Common Logical Flaws & Fallacies

1. What You Need to Know

Logical flaws are predictable ways arguments go wrong: the evidence doesn’t justify the conclusion because the reasoning makes an illicit jump, ignores an alternative, misuses a principle, or shifts meaning.

On the LSAT, flaws show up most directly in Flaw, Parallel Flaw, and Method of Reasoning questions—but also as wrong answers (or correct weaken/strengthen answers) in Strengthen/Weaken, Assumption, and Evaluate.

The core rule

An argument is flawed when:

The premises could be true and yet the conclusion could still be false because the reasoning doesn’t bridge the gap.

What you’re actually doing on test day

You’re not writing an essay about “fallacies.” You’re doing two things fast:

Describe the gap between evidence and conclusion in plain language.
Match that description to the choice that says the same thing (often in abstract, “LSAT-speak”).

Critical reminder: Many correct flaw answers are abstract (e.g., “confuses a necessary condition with a sufficient one”). Your job is to translate the stimulus into that abstraction.

2. Step-by-Step Breakdown

Use this anytime you suspect the reasoning is doing something “too quick” (especially in Flaw / Parallel Flaw).

The 6-step flaw-finding routine

Find the conclusion (look for “therefore,” “thus,” “so,” “hence,” “clearly”).
List the support (premises, data, facts).
Ask: What would have to be true for this to work? (the missing link).
Identify the type of jump (causal, conditional, comparison, sampling, etc.).
Prephrase the flaw in simple words.
Match, don’t “prove”: pick the choice that best captures your prephrase, even if it uses different wording.

Mini worked walkthrough (annotated)

Stimulus: “Every time the city increases parking fines, downtown sales drop the next month. So the fine increases cause the drop in sales.”

Conclusion: “fine increases cause drop in sales.”
Evidence: correlation over time.
Missing link: no other cause; direction is correct.
Flaw type: correlation $\rightarrow$ causation (plus possible post hoc timing assumption).
Prephrase: “Assumes because B follows A, A caused B; ignores other explanations.”

Decision points (fast diagnostics)

If you see cause/effect language: check for alternative causes, reverse causation, mere correlation, selection bias.
If you see conditional words (“if,” “only if,” “unless,” “requires”): check for necessary/sufficient confusion, affirming the consequent, denying the antecedent.
If you see numbers/percentages/surveys: check representativeness, sample size, base rates, changing denominators.
If you see comparisons/analogies: check whether the things compared are similar in relevant respects.
If you see recommendations/plans: check feasibility, unintended consequences, tradeoffs, baseline.

3. Key Formulas, Rules & Facts

A. Conditional/logic-related flaws (high yield)

Flaw (label)	What it looks like	Why it’s flawed / what’s missing
Necessary vs. sufficient confusion	Treats “requires/only if” as if it’s “guarantees/if” (or vice versa)	Having a necessary condition doesn’t ensure the result; having a sufficient condition isn’t required
Affirming the consequent	If $A \rightarrow B$ . $B$ . Therefore $A$ .	$B$ could happen for other reasons
Denying the antecedent	If $A \rightarrow B$ . Not $A$ . Therefore not $B$ .	$B$ might still happen without $A$
Mistaken negation / mistaken reversal	Flips or negates the wrong part of a conditional	Changes meaning; creates unsupported inference
Quantifier shift	“Some” $\rightarrow$ “most/all,” or “not all” $\rightarrow$ “none,” etc.	Illicit move between quantities

Conditional language triggers: only if (necessary), if (sufficient), unless (often introduces a necessary condition), without, requires, depends on.

B. Causation flaws (most tested family)

Flaw	Classic pattern	Fast fixes you’d look for in Strengthen/Weaken
Correlation $\rightarrow$ causation	Two things vary together, so one causes the other	Rule out third cause, establish mechanism, show temporal order
Post hoc (after therefore because)	B happened after A, so A caused B	Show B could be coincidence; alternative causes
Reverse causation	Assumes direction wrong	Show effect could cause the “cause”
Causal oversimplification	Single cause claimed where multiple factors likely	Show additional causes; multifactor explanation
Confounding / omitted variable	Ignores variable influencing both	Identify variable explaining both
Causal overgeneralization	Cause in one setting $\rightarrow$ cause everywhere	Show context differences

C. Sampling / statistics / representation flaws

Flaw	What it sounds like	What to watch for
Unrepresentative sample	“We surveyed our app users…” $\rightarrow$ “people in general…”	Selection bias, self-selection, coverage bias
Small sample	Very few observations $\rightarrow$ broad conclusion	Randomness / volatility
Percentage vs. number shift	“Percent increased” $\rightarrow$ “more people”	Denominator changed; total size changed
Base rate neglect	Focuses on a vivid stat, ignores overall rates	Need background prevalence
Survivorship bias	Looks only at “successes” still visible	Missing failed cases

D. Argument-structure & reasoning flaws

Flaw	Definition in LSAT terms	Common clue words
Circular reasoning (begging the question)	Conclusion is assumed in a premise	Restatement, “obviously,” “clearly” without new support
Ad hominem	Attacks person, not claim	“He’s dishonest, so his argument is wrong”
Straw man	Misrepresents opponent’s view, then attacks it	“They say we should… (extreme version)”
Equivocation (shift in meaning)	Same word/phrase used in two senses	“theory,” “right,” “natural,” “free,” “power”
Composition	Parts have trait $\rightarrow$ whole has trait	“Each player is a star, so the team is unbeatable”
Division	Whole has trait $\rightarrow$ parts have trait	“Company is profitable, so each division is profitable”
False dilemma	Treats few options as the only options	“Either we do X or disaster”
Appeal to authority (weak authority)	Cites non-expert/irrelevant expert	Check expertise relevance, consensus
Appeal to popularity	Many believe/do it $\rightarrow$ true/best	Popularity isn’t truth
Appeal to ignorance	Not disproven $\rightarrow$ true (or vice versa)	Lack of evidence isn’t evidence
Loaded/leading question	Presupposes controversial point	“Have you stopped…?”
Relative vs. absolute confusion	“Less than before” $\rightarrow$ “low,” etc.	Baseline matters

