“Famous Flaws” is not terminology used in LSAT questions. Manhattan Prep uses the name for a specific ten-item framework in its logical-reasoning materials. This guide follows Manhattan Prep’s published Famous Flaws list and gives an example, typical answer-choice wording, and a spotting cue for each item.
The canonical 10 Famous Flaws
| Famous Flaw | Core mistake |
|---|---|
| Necessary vs. sufficient | Reverses or otherwise misuses a conditional relationship |
| Part vs. whole | Transfers a property between a group and its members |
| Correlation vs. causality | Treats an association as proof of causation |
| Unproven vs. untrue | Equates lack of proof with truth or falsity |
| Sampling | Generalizes from an unrepresentative sample |
| Ad hominem | Attacks a source instead of addressing the claim |
| Inappropriate authority | Relies on expertise that is irrelevant or unestablished |
| Equivocation | Uses one term in two different senses |
| Circular reasoning | Assumes or restates the conclusion |
| Internal contradiction | Uses claims that cannot all be true |
False dilemmas, appeals to popularity, percentage-versus-number errors, straw-man arguments, and intent-versus-result errors can also appear in LSAT reasoning. They are useful patterns, but they are not part of Manhattan Prep’s named ten-item list.
Two skills, not one
Flaw questions require you to recognize the gap between the premises and conclusion, then match that gap to abstract answer-choice language. The quoted formulations below are representative paraphrases, not fixed or verbatim LSAT wording.
When an argument does not fit a familiar label, describe the support gap in your own words before reading the choices.
The recurring flaw patterns
Confusing sufficient and necessary conditions
The argument treats a guarantee as a requirement, or a requirement as a guarantee.
Example: “Anyone who studied formal logic passed. Maria passed, so she studied formal logic.” Logic study may not be the only route to passing.
Typical wording (paraphrased): “Mistakes a sufficient condition for a necessary one.” Cue: Words such as if, only if, unless, or requires, followed by a reversed conditional.
Correlation mistaken for causation
Two things occur together, and the argument concludes that one caused the other.
Example: “Students who eat breakfast score higher, so breakfast improves test performance.” A stable routine could contribute to both.
Typical wording (paraphrased): “Infers causation from a correlation.” Cue: An observed association or sequence followed by causes, leads to, or is responsible for.
Attacking the source
A claim is rejected because of who made it rather than because of a defect in the claim or its support.
Example: “The senator supports stricter emissions rules, but she owns solar-company stock, so her argument is wrong.”
Typical wording (paraphrased): “Criticizes the proponent instead of the argument.” Cue: The conclusion rejects a claim while the premises discuss only the claimant.
Circular reasoning
The supposed support merely assumes or restates the conclusion.
Example: “This policy is unjust because it treats people unfairly.” The premise supplies no independent reason.
Typical wording (paraphrased): “Presupposes what it sets out to establish.” Cue: Rephrase the premise and conclusion; if they make the same claim, suspect circularity.
Unrepresentative sample
A conclusion about a population rests on a sample that may differ systematically from that population.
Example: “Most jazz-festival visitors value live music, so most city residents value it.” Festival visitors are self-selected.
Typical wording (paraphrased): “Generalizes from a sample unlikely to be representative.” Cue: A survey of one subgroup supports a conclusion about a broader group.
Part-to-whole and whole-to-part
A property is transferred without justification from members to a group or from a group to its members.
Example: “Every player is excellent, so the team must be excellent.” Strong individuals may work poorly together.
Typical wording (paraphrased): “Assumes what is true of each member is true of the group.” Cue: The argument moves between each member and the group itself.
Equivocation
A key term changes meaning during the argument.
Example: “Feathers are light. Anything light cannot be dark. Therefore, feathers cannot be dark.” Light first means low in weight and then bright in color.
Typical wording (paraphrased): “Uses a key term in two different senses.” Cue: Give the repeated word a definition at each appearance and check whether the definitions match.
Appeal to inappropriate authority
An endorsement is treated as evidence even though the cited person’s relevant expertise is absent or unestablished.
