What You'll Learn This Hour
- Precisely define Strengthen and Weaken questions and identify them from stem language
- Apply the Negation Test to expose hidden assumptions in causal and statistical arguments
- Recognize the three classic causal weakeners: alternate cause, reverse causation, correlation-only
- Avoid the most common traps: out-of-scope choices and choices that prove rather than just affect
Core Concepts
Strengthen
- + Supports the conclusion by bolstering the assumption
- + Adds new evidence that makes the conclusion more likely to be true
- + Eliminates an alternative explanation that would undermine the argument
- + Stem language: "which of the following, if true, most strengthens..."
Weaken
- - Attacks the assumption the argument depends on
- - Provides an alternate explanation for the evidence presented
- - Shows the evidence is unreliable or the conclusion has exceptions
- - Stem language: "which of the following, if true, most seriously weakens..."
The Negation Test (for Assumptions)
To confirm a hidden assumption: negate the answer choice. If the negated version destroys the argument, the answer IS the assumption. This also helps in Strengthen/Weaken because strengthening an argument is equivalent to confirming an assumption, and weakening is equivalent to negating one.
Example:
Argument: Company X switched to remote work, so productivity must have increased.
Candidate assumption: "Remote work does not decrease productivity for this company's employees."
Negated: "Remote work DOES decrease productivity." — This destroys the conclusion. The assumption is valid.
Causal Argument Weakeners
When an argument concludes that A caused B, there are three classic ways to weaken it:
Alternate Cause
Something else (C) could have caused B, not A. The argument ignores other explanations.
Reverse Causation
B actually caused A, not the other way around. The direction of causation is reversed.
Correlation Only
A and B merely occur together (correlation) but neither causes the other — a third factor causes both.
Visual Model: The Argument Balance Scale
Every CR argument is a scale. Evidence sits on one side, the Conclusion on the other. Strengthen adds weight to the Conclusion side; Weaken tips the scale away from it.
Worked Examples
Stimulus: The city of Merriton introduced a new subway line last year. Since then, traffic congestion on Route 7 — the highway that runs parallel to the subway — has decreased by 18%. The city council concludes that the subway line caused the reduction in traffic congestion.
Which of the following, if true, most strengthens the city council's conclusion?
Step-by-Step Solution:
Argument structure: Evidence = subway opened + congestion fell 18%. Conclusion = subway caused the reduction.
Hidden assumption: No other factor caused the traffic reduction during that period.
Why (C) strengthens: If congestion stayed constant on all OTHER highways but fell specifically on Route 7, this rules out a city-wide trend (like fewer drivers overall). It isolates the subway as the likely cause, directly supporting the assumption.
Why others fail: (A) actually weakens by providing an alternate cause (road widening). (B) is tempting but 22% of current riders is not enough to explain an 18% reduction without knowing baseline numbers. (D) and (E) are irrelevant to the causal claim.
Stimulus: A study found that children who regularly eat breakfast perform better on standardized math tests than children who skip breakfast. Therefore, eating breakfast improves mathematical ability in children.
Which of the following, if true, most seriously weakens the argument above?
Step-by-Step Solution:
Argument structure: Evidence = breakfast eaters score higher on math. Conclusion = breakfast CAUSES better math performance.
Assumption: No third factor causes both eating breakfast AND higher math scores.
Why (C) weakens: This is a classic "third factor" / confounding variable. Income explains both variables — wealthier kids eat breakfast more AND get tutoring. The correlation between breakfast and math scores is not a causal relationship; both are driven by income. This is the "correlation does not equal causation" trap.
Why others fail: (A) does not weaken (exceptions don't disprove a general claim). (B) is irrelevant. (D) is irrelevant. (E) introduces another factor but does not explain WHY breakfast eaters score higher.
Stimulus: Northville High School switched from paper textbooks to tablet computers for all students. The principal argues that this will save the school money in the long run because tablets can store hundreds of digital textbooks at a fraction of the cost of physical books.
The principal's argument relies on which of the following assumptions?
Step-by-Step Solution using the Negation Test:
Argument: Tablets store cheap digital books, therefore tablets save money long-term.
Apply negation test to (C): "The total cost of tablets DOES exceed the savings." — If true, the whole "saves money" conclusion collapses. The negation destroys the argument. Answer: (C) is the assumption.
Apply negation test to (B): "Tablets DO require frequent software updates." — This is closer to a weaken answer than a core assumption; the argument could still work if updates are cheap. Not the primary assumption.
Key insight: The assumption must be the logical bridge the argument CANNOT survive without. (C) is that bridge: net savings only occur if total tablet costs are less than textbook savings.
GMAT Traps to Avoid
Trap 1: "Prove vs. Affect"
Strengthen answers do NOT need to prove the conclusion. They only need to make it more likely. Similarly, Weaken answers do not disprove — they just make the conclusion less likely. Many students eliminate the right answer because it doesn't feel strong enough.
Trap 2: Out-of-Scope Choices
An answer may discuss a related topic that sounds relevant but does not actually affect THIS specific argument's conclusion. Always ask: "Does this directly impact the logical link between evidence and conclusion?" If not, eliminate it.
