LSAT Logical Reasoning: Weaken

Rank 4 by frequency | 402 questions in corpus (8.9% of all questions)

A Weaken question presents an argument and asks which answer choice, if true, would most undermine, damage, or call into question the argument's conclusion. The answer does not need to destroy the argument entirely – it just needs to reduce confidence in the conclusion more than any other choice. The correct answer introduces a NEW fact that makes the conclusion LESS LIKELY to follow from the premises.

Your ability to identify what an argument assumes (its vulnerabilities) and recognize which new information would exploit those vulnerabilities. The correct answer always attacks the unstated assumption linking premises to conclusion, not the premises themselves.

The Task

Identify the answer that hurts the argument the most – by introducing a fact that exposes a gap, presents a counter-example, undermines a key assumption, or provides an alternative explanation.

What It Tests

Your ability to identify what an argument assumes (its vulnerabilities) and recognize which new information would exploit those vulnerabilities. The correct answer always attacks the unstated assumption linking premises to conclusion, not the premises themselves.

A. EXACT LOGICAL FLOW

Step-by-Step Stimulus Structure

1. Background/Context (optional): Topic setup. Not a premise or conclusion. 2. Premises/Evidence: Factual claims accepted as true. 3. Conclusion: The author's main claim that goes beyond what the premises strictly prove. The conclusion introduces a logical leap. 4. The Gap: The space between what the premises establish and what the conclusion claims. This gap is the argument's vulnerability – it represents one or more unstated assumptions.

How the Conclusion Relates to the Premises

The conclusion always overreaches the premises in at least one way: - Scope shift: Conclusion uses a term/category not in the premises - Causal leap: Premises show correlation; conclusion asserts causation - Degree/certainty escalation: Premises support "possible"; conclusion states "certain" - Temporal mismatch: Premises describe past; conclusion extends to future - Comparison without basis: Premises describe one situation; conclusion compares to another

The Nature of the Gap

The gap is always an unstated assumption. The correct answer attacks this assumption: - Premises are accepted as true and CANNOT be attacked - The conclusion's truth value is what is at issue - The correct answer introduces NEW information that damages the connection between premises and conclusion - The answer does NOT need to disprove the conclusion – it only needs to make it LESS LIKELY

B. ALL WITHIN-TYPE VARIATIONS / SUBTYPES

Variation 1: Standard Weaken

  • Stem: "Which one of the following, if true, most seriously weakens the argument?"
  • Single-speaker stimulus with a clear argument. Most common form.

Variation 2: Weaken EXCEPT

  • Stem: "Each of the following, if true, weakens the argument EXCEPT:"
  • Four answers weaken; one does NOT.
  • Critical insight (PowerScore): The correct answer is NOT necessarily a strengthener – it is simply "not a weakener." Could be neutral, irrelevant, or even a strengthener. The logical opposite of "weaken" is "not weaken," not "strengthen."

Variation 3: Dialogue/Two-Speaker Format

  • Stem: "Which one of the following, if true, would provide [Speaker A] with the strongest counter to [Speaker B]'s response?"
  • Must track two separate arguments and identify which to attack. Generally harder.

Variation 4: Principle-Based Weaken

  • Stem: "Which one of the following principles, if valid, most seriously undermines the argument above?"
  • The answer is an abstract principle whose application damages the argument.

Variation 5: Weaken with Causal Stimulus (~30% of weaken questions)

Five classic attack vectors (PowerScore): 1. Alternate cause: A different factor (Z) explains Y 2. Cause without effect: X occurred but Y did not 3. Effect without cause: Y occurred but X did not 4. Reversed causation: Y actually causes X 5. Data/methodology problem: Study is flawed (no control group, confounding variables)

Variation 6: Weaken with Analogy/Comparison Stimulus

  • Attack vector: Show a relevant difference between compared items

C. ANSWER CHOICE CONSTRUCTION

How the Correct Answer Is Designed

1. Introduces new information not in the stimulus 2. Targets an unstated assumption 3. Makes the conclusion less likely (need not disprove it) 4. Accepted as true per "if true" clause 5. Often uses stronger wording ("all," "most," "none") for decisive weakening 6. Does NOT contradict stated premises – attacks the inferential LINK

