Masterclass: Critical Reasoning – Weaken • GMATCourse.mba

Core Philosophy: How Arguments Break

A weaken question asks for new information that makes the conclusion less likely to follow from the evidence. You are not proving the conclusion is false — you are reducing the probability that the evidence supports the conclusion.

Arguments are most vulnerable at three points: the causal bridge (where assumptions live), alternative explanations (competing causes), and quantitative claims (where data can be misinterpreted).

Core Insight: The correct weakener is the one that most damages the link between the evidence and the conclusion — not the one that most directly challenges the evidence itself.

Three Ways to Attack an Argument

Attack Vectors

Attack the Assumption

Show the hidden bridge is false or unreliable

P + [broken bridge] ≠ C

Offer an Alternative

Provide another explanation for the evidence

P could be caused by X, not C

Undermine the Data

Show the evidence doesn't mean what the author thinks

P is misinterpreted → C fails

The Weaken Strategy

Identify the conclusion

Isolate the author's specific claim. Define its exact scope.

Find the central assumption

Ask what must be true for the evidence to support the conclusion. This is where the argument is most vulnerable.

Pre-phrase what would hurt

Before reading choices, imagine the kind of information that would make you less confident in the conclusion.

Choose the greatest damage

Multiple choices may weaken. Pick the one that most significantly reduces the probability the conclusion is correct.

Worked Examples

Example 1 — Alternative Cause

Argument: "Sales of winter coats in our stores rose 30% after we launched our new marketing campaign. The marketing campaign is working."

Assumption: The marketing campaign caused the sales increase.

Best Weakener: The winter of the sales increase was the coldest on record in the past 50 years. (The extreme weather, not the campaign, explains the spike.)

Example 2 — Broken Bridge

Argument: "City Y installed air quality monitors throughout the city. Within a year, respiratory illness rates dropped by 15%. The monitors improved air quality."

Assumption: Monitors improve air quality (they measure it, not fix it!).

Best Weakener: Air quality monitors can only measure air pollution — they have no mechanism to reduce pollution levels. (The assumption that monitors caused improvement is simply false.)

Example 3 — Misinterpreted Data

Argument: "Students at private schools score 20% higher on standardized tests than students at public schools, proving that private school education is superior."

Assumption: The score gap is caused by school quality, not student selection.

Best Weakener: Private school students come disproportionately from high-income households with educated parents who provide extensive tutoring and enrichment outside of school. (The data doesn't prove school quality — it may reflect family background.)

10 Common Weaken Traps

1. Opposite-direction trap

A choice that actually strengthens the argument. Always verify the direction of the effect.

2. Out-of-scope weakener

The choice challenges something not claimed in the conclusion.

3. Too-strong weakener

A choice that completely destroys the argument may be too extreme for "most weakens."

4. Premise attacker

Weakening the evidence doesn't necessarily weaken the argument if the conclusion doesn't depend on that evidence.

5. Irrelevant data

Statistics about different populations, geographies, or time frames may not apply to the specific argument.

6. Circular attack

A choice that just repeats the opposite of the conclusion without providing new evidence.

7. Temporal mismatch

Evidence from a different time period may not apply to the stated claim.

8. Alternative weakener

A choice that provides a different explanation but doesn't damage the original causal link.

9. Degree confusion

A choice that says "some X" when the conclusion relies on "most X."

10. Mechanism vs. cause trap

Explaining the mechanism of how something works doesn't undermine why it is observed.

Weaken vs Strengthen — The Key Differences

Feature	Weaken	Strengthen
Goal	Make conclusion less probable	Make conclusion more probable
Best attack point	The assumption / causal bridge	The assumption / gap in support
Power move	Introduce alternative cause	Eliminate alternative causes
Wrong direction	Choice that actually supports	Choice that actually attacks

10 GMAT-Style Practice Questions

Select your answer, then reveal the step-by-step explanation. Each question reflects real GMAT difficulty and format.

Question 1 of 10 GMAT Verbal

A company installed standing desks for all employees and, six months later, reported a 22% decrease in reported back pain among employees. The company's management concluded that standing desks reduced back pain. Which of the following, if true, most weakens the management's conclusion?

