaw · Level 4

Tech columnist: A program is said to be "open" when its source code is published for anyone to inspect. Some critics charge that our city's new messaging app is not open, because the company that makes it tightly restricts which outside developers may release add-ons for it. This charge does not hold up. The company has in fact posted the app's complete source code online for all to see. So the app is open after all, and the critics are simply mistaken.

Which one of the following most accurately describes a flaw in the argument's reasoning?

1. It treats the public availability of a program's source code, which is enough to make a program open, as though that availability were also required for a program to be open.
2. It draws a conclusion about messaging apps in general from considerations concerning only one particular app.
3. It rejects the critics' objection only by establishing that the app is open in a sense of "open" other than the one on which the objection actually rests.
4. It presupposes, among its premises, the very claim that the app is open that the argument purports to establish.
5. It dismisses the critics' objection by calling into question the motives of those who raise it rather than its content.

Show the answer

C is correct. The word 'open' carries two different meanings here: open in the sense of having published source code, and open in the sense of letting outside developers freely release add-ons. The critics' charge uses the second sense; the columnist's reply establishes only the first. Because the reply changes which sense of 'open' is in play, it does not actually engage the critics' objection, even though it looks like a direct refutation.

A. Attractive, because the columnist opens with a definition of 'open' and then reasons from the source code being public. But the columnist's inference (source code published, therefore open) uses that condition only as sufficient, which is legitimate, and the conclusion never claims the condition is required. So this misdescribes the move: the defect is the shift in the meaning of 'open,' not a confusion of sufficient with necessary.

B. The conclusion is about this one messaging app, not about messaging apps generally, so the argument commits no hasty generalization across apps.

C. The critics call the app not 'open' in the sense of being open to outside developers, who are restricted from freely releasing add-ons. The columnist refutes them by showing the app is 'open' in a different sense, that its source code is published. Establishing openness in the columnist's sense does nothing to rebut a charge made in the developer-access sense, so the rebuttal works only by quietly switching which sense of 'open' is at issue.

D. The argument does not assume the app is open among its premises; its premise is the independent fact that the company posted the source code online. The conclusion that the app is open is drawn from that fact, so the reasoning is not circular.

E. The columnist never attacks the critics' motives or character; the critics' objection is answered (however faultily) on its content, by pointing to the published source code. This names an ad hominem move the argument does not make.

Forge180 practice question · Logical Reasoning · Flaw · Level 5

Historian: No society has ever maintained a large city without an extensive network of roads bringing in food from the surrounding countryside. Excavations at Cathal have now revealed just such a network: several wide, paved avenues running far out into the farmland around the site. So Cathal must once have maintained a large city.

Which one of the following most accurately describes a flaw in the argument's reasoning?

1. It treats the fact that a condition is required for a situation as showing that the condition's presence guarantees the situation.
2. It presumes the truth of the very conclusion that it sets out to establish.
3. It treats the fact that a condition would be enough to produce a situation as showing that the situation could not arise without that condition.
4. It treats the fact that two things are found together as showing that one of them produced the other.
5. It bases a broad claim about all societies on what happens to have been found at a single site.

Show the answer

A is correct. The premise 'no large city without an extensive road network' makes the network necessary for a large city (city implies network). Finding the network and concluding a city existed affirms the consequent: it treats a necessary condition as though it were sufficient. A states this. C is the mirror-image error (treating a sufficient condition as necessary), which the argument does not commit, though its 'could not arise without' wording mimics the stimulus. B (circularity), E (overgeneralizing from one site), and D (correlation to causation) each name a flaw the argument does not make.

A. The first sentence says a large city cannot exist without an extensive road network, so the network is a necessary condition for the city (city implies network). The historian finds the network at Cathal and concludes there was a city, which treats that necessary condition as if its presence were enough to guarantee a city. A road network could exist for other reasons with no large city, so the inference is invalid. This is exactly the described error.

B. The conclusion (a large city existed at Cathal) is nowhere assumed among the premises; the premises are the general rule and the finding of the roads. The reasoning is invalid but not circular.

C. The intended trap. It reverses the direction of A. C describes taking a SUFFICIENT condition ('would be enough to produce') and wrongly making it necessary ('could not arise without'). Its 'could not arise without' phrasing echoes the stimulus's 'without,' which makes it tempting. But the network is presented as necessary, not sufficient, and the conclusion asserts that a city existed, not that a city required the network. So C misdescribes both the premise's logical role and the conclusion.

D. There is no appeal to two things merely co-occurring; the argument runs on a stated necessary-condition rule, not on a correlation, so no correlation-to-causation error is committed.

E. The argument does not derive a broad claim about all societies from the Cathal site; the universal claim is a stated premise, and the conclusion is only about Cathal. This mischaracterizes what is inferred from what.

What to do with the ones you missed

"I got it wrong" is not a diagnosis. For each miss, find which step broke:

• Wrong conclusion. Everything downstream was doomed. Fix: underline the conclusion before reading the stem.
• No prediction. You understood the argument but went to the choices with nothing to compare them against. The most common failure, and the most fixable.
• Abandoned prediction. You had the gap right, then talked yourself out of it. Usually the credited answer was there in unfamiliar wording. Reread the choices with your prediction in front of you.
• Mirror-image prediction. Right distinction, wrong direction. That is a concept gap, not a technique gap, and it will keep costing you until you close it.
• A real flaw, but not this one. Ask what phrase in the stimulus made it seem present. Noticing the suggestion without accepting it is most of the improvement on this type.

Keep a tally of which one it was. After thirty or forty questions the pattern tells you what to practice in a way your accuracy percentage never will. Review the ones you got right but felt shaky on too: a lucky guess and a confident hit look identical in your stats.

Common questions

How common are Flaw questions on the LSAT?

It is the most common single type in Logical Reasoning, typically appearing four to six times in a 25-question section. Across the two scored LR sections, that is roughly eight to twelve points of your score. Weaken, Necessary Assumption and Parallel Flaw lean on the same skill, so the real share is higher.

What is the difference between a Flaw question and a Weaken question?

The analysis is identical; the answers do different jobs. A Flaw answer describes the gap: "it treats a correlation as though it established causation." A Weaken answer exploits it with a new fact: "the regions with the most wind power also have the least heavy industry." Same first three steps, different fourth one.

Do I need to memorize the fallacy names?

No. The exam never asks you to name a fallacy, and hunting for a memorized label is exactly how the decoys catch people. Learn the twelve families as shapes of bad reasoning, so your plain-language description comes faster. That is all they are for.

What if I cannot name the flaw at all?

Describe it instead. "The evidence is that no study found it and the conclusion is that it isn't happening" is not a fallacy name, and it is enough to eliminate four choices. Failing that, fall back on the conclusion: check each choice against it and cut the ones describing an argument that concluded something else.

Are Flaw questions worth prioritizing?

Yes, and earlier than feels natural. They are the most frequent type, the skill transfers straight to Weaken, Assumption and Parallel Flaw, and seeing what an argument leans on is what the whole section tests. Improvement here shows up in your score faster than improvement on any narrower type.