Volume II: Logical Reasoning · Chapter 17
The job: Fix It or Break It. Name the reasoning error the argument commits.
Frequency: roughly 4 per section. Difficulty: medium, with a hard tail.
A Flaw question hands you a broken argument and asks you to put your finger on exactly what is broken. You are not asked whether the conclusion is true, or whether the premises are true. You are asked: granting the premises, why does the conclusion fail to follow? Your answer names the defect in the move from one to the other.
This is the same skill you trained for Necessary Assumption in Chapter 11, viewed from the other side. Every argument with a gap can be described two ways. As an assumption: the author takes for granted that the gap is closed. As a flaw: the author fails to consider that the gap might be open. The two are sides of one coin. If the assumption is “the survey respondents are representative of the city,” the flaw is “treats an unrepresentative sample as if it spoke for the whole city.” When you can find the assumption, you can name the flaw, and vice versa.
Hold tight to one rule above all others. You evaluate the link from premise to conclusion, never the truth of a premise on its own. An answer that says the argument “relies on a false premise” or “depends on an inaccurate figure” is almost always wrong, because on the LSAT you accept the premises as given. The error is never that a premise is false. It is that the author used true-as-given premises to reach a conclusion they do not support. In everyday terms, when a friend says, “My phone lasted all day on a single charge yesterday, so it has the best battery of any phone on the market,” you do not dispute that it lasted all day; you grant that and aim straight at the leap, because one good day cannot establish the best battery of any phone. Attack the move, not the premise.
Flaw is the second most common Logical Reasoning type, just behind Assumption, so the payoff for getting good at it is high. The underlying gap-finding skill is the one you have built since Chapter 2. The work here is learning to pre-name the error, then match your plain-English name to the abstract language the test hides it behind.
The stem tells you a flaw exists and asks you to identify it. Look for:
Two look-alikes. Method of Reasoning (Chapter 18) asks you to describe the structure neutrally, with no claim that anything is wrong. Flaw is a Method question where the stem has told you in advance that the structure is defective, so you go in hunting for the error. Necessary Assumption (Chapter 11) asks for the unstated premise the argument needs; Flaw asks you to name the hole that premise would have filled. If you can do one, you can do the other. Note also that “most vulnerable” still means exactly one answer is a genuine flaw the argument committed. The hedge in the stem does not loosen the standard.
Fix your job. You are about to read a broken argument and name the break. New information is not your concern here; unlike Strengthen or Weaken, the right Flaw answer describes what is already on the page, it does not add facts.
Use the Therefore Test from Chapter 2. Nail down the author’s main opinion and the support offered for it. You cannot name a flaw until you know which claim is being defended and with what.
This is the heart of the method, and it is non-negotiable. Before you read a single answer choice, say in your own plain words what the author did wrong. Not in test language, in kitchen-table language: “She assumed that because two things happen together, one causes the other.” “He treated a survey of his own friends as if it spoke for everyone.” If you walk into the choices without a pre-named flaw, the abstract wording will push you around. With one in hand, you are matching, not searching, and matching is far faster and safer.
To pre-name the flaw, run it through two lenses (try both; you need not decide which applies):
Causation is the single most common flaw on the test, so make it your first reflex. Any time the conclusion claims A caused, produced, or explains B, or the premise-to-conclusion link quietly assumes it, run the three-question checklist from Chapter 5:
If any door is open and the author walked past it, that is your flaw. You do not need to name the specific alternative cause. You only need to recognize that the author leapt from a correlation or a sequence to a cause.
The LSAT rarely says “the author confused correlation with causation.” It says “infers a causal relationship from the mere fact that two phenomena are correlated.” Same flaw, formal dress. Find the choice that says the same thing in test language, even from a different angle. The right answer often describes your flaw from a viewpoint you did not anticipate: “fails to recognize that two methods can produce the same result” is just another way of saying “assumes different methods must produce different results.” Translate, do not pattern-match on surface words.
The full inventory of flaws, with each one’s plain name, abstract disguise, and a micro-example, lives in Chapter 6, the Flaw Catalog. Keep it at your elbow while you drill this chapter. This chapter teaches how to attack the question; Chapter 6 is your reference for what the flaws are.
Before you bubble, look back and point to where in the argument the flaw occurs. If you cannot point to it, it is not your answer. This single discipline kills the three big traps below.
These three account for most missed Flaw questions. Memorize them.
1. Strong presumption against “circular reasoning.” Answers that say the argument “assumes what it sets out to prove” or “presupposes the truth of its conclusion” are dangled constantly and are almost never right. True circular reasoning, where the conclusion merely restates a premise in new words, occurs in fewer than one in twenty flawed arguments. Only pick it when you can point to a premise and conclusion that genuinely say the same thing. If the argument moves from one idea to a different idea, even by a bad move, it is not circular.
2. Reject the inapplicable flaw. The wrong answer describes a real, recognizable flaw type accurately. It just is not the flaw this argument committed. “Confuses a necessary condition with a sufficient one” is a perfect description of a real error, but if the argument contains no conditional reasoning, it is a trap. Your defense is Step 6: point to where the flaw occurs, or eliminate.
3. Reject answers that describe legitimate reasoning as a flaw. Some choices describe a perfectly fine move dressed up to sound like a defect. “Rejects a claim because it has been inadequately supported” is not a flaw, that is ordinary good reasoning. “Concedes a point made by the opposing side” is legitimate. “Relies on a premise some would find controversial” is not a flaw, because you accept premises as given. Only pick a move you genuinely agree is an error.