Example: “A celebrated physicist says the zoning law is unconstitutional, so it probably is.” Physics credentials do not establish constitutional-law expertise.
Typical wording (paraphrased): “Relies on an authority outside the relevant field.” Cue: The main support is someone’s status; ask whether that status bears on this subject.
Absence of evidence treated as evidence of absence
A claim is declared false because it has not been proved, or true because it has not been disproved.
Example: “No study has shown that the additive is harmful, so it is safe.”
Typical wording (paraphrased): “Treats a lack of evidence against a claim as proof of it.” Cue: No one has shown or there is no proof, followed by a definite conclusion.
Internal contradiction
The argument relies on claims that cannot all be true.
Example: “Every application was reviewed, but Jordan’s application was never reviewed. Therefore, the review process was reliable.” The two premises conflict.
Typical wording (paraphrased): “Relies on mutually inconsistent claims.” Cue: Test whether the stated premises can all be true at the same time.
The two-part test for answer choices
Run every tempting choice through two checks:
- Accuracy: Does the argument actually do what the choice describes? If a choice mentions a sample and the stimulus contains none, eliminate it.
- Relevance: Is that error the reason the premises fail to support the conclusion?
A correct choice must pass both checks.
A worked mini-question
Stimulus: Last year the city added protected bike lanes to Main Street. Accidents subsequently declined there. Therefore, the bike lanes made Main Street safer.
Question: The reasoning is most vulnerable because it:
- (A) assumes its conclusion
- (B) attacks the source of a claim
- (C) infers causation from a sequence without excluding alternatives
- (D) generalizes from Main Street to every city street
- (E) reverses a conditional relationship
Analysis: (A), (B), and (E) describe patterns absent from the stimulus. (D) is inaccurate because the conclusion concerns Main Street only. (C) identifies the support gap: traffic volume, other road changes, or ordinary variation might explain the decline. (C) is correct.
Brief comparison drills
Necessary versus sufficient: “Only registered students may enter the library. Devon is registered, so Devon may enter.” Registered is necessary under the rule, but the rule does not say registration guarantees entry. A separate restriction could apply.
Unproven versus false: “Investigators have not proved that Lee altered the file, so Lee did not alter it.” Failure to prove a claim does not establish its negation. Reverse the conclusion—“therefore Lee did alter it”—and the reasoning remains defective unless the absence of evidence is itself conclusive under the circumstances.
When two labels seem plausible, state the precise move: “The argument reversed the rule” or “The argument converted not proved into false.” That description is more dependable than the label alone.
FAQ
Is there an official list of LSAT flaws?
“Famous Flaws” refers here to Manhattan Prep’s published framework, not wording used in LSAT questions. Other prep systems may group or name recurring errors differently.
Do I need to memorize the formal names?
No. Flaw questions ask you to select a description, not supply a label. Learn what each pattern does and how paraphrased answer choices express it.
Is circular reasoning a trap answer?
It can be tempting whenever an argument feels weak. Choose it only when the premise assumes or restates the conclusion rather than merely giving inadequate evidence.
Do these flaws help with the GRE argument essay?
They help with general argument analysis, but the current GRE General Test contains one Analyze an Issue task, not the former Analyze an Argument task, according to ETS’s current Analytical Writing overview.
How should I drill flaw questions?
Predict the flaw before viewing the choices, then apply the accuracy-and-relevance test. Practice with PurrLearn’s LSAT Logical Reasoning flaw quiz, then isolate conditional errors in the necessary-versus-sufficient drill. Once flaw questions feel routine, extend to the types that share the same first step: assumption questions and the negation test, strengthen and weaken, and method of reasoning — all of them turn on the same gap, just asked differently or causal errors in the correlation-versus-causation drill.
Conclusion
The Famous Flaws are Manhattan Prep’s ten-pattern study framework, not an exhaustive taxonomy. Learn the precise reasoning move behind each name, translate abstract choices with the two-part test, and practice distinguishing easily confused patterns. Then use the interactive flaw quiz to name each error before looking at the answer choices.