E. Analogy & comparison flaws

Flaw	Pattern	What makes it wrong
Weak analogy	A and B share some traits $\rightarrow$ share key trait	Similarities not relevant; key differences ignored
False comparison	Compares groups with different baselines	Different contexts invalidate inference

4. Examples & Applications

Example 1: Necessary vs. sufficient (classic)

Stimulus: “To be admitted, an applicant must submit two recommendations. Jamie submitted two recommendations, so Jamie will be admitted.”

Gap: Two recommendations are necessary, not sufficient.
Correct flaw description: “Treats a condition required for admission as though it guarantees admission.”

Example 2: Conditional fallacy (affirming the consequent)

Stimulus: “If a painting is by Kline, it will have heavy black lines. This painting has heavy black lines, so it must be by Kline.”

Form: If $A \rightarrow B$ . $B$ . Therefore $A$ .
Gap: Many painters could use heavy black lines.

Example 3: Sampling / survey trap

Stimulus: “A poll of visitors to a luxury gym found that most support raising city property taxes. Therefore, most city residents support raising property taxes.”

Gap: Luxury gym visitors aren’t representative of city residents.
Correct flaw: “Generalizes from an unrepresentative sample.”

Example 4: Weakening a causal claim (typical LSAT move)

Claim: “After the school banned sugary drinks, nurse visits declined. So the ban caused better student health.”

Best weaken types:
- Alternative cause: “At the same time, the school hired a full-time counselor who reduced stress-related visits.”
- Reverse causation: less likely here, but could be “nurse visit reporting policy changed.”
- Measurement change: “Nurse visits were logged differently after the ban.”

5. Common Mistakes & Traps

Mistake: Treating a ‘flaw’ question like it’s asking for the assumption.
- What goes wrong: You hunt for a statement that would “fix” the argument.
- Why wrong: Flaw answers describe what’s wrong; they usually don’t patch it.
- Fix: Prephrase in the form “The argument fails to… / assumes that…” not “It would be true if…”
Mistake: Missing the conclusion, so you misdiagnose the flaw.
- What goes wrong: You attack a premise or take background as the point.
- Fix: Force yourself to identify the conclusion first; everything else is support or context.
Mistake: Falling for extreme answer choices that exaggerate the flaw.
- What goes wrong: You pick “proves,” “guarantees,” “completely rules out,” when the stimulus didn’t.
- Fix: Match strength: if the argument is mildly flawed, the right answer is usually moderate and precisely worded.
Mistake: Confusing correlation $\rightarrow$ causation with “circular reasoning.”
- What goes wrong: You label any bad argument as circular.
- Fix: Circular = conclusion basically restated as evidence. Correlation/causation = evidence is a relationship or timing, not a restatement.
Mistake: Not distinguishing ‘some’ vs ‘most’ vs ‘all’.
- What goes wrong: You accept a shift from “some” to “many/most” as if it’s harmless.
- Fix: Quantifiers are math-like on LSAT: “some” does not license “most.”
Mistake: Overfocusing on real-world truth.
- What goes wrong: “But that’s true in real life!”
- Why wrong: LSAT cares whether the premises support the conclusion, not whether the conclusion happens to be true.
- Fix: Ask “Could premises be true and conclusion false?”
Mistake: In Parallel Flaw, matching topic instead of structure.
- What goes wrong: You pick the option about similar subject matter.
- Fix: Abstract the core move (conditional error? causal leap? sampling?) and match that pattern.
Mistake: Missing an equivocation because the word shift is subtle.
- What goes wrong: You treat “natural,” “theory,” “free,” “right,” “efficient” as stable.
- Fix: When a key term appears in both premise and conclusion, ask: “Same meaning both times?”

6. Memory Aids & Quick Tricks

Trick / mnemonic	Helps you remember	When to use
C-A-R: Correlation $\rightarrow$ Alternative cause / Reverse causation	The 2 biggest causal gaps	Any correlation or before/after evidence
N $\neq$ S: Necessary $\neq$ Sufficient	Don’t treat “required” as “enough”	Any “must/only if/requires” language
If $A \rightarrow B$ , then $B$ doesn’t imply $A$	Affirming the consequent	Conditional arguments
Parts $\neq$ Whole	Composition/division	Group vs individual claims
Two meanings? Circle the word.	Equivocation	Same term in premise + conclusion
Percent? Ask ‘Out of what?’	Denominator shifts	Any percent/ratio statistic
Either/or? Look for ‘both/other.’	False dilemma	Any “only two options” framing

Quick translation: “Takes for granted” = assumes. “Fails to consider” = ignores an alternative. “Presumes without justification” = unsupported leap.

7. Quick Review Checklist

Identify conclusion and support before labeling any flaw.
If you see causation, ask: alternative cause? reverse direction? mere correlation?
If you see conditional language, check: necessary vs sufficient and the two classic invalid forms.
Track quantifiers: some/most/all; not all vs none.
For surveys and studies: representative sample? sample size? denominator? base rates?
For analogies: are similarities relevant, and are key differences ignored?
Watch for word shifts (equivocation) in repeated terms.
In answer choices, match the abstract description to your prephrase; don’t demand identical wording.

You’ve got this: if you can name the gap quickly, the right answer usually becomes the only one that “talks like” your prephrase.