Common Correct Answer Strategies

| Strategy | How It Works | |———-|————-| | Alternative explanation | Different cause for the observed effect | | Undermines data | Evidence is flawed, biased, or unrepresentative | | Breaks the analogy | Relevant difference between compared items | | Counterexample | Case where premises hold but conclusion fails | | Timing mismatch | Cause and effect don't align temporally | | Scale/scope issue | Numbers vs. percentages or part-to-whole error | | Reverses causation | Causal direction is opposite to claimed |

Common Wrong Answer Patterns

1. Irrelevant/Out of Scope: Tangential topic; mentions stimulus terms but doesn't address conclusion's logic 2. Opposite (Strengthener): Makes the conclusion MORE likely 3. Premise Restater: Repeats existing evidence; no net effect 4. Too Extreme: Sweepingly absolute claim beyond what's needed 5. Addresses Wrong Conclusion: Weakens a claim that sounds like but isn't the actual conclusion 6. Requires Extra Assumptions: Could theoretically weaken, but only through a long chain of inferences 7. Attacks Premises: Tries to prove a stated premise false (premises are accepted as true) 8. Addresses Background Only: Relates to context rather than the actual argument

D. COMMON PATTERNS AND TRAPS

Most Common Argument Structures (~by frequency)

1. Causal claims based on correlation (~30%) 2. Prediction/recommendation based on evidence 3. Generalization from sample 4. Argument by analogy 5. Rejection of alternative ("the only explanation for X is Y") 6. Numbers-vs-percentages confusion 7. Comparison without basis 8. Conditional reasoning

How LSAC Designs the Hardest Versions

1. Buried/implicit conclusion: Not signaled by indicator words 2. Multiple possible weakeners: Several seem to weaken, but only one attacks the CENTRAL assumption 3. Abstract/unfamiliar topic: Obscure scientific, legal, or philosophical content 4. Subtle scope shifts: Tiny but critical word changes ("some" in premises vs. "most" in conclusion) 5. Correct answer seems tangential: Introduces a seemingly unrelated fact that devastatingly undermines the assumption 6. Decoy strengtheners: An answer that strengthens catches students who confuse direction 7. Numbers/percentages traps: Stimulus conflates absolute numbers with rates; correct answer exposes this

E. THE "ANATOMY" OF THE QUESTION

What Makes It Unique

  • vs. Flaw: Both require identifying a weakness, but Weaken asks you to PROVIDE new information (concrete fact); Flaw asks you to DESCRIBE the existing error (abstract characterization)
  • vs. Necessary Assumption: The negation of a necessary assumption will weaken. But weaken answers are not limited to negated assumptions
  • vs. Strengthen: Mirror image – identical structure, opposite direction
  • vs. Sufficient Assumption: Both introduce new information, but SA answers GUARANTEE the conclusion while Weaken answers DAMAGE it

The Exact Cognitive Steps

1. Read the stem first to confirm it's a Weaken question 2. Read the stimulus: find conclusion, premises, and gap 3. Identify the conclusion (use indicator words) 4. Identify the premises 5. Articulate the gap: What does the author assume? 6. Prephrase broadly: What TYPE of information would hurt? 7. Evaluate each answer: Accept as true. "Does this make the conclusion less likely?" 8. Eliminate wrong answers: irrelevant > strengtheners > premise restaters 9. Compare remaining: Pick the one causing MORE damage with FEWER additional assumptions 10. Adopt an "opposing counsel" mindset: What's the most devastating rebuttal?

Official Content Examples

Example 1: Alternative Explanation (Difficulty 3)

Source: PT47, Section 3, Question 24

Stimulus: A police commissioner argues that a 15% decrease in violent crime was caused by a new mandatory sentencing law enacted at the beginning of last year, since no other major policy changes were made.

Example 2: Counterexample (Difficulty 3)

Source: PT64, Section 1, Question 8

Stimulus: The "bottom-up" theory holds that edible plant availability primarily determines ecosystem characteristics, and that a reduction in predators will have little impact on the rest of the ecosystem.

Example 3: Breaking a Causal Link (Difficulty 3)

Source: PT57, Section 2, Question 14

Stimulus: Older bees (foragers) have larger brains than younger bees (non-foragers). Since foraging requires greater cognitive ability, it appears that foraging leads to increased brain size.

Practice LSAT Logical Reasoning Questions