Trap 3: Opposite Answers
GMAT often includes a "reverse" trap: one answer that strengthens and a very similar one that weakens. If you misread the question stem (strengthen vs. weaken), you will confidently pick the wrong answer. Always reread the stem before selecting.
Trap 4: New Assumptions in Strengthen
A Strengthen answer that introduces a brand new, unrelated claim about a different topic is wrong even if it sounds positive. It must connect directly to the argument's specific conclusion — not to a general principle that would be nice if true.
Practice Questions
12 GMAT-style questions. Attempt each before revealing the answer.
A pharmaceutical company claims that its new drug, Veritine, reduces migraine frequency. In a clinical trial, patients who took Veritine for three months reported 40% fewer migraines than before. Therefore, Veritine is an effective migraine treatment.
Which of the following, if true, most strengthens the conclusion?
Show Answer
Correct Answer: (C)
The argument concludes Veritine caused the reduction. The key assumption is that the reduction wasn't due to placebo effect or natural variation. A control group that showed no improvement while Veritine patients improved isolates Veritine as the cause — the strongest possible strengthen for a clinical trial argument.
Why not (B)? This actually weakens the argument by introducing an alternate cause (seasonal variation).
A city reports that crime rates in the downtown district fell by 30% in the year after streetlights were upgraded. The mayor concludes that improved street lighting reduces crime.
Which of the following, if true, most strengthens the mayor's conclusion?
Show Answer
Correct Answer: (B)
Comparing the upgraded district to neighboring districts that did NOT get upgraded (and saw no crime reduction) provides a natural control group. This rules out city-wide trends (economy, policing changes) as alternate causes, directly supporting the causal link between lighting and crime reduction.
Why not (D)? Criminal self-reports are anecdotal and do not demonstrate that the specific upgrade caused the 30% reduction in this district.
The town of Lakewood is considering implementing a plastic bag ban similar to the one adopted by the town of Riverside three years ago. Riverside's ban led to a 60% reduction in plastic bag litter in local parks. Therefore, a plastic bag ban in Lakewood will also significantly reduce plastic bag litter.
Which of the following, if true, most strengthens the argument?
Show Answer
Correct Answer: (A)
Analogy arguments assume the two situations are sufficiently similar. The main vulnerability is that Lakewood and Riverside might differ in plastic bag usage habits. (A) directly closes this gap by establishing that baseline plastic bag usage is comparable, making the analogy stronger.
Why not (E)? If Riverside's success depended on heavy fines and Lakewood doesn't plan similar enforcement, this would actually be information that could weaken the analogy if Lakewood doesn't implement the same fines.
A marketing study showed that customers who receive personalized email recommendations spend 25% more per transaction than customers who receive generic emails. Retailer Xova plans to implement a personalization algorithm and expects a significant increase in average transaction value.
Which of the following, if true, most strengthens Xova's expectation?
Show Answer
Correct Answer: (A)
The argument applies a study's findings to Xova specifically. The assumption is that Xova resembles the retailers in the study. Confirming that the study retailers have a similar product mix closes the gap between the general study and Xova's specific situation.
Why not (E)? Competitors not having personalization is irrelevant to whether personalization will work for Xova — it addresses competitive advantage, not the effectiveness of personalization itself.
After Greenfield Corporation implemented a mandatory wellness program, employee sick days fell by 35% over the following year. The HR director concluded that the wellness program improved employee health.
Which of the following, if true, most seriously weakens the HR director's conclusion?
Show Answer
Correct Answer: (C)
This is a textbook alternate cause. Employees may have stopped taking sick days out of job insecurity (fear of layoffs), not because they became healthier. This explains the 35% reduction without giving credit to the wellness program. The HR director assumed no other factor changed, but (C) shows a major factor did change.
Why not (E)? Natural fluctuation is vague and doesn't specifically explain a 35% DROP. (C) provides a concrete, specific alternate mechanism.
Researchers found that people who own pets have lower average blood pressure than people who do not own pets. They conclude that owning a pet reduces blood pressure.
Which of the following, if true, most seriously weakens this conclusion?
Show Answer
Correct Answer: (B)
This demonstrates reverse causation. Rather than pets causing low blood pressure, HIGH blood pressure causes people NOT to own pets (doctors advise against it). The direction of causation is reversed — the health condition determines pet ownership, not the other way around.
Note on (A): This introduces an alternate cause (exercise), which also weakens, but (B) more directly attacks the causal direction claimed in the argument. On GMAT, choose the answer that most directly attacks the argument's specific claim.
Maplewood University plans to eliminate its foreign language requirement for graduation, arguing that since only 15% of graduates use a foreign language professionally after graduation, the requirement wastes students' time and resources.
Which of the following, if true, most seriously weakens the university's argument?
Show Answer
Correct Answer: (B)
The argument assumes the only value of foreign language study is direct professional use. (B) attacks this assumption by providing an alternative benefit (cognitive skills) that applies to ALL professions — making the requirement valuable even for the 85% who don't use a language professionally.