The unifying rule behind all three: only pick a flaw you genuinely agree the argument committed and can point to. The abstract language is engineered to make you doubt your own plain reading. Trust the plain reading first.
Last year a regional hospital began offering a new outpatient physical-therapy program to patients recovering from knee surgery. Patients who enrolled in the program recovered full mobility, on average, three weeks faster than patients who did not enroll. The hospital’s director concluded that the new program is more effective than conventional at-home recovery, and she has recommended that every knee-surgery patient be required to enroll.
The reasoning in the director’s argument is most vulnerable to criticism on the grounds that it
(A) presumes, without justification, that a program effective for knee-surgery patients will be equally effective for patients recovering from other surgeries.
(B) fails to consider that patients who chose to enroll in the program may have differed beforehand from those who did not in ways that affect recovery speed.
(C) takes for granted that no recovery program could help patients regain mobility in less than the average time reported.
(D) draws a conclusion about the program’s effectiveness that simply restates the claim that the program works better than at-home recovery.
(E) rejects the option of conventional at-home recovery merely because that option has been inadequately studied.
Step 1, stem. “Most vulnerable to criticism.” This is a Flaw question. I am naming the break in the director’s reasoning.
Step 2, conclusion and premises. Conclusion: the new program is more effective than conventional at-home recovery (and so should be required). Premise: enrolled patients recovered three weeks faster, on average, than patients who did not enroll.
Step 3 and 4, pre-identify with the lenses and the checklist. This is a causal conclusion: the program caused faster recovery. Run the checklist. Reversed? No. Coincidence or bad data? Possible, but there is a sharper door. Third factor? Yes, the classic one. The patients chose whether to enroll. The two groups are self-selected, not randomly assigned, so the people who signed up may have been more motivated or better positioned to recover fast to begin with, and that pre-existing difference, not the program, could explain the three-week gap. My plain-English flaw: “the two groups may have already been different, so the program might not be what caused the faster recovery.” That is a “what else?” flaw and a third-factor flaw at once.
Step 5 and 6, match and confirm.
One keeper survives. The answer is (B). The causal checklist found the third-factor door (self-selection) before I read any choice, so (B) matched instantly and the three engineered traps, the inapplicable flaw in (A), the circular bait in (D), and the describes-a-non-flaw in (E), could not impersonate it.
Pre-name the flaw in plain words first, then pick. Answers follow.
1. A researcher reports: “In our nationwide study, regions where households own more books per capita have markedly higher student test scores. Clearly, having more books in the home improves children’s academic performance, so the government should distribute free books to every household.” The reasoning is flawed because it
(A) confuses a percentage with an absolute number of books.
(B) relies on a sample of regions too small to support a nationwide conclusion.
(C) overlooks the possibility that a factor such as household wealth or parental education could produce both more books and higher scores.
(D) takes for granted that what is true of some households is true of every household.
(E) infers that books cause high scores merely because a famous educator endorses reading.
2. A manager argues: “Our most productive employees all report that they take a short walk every afternoon. If we require every employee to take an afternoon walk, our whole workforce will become more productive.” The reasoning is most vulnerable to the objection that it
(A) confuses a condition necessary for productivity with one that is sufficient for it.
(B) overlooks the possibility that the walks are an effect of high productivity rather than a cause of it, for instance because productive employees finish early enough to take breaks.
(C) provides no evidence that any employee enjoys walking.
(D) rejects alternative productivity measures merely because they are expensive.
(E) treats the manager’s own opinion as though it were established fact.
3. An editorial states: “The proposed museum tax cannot be a good idea. After all, it was first suggested by the developer who owns the land next to the proposed museum site, and she obviously stands to profit if the museum is built.” The reasoning is flawed because it
(A) presumes that two alternatives are the only ones available.
(B) treats the fact that a proposal’s author may benefit from it as sufficient grounds to conclude the proposal is bad.
(C) draws a conclusion about the present from evidence about the past.
(D) mistakes the popularity of a proposal for evidence of its merit.
(E) relies on premises that contradict one another.
1. (C). Plain flaw: a causal conclusion from a correlation; the third-factor door is wide open. Wealth or education could drive both book ownership and scores. (C) names that third factor. (A) is numbers-versus-percentages, not present, inapplicable. (B) attacks sample size, but the study is nationwide and nothing suggests it is too small: a describes-a-non-flaw inventing a defect that is not asserted. (D) is part-to-whole composition, not present, inapplicable. (E) is appeal to inappropriate authority; no authority is cited, inapplicable and built to sound on-topic with “reading.”
2. (B). Plain flaw: causal conclusion from a correlation, with two open doors: reversed causation (productivity leaves time for walks) and a third factor. (B) states the reversal concretely. (A) is necessary-versus-sufficient confusion, not present, inapplicable, since there is no conditional claim that walking is required. (C) is a describes-a-non-flaw; the argument need not show employees enjoy walking. (D) is inapplicable; no alternative measures are rejected. (E) is a describes-a-non-flaw; asserting an opinion is what every arguer does.
3. (B). Plain flaw: the editorial rejects the proposal solely because its author would profit, the self-interest temptation made into the actual error. A motive does not show the proposal is bad, so concluding it “cannot be a good idea” on that basis is the flaw. (B) names it. (A) is false dichotomy, (C) is time-shift, (E) is internal inconsistency, all not present, inapplicable. (D) is appeal to popularity, not present and inverted (the proposal is unpopular if anything). Note that a motive is not sufficient to show a claim is wrong: this is the self-interest trap pattern, picked here only because the motive is the argument’s entire support.
Put it to work: drill Flaw questions in the practice platform, free during early access, with every answer choice explained.
