Common Logical Flaws & Fallacies

What You Need to Know

Logical Flaw questions (and many Strengthen/Weaken/NA/Sufficient Assumption questions) hinge on spotting how an argument goes wrong. On the LSAT, most wrong arguments fail in a small set of repeatable ways: bad conditional reasoning, bad causal reasoning, bad sampling, shifting terms/quantifiers, or confusing what was proven.

Your core job: separate premises (support) from conclusion (claim), then name the gap: what must be true for the premises to justify the conclusion?

The “flaw vocabulary” you must recognize

• Necessary vs. sufficient confusion (especially with conditionals)
• Correlation vs. causation (and its cousins: reverse causation, confounders)
• Quantifier shifts (some/most/all; “not all” vs “none”)
• Scope shifts / new term introduced (conclusion is about something broader/different)
• Representativeness & sampling (small, biased, self-selected)
• Attacking the source instead of the claim (ad hominem)
• Misdescribing an opponent’s view (straw man)
• Bad comparisons/analogies (ignoring relevant differences)

Critical reminder: a “flaw” answer choice can be worded abstractly. If you can translate it into what happened in the stimulus, it’s a contender.

Step-by-Step Breakdown

Use this for Flaw, Weaken, Strengthen, Necessary Assumption, and Parallel Flaw.

1. Find the conclusion (the author’s main point).

   • Look for: therefore, thus, hence, so, consequently, clearly.
   • Or ask: “What is the author trying to prove?”

2. List the premises (the support).

   • Evidence, facts, studies, statistics, comparisons, expert statements.

3. Put the reasoning in a simple form.

   • Conditional? Causal? Statistical? Analogy? Plan/proposal? Elimination?

4. Articulate the gap in one sentence.

   • “This assumes ____.”
   • “This confuses ____ with ____.”
   • “This rules out ____ without justification.”

5. Pre-phrase the flaw (predict it before reading answers).

   • Even a rough prediction helps you avoid trap answers that sound logical.

6. Match your pre-phrase to an answer choice.

   • Correct answers often use standard phrasing like “takes for granted,” “presumes,” “fails to consider,” “confuses,” “infers… solely from,” etc.

Micro-worked example (conditional)

Stimulus: “If a product is unsafe, it will be recalled. This product was recalled. Therefore it is unsafe.”

• Form:
  $\text{Unsafe} \rightarrow \text{Recalled}$
  $\text{Recalled}$
  $\therefore \text{Unsafe}$
• Flaw: Affirming the consequent (treating a necessary condition as sufficient).

Key Formulas, Rules & Facts

A. Conditional reasoning fallacies (super high-yield)

Valid moves

• Modus Ponens:
  $A \rightarrow B$
  $A$
  $\therefore B$
• Modus Tollens:
  $A \rightarrow B$
  $\neg B$
  $\therefore \neg A$

Invalid moves (classic flaws)

• Affirming the consequent:
  $A \rightarrow B$
  $B$
  $\therefore A$
• Denying the antecedent:
  $A \rightarrow B$
  $\neg A$
  $\therefore \neg B$

B. Fallacy map (what it is, LSAT tells, how to attack)

Examples & Applications

Example 1: Causation (classic weaken targets)

Stimulus: “In neighborhoods where more parks were built, crime fell. Therefore, building parks reduces crime.”

• Type: Correlation → causation.
• Key insight: Need to consider:
  • Confounder: city also increased policing or economic investment.
  • Reverse causation: city built parks because crime was already falling.
  • Selection: parks built in neighborhoods already trending safer.
• Best weaken answer shapes: introduces alternative cause, shows timing mismatch, or shows crime fell equally where no parks were built.

Example 2: Necessary vs. sufficient (common flaw answer wording)

Stimulus: “To get licensed, a contractor must pass an exam. Rivera passed the exam, so Rivera is licensed.”

• Form: $\text{Licensed} \rightarrow \text{Passed}$ (passing is necessary, not sufficient)
• Flaw: Treats a necessary condition as sufficient.
• What would fix: add that passing the exam is enough, or that Rivera completed all other licensing requirements.

Example 3: Quantifier shift + sampling

Stimulus: “A survey of 50 customers found that some prefer Brand A to Brand B. Therefore, most customers prefer Brand A.”

• Flaws:
  • Some → most (quantifier shift)
  • Small/possibly unrepresentative sample
• Trap answers: choices that only say “survey size is small” might be incomplete if the bigger problem is the quantifier leap (depends on wording).

Example 4: Equivocation

Stimulus: “A ‘theory’ is just a guess. Evolution is a theory. So evolution is just a guess.”

• Flaw: “Theory” shifts meaning (scientific theory vs casual guess).
• Correct flaw answer often says: uses a key term in an inconsistent or ambiguous way.

Common Mistakes & Traps

1. Boldly labeling a fallacy without matching the stimulus.

   • What goes wrong: You see “correlation” and scream “causation flaw,” but the argument only claims “this is evidence of,” not “this proves.”
   • Fix: Match the strength of the conclusion: “suggests” vs “establishes.”

2. Mixing up conditional indicators (“only if” vs “if”).

   • What goes wrong: You reverse the arrow.
   • Rule: “$A$ only if $B$” means $A \rightarrow B$ (B is necessary).
   • Fix: Translate slowly, then look for mistaken reversal.

3. Choosing an answer that’s true but not the flaw.

   • What goes wrong: You pick a criticism of the evidence that doesn’t explain why the conclusion doesn’t follow.
   • Fix: Ask: “If I fixed this issue, would the reasoning become valid?” If no, it’s not the core flaw.

4. Falling for “criticizes the author” trap language.

   • What goes wrong: You pick harsh-sounding answers (e.g., “ignores all evidence”) when the stimulus does something milder.
   • Fix: Calibrate: LSAT flaw answers must be accurate, not just negative.

5. Confusing “attacks source” with “attacks argument.”

   • What goes wrong: You call it ad hominem whenever someone mentions bias.
   • Fix: It’s ad hominem only if the argument goes: “Source is bad → claim is false,” without addressing the claim’s support.

6. Missing the “new term” in the conclusion (scope shift).

   • What goes wrong: Premises discuss “reducing emissions,” conclusion recommends “banning cars.”
   • Fix: Circle new concepts in the conclusion; ask where they were supported.

7. Overlooking denominator/base-rate issues in stats.

   • What goes wrong: “More incidents” is inferred from “higher rate” (or vice versa).
   • Fix: Always ask: “Out of how many?” “Compared to what total?”

8. On Parallel Flaw, matching topic instead of structure.

   • What goes wrong: You pick the choice with similar subject matter rather than the same logical mistake.
   • Fix: Abstract it: replace content with letters, then match the pattern.

Memory Aids & Quick Tricks

Quick Review Checklist

• Can you underline the conclusion and list premises in under $15$ seconds?
• Did the argument switch from correlation to causation (or ignore reverse/third-variable explanations)?
• Did it confuse necessary and sufficient (especially via “only if,” “must,” “requires”)?
• Did it make an illegal conditional move (affirming consequent / denying antecedent)?
• Did it shift quantifiers (some → most/all; not all → none)?
• Did it broaden scope or add a new term in the conclusion?
• Did it rely on a bad sample (small, biased, self-selected) or an anecdote?
• Did it equivocate (same word, different meaning) or argue in a circle?
• For Parallel Flaw: did you match the structure rather than the topic?

You don’t need to memorize every name—just get fast at spotting the pattern and stating the gap clearly.