Why not (A)? Enjoyment is irrelevant to the argument about wasting time and resources — the argument is about utility, not pleasure.
A study found that regions with more fast-food restaurants per capita also have higher obesity rates. Health researchers concluded that the availability of fast food causes higher obesity rates in those regions.
Which of the following, if true, most seriously weakens the conclusion?
Show Answer
Correct Answer: (C)
Classic third-variable / correlation-is-not-causation weakener. Income drives BOTH fast-food density (fast food thrives in lower-income markets) AND obesity (less access to fresh produce). The correlation between fast food and obesity may be entirely explained by the third factor — income — with neither causing the other.
Why not (D)? Some people not eating fast food is consistent with the argument — you don't need everyone to eat fast food to have higher obesity rates. It doesn't address the causal claim.
The board of directors voted to increase advertising spending by 40% next quarter. The CEO argues that this will increase total revenue, because historically, every 10% increase in advertising has corresponded with a 5% increase in sales.
The CEO's argument depends on which of the following assumptions?
Show Answer
Correct Answer: (B)
Negation test on (B): "The relationship between advertising and sales will NOT continue." If this is true, the entire argument collapses — past data is irrelevant if conditions have changed. This is the core bridge the CEO relies on: that historical trends will predict future results.
Negation test on (C): "Competitors WILL increase advertising." Does this destroy the argument? Not necessarily — the CEO's claim is about the historical relationship, not about competitive dynamics specifically. (C) is a weaken consideration but not the primary unstated assumption.
Economists argue that the proposed minimum wage increase will cause unemployment to rise, because when the cost of labor rises, businesses will hire fewer workers to maintain profitability.
Which of the following is an assumption the economists' argument requires?
Show Answer
Correct Answer: (C)
Negation test on (C): "Businesses CAN offset higher labor costs by raising prices or increasing worker productivity." If this is true, businesses maintain profitability without cutting workers — and the economists' unemployment conclusion fails entirely. The argument assumes that layoffs are the only way to protect profits.
Negation test on (A): "Most businesses do NOT operate with thin profit margins." This weakens but doesn't fully destroy the argument — even profitable businesses have incentives to minimize labor costs. (C) is the more fundamental assumption.
Paloma Energy plans to switch its vehicle fleet from gasoline-powered to electric vehicles. The company's sustainability director claims this will significantly reduce the company's carbon footprint, since electric vehicles produce zero direct emissions.
Which of the following is an assumption the sustainability director's argument requires?
Show Answer
Correct Answer: (B)
The director's claim is about TOTAL carbon footprint. EVs have zero direct emissions but may cause significant indirect emissions if charged with coal-generated electricity. Negation of (B): "The electricity WILL primarily come from high-emission sources." In that case, switching to EVs might not reduce (and could even increase) the overall carbon footprint — destroying the argument.
Why not (D)? Availability affects feasibility of the switch, not whether it WOULD reduce emissions if completed. The argument's logic is conditional ("if they switch..."), so availability is a practical concern, not an assumption about the carbon reduction claim itself.
A school district eliminated recess periods to add 30 more minutes of daily academic instruction. The superintendent claims this will improve standardized test scores, because more instructional time allows teachers to cover more material and reinforce key concepts.
Which of the following is an assumption the superintendent's argument requires?
Show Answer
Correct Answer: (C)
The argument assumes that MORE instructional time translates into MORE learning. But if students lose focus without breaks, the extra 30 minutes may be ineffective or counterproductive. Negation of (C): "Students will NOT remain sufficiently focused." If true, the additional instruction time produces no benefit — and eliminating recess may even hurt performance by reducing students' cognitive refresh. The argument collapses.
Why not (B)? Teacher preparedness is a reasonable concern, but the argument's core leap is about whether MORE time = BETTER scores. (C) attacks the assumption more directly by questioning whether learning is retained during extended unbroken sessions.
Quick Reference Card
# CR Strengthen & Weaken — Key Rules
## Question Identification
STRENGTHEN stems → "most supports", "most strengthens", "best supports the conclusion"
WEAKEN stems → "most undermines", "most seriously weakens", "most seriously calls into question"
## Core Process (applies to both)
1. Identify the CONCLUSION (what is being argued FOR)
2. Identify the EVIDENCE (the premises supporting it)
3. Find the GAP / ASSUMPTION (the unstated bridge)
4. Pick the answer that most directly affects that gap
## Strengthen Moves
+ Confirm the assumption directly
+ Eliminate an alternate cause or explanation
+ Provide additional evidence in the same direction
## Weaken Moves
- Attack the assumption directly
- Introduce alternate cause (third factor)
- Show reverse causation (B caused A, not A caused B)
- Show correlation only (confounding variable explains both)
## Negation Test
Negate the answer choice → if the argument COLLAPSES, it IS the assumption
## Critical Reminders
! Strengthen ≠ Prove | Weaken ≠ Disprove
! Accept "if true" — never question whether the answer choice is realistic
! Out-of-scope answers look tempting — must connect to THIS argument's specific gap
! Re-read the stem — strengthen vs. weaken traps are common