Correct Answer: (B)

(B) is correct. If a stretching program was introduced at the same time, it provides an alternative explanation for the reduction in back pain. The company cannot credit standing desks alone. (A) describes an expected adjustment period — doesn't weaken the long-term conclusion. (C) is about cost, irrelevant to whether they reduce back pain. (D) establishes stakes but doesn't address causation. (E) describes usage flexibility, which might even slightly strengthen the conclusion.

Question 2 of 10 GMAT Verbal

A tech startup claims its productivity software increased its employees' output by 40% within three months of adoption, and therefore the software is highly effective. Which of the following most weakens this claim?

Correct Answer: (B)

(B) is correct. If total output rose 40% but staff size also increased by 35%, the software may have had little to no effect on per-employee productivity. The aggregate output number is misleading. (A) notes a learning curve but doesn't challenge the long-term conclusion. (C) shows industry norms — doesn't address this company's specific claim. (D) is about origin, not effectiveness. (E) introduces another confound (time management training), which also weakens but is less direct than (B).

Question 3 of 10 GMAT Verbal

An environmental group argues that the new coastal protection law will significantly reduce beach erosion because it bans construction within 500 meters of the coastline. Which of the following, if true, most weakens this argument?

Correct Answer: (B)

(B) is correct. The argument assumes construction near the coast causes most of the erosion. If wave action and storms are the primary causes, banning construction won't significantly address the problem. (A) suggests broader global trends but doesn't directly address the effectiveness of this law. (C) establishes relative scope but doesn't weaken causation. (D) introduces a different provision — irrelevant to erosion. (E) is about opposition, not effectiveness.

Question 4 of 10 GMAT Verbal

A university report found that students who use the campus gym regularly achieve higher GPAs than those who do not. The report concluded that regular exercise improves academic performance. Which of the following most weakens this conclusion?

Correct Answer: (B)

(B) is correct. This introduces a common-cause explanation: self-discipline drives both gym use and high GPAs. The correlation between gym and GPA may not be causal — both could be effects of a third variable (self-discipline). (A) notes access limitations — doesn't affect the causal interpretation. (C) actually strengthens (provides mechanism). (D) notes low prevalence — doesn't weaken causation. (E) introduces another potential mediating variable — less impactful than (B).

Question 5 of 10 GMAT Verbal

A nutritional supplement company argues that its Vitamin D supplement reduces depression symptoms, citing a study showing people who take the supplement report 30% fewer depressive episodes. Which of the following most weakens this argument?

Correct Answer: (B)

(B) is correct. If supplement users are self-selecting for health motivation (they're willing to buy supplements, track health outcomes, etc.), the reduction in depressive episodes could result from this general motivation to improve health, not the supplement itself. (A) is about alternative sources, not effectiveness. (C) describes sample composition — potentially relevant but less directly damaging. (D) introduces natural fluctuation — partially weakens but doesn't target the causal link as directly. (E) raises the other-ingredient issue, which weakens attribution but is less impactful.

Question 6 of 10 GMAT Verbal

A car manufacturer argues that its new electric vehicle model will be commercially successful because pre-orders have exceeded 50,000 units in the first week. Which of the following most weakens this claim?

Correct Answer: (B)

(B) is correct. Pre-orders translate to commercial success only if they convert to actual purchases. A 60% cancellation rate in the industry means 50,000 pre-orders may yield only 20,000 actual sales — potentially not enough to call it a success. (A) is a strengthener. (C) raises cost concerns but doesn't address whether existing pre-orders translate to success. (D) notes future competition but doesn't address current conversion. (E) is a strengthener.

Question 7 of 10 GMAT Verbal

A city government credits its new public transit expansion for reducing traffic congestion by 18% over two years. Which of the following most weakens this conclusion?

Correct Answer: (A)

(A) is correct. New road capacity is a direct alternative explanation for reduced congestion. If more lanes were added simultaneously, the transit expansion cannot take sole credit for the improvement. (B) actually strengthens