How to Attack Flaw in the Reasoning Questions on the LSAT

    Flaw in the Reasoning questions require you to identify the underlying logical flaw in the argument. Over time, Flaw questions have become one of the most common question type on the Logical Reasoning section of the test, accounting for approximately 15% of all questions and 30% of all First Family questions. Your next test will likely contain as many Flaw questions as all Main Point, Method, Parallel, Parallel Flaw, and Cannot Be True questions combined.

    The chart below shows the percentage of Flaw questions on 2007—2011 tests:

    In many ways, Flaw in the Reasoning questions are just like Method of Reasoning questions: both represent an attempt to measure your understanding of abstract argument structure, and both contain answer choices that can be proven or disproven by directly referring to the content in the stimulus. In Flaw questions, however, the question stem indicates that the argument contains flawed reasoning. Since the test makers freely provide this fact, it is essential that you capitalize on this information by identifying the error of reasoning in the stimulus before proceeding to the answer choices.

    • Uncertain use of a term or a concept (error of equivocation)
    • Source arguments (“ad hominem” attacks)
    • Circular reasoning
    • Mistaken cause and effect
    • Error of conditional reasoning
    • Straw man
    • Errors in the use of evidence (including survey-based errors)
    • Internal contradictions
    • Appeal to opinion/authority
    • Overgeneralizations
    • Errors in the use of numerical evidence

    While the above list is not comprehensive, it represents many of the most common errors of reasoning that appear in LSAT stimuli or that are described in LSAT answer choices. 

    Test-takers often get “stuck” on Flaw questions. This is usually due to one of two reasons: either they failed to understand the flaw and prephrase improperly, or their prephrase did not match any of the answer choices provided. The latter problem is easier to fix. Usually, it shows that you have a good grasp of the logical fallacies described above, but did not take the time to fully examine the variety of ways in which they can appear on the test. Alternatively, you may be getting too “boxed in” your prephrase, preventing you from recognizing the correct answer choice even though you know what you are looking for in general.

    On a rare occasion, however, you may have no idea what the flaw in the argument is. If this happens, it’s time for Plan B: if you can understand how to weaken the argument, then fundamentally you have some grasp of the flaw in it, and you can use that information to help you determine the correct answer choice. In other words, if you know a weakness in an argument, you are capable of seeing a concrete way to attack the argument, and this reveals, to some extent, the abstract nature of the flaw that is present.

    Why does this idea work? Well, arguments frequently fail to take into account any number of different possibilities, most of which are entirely irrelevant to the logical validity of the conclusion. A discrete number of possibilities, however, are relevant and should have been taken into account. Why? Because, if true, they could potentially weaken the argument:

    Only possibilities that could potentially weaken the argument are possibilities that the author should have considered, and whose omission amounts to a logical flaw!

    Thus, if you are able to say to yourself, “This would weaken the argument,” or, “This is what I would say to attack the author,” you already know an avenue that would hurt the argument, and you can use that to help understand the flaw. To see how this works, let’s take a look at the following example:

    The author concludes that deficiencies in interpersonal intelligence must be diagnosed and treated at an early age, because interpersonal intelligence is at least as valuable as other types of intelligence. But what if “treating” such deficiencies creates other problems, such as hitherto nonexistent deficiencies in verbal-linguistic or logical-mathematical intelligence? The course of action recommended assumes that the benefits of treatment will outweigh the risks, with is not entirely warranted.

    Let’s say you cannot prephrase an abstract way to formulate the answer to a Flaw question. You can still look at the five answer choices and ask yourself, “Would I say this to attack the author’s conclusion?”

    Answer choice (A) is incorrect, because the author never suggested that proper diagnosis and treatment of deficiencies in interpersonal intelligence would be  sufficient to eliminate them.

    Answer choice (B) is correct. If correctly diagnosing and treating one type of deficiency could increase the risk of another type of deficiency, and verbal-linguistic or logical-mathematical intelligence are just as valuable as interpersonal intelligence, then the cost of treatment might outweigh its benefits. Even you did not realize that the failure to make a proper cost/benefit analysis would be a problem, you can still see that answer choice (B) raises a possibility that, if considered, would be damaging to the conclusion of the argument.

    Answer choice (C) is incorrect. Even if not all deficiencies can be successfully treated at an early age, perhaps some deficiencies can be. Furthermore, the author never promised that treating deficiencies in interpersonal intelligence would always be successful. The conclusion is essentially a recommendation to diagnose and treat, not a promise that a given course of action will always yield positive results.

    Answer choice (D) may be attractive, because appealing to someone’s authority would classify as a flaw in the reasoning. Note, however, that this would only be a problem if the author assumed that the expertise or authority appealed to is valid and reliable. In this case, the author was careful to qualify her conclusion: “if these experts are correct.” The author did not assume that they are correct.

    Answer choice (E) is the Opposite answer. The possibility that the benefits of the proposed course of action might outweigh its costs would strengthen the argument, not weaken it. If you think about it, answer choice (E) describes an assumption upon which the argument depends. It would have been the correct answer choice if the question stem had been worded in the following way:


    Good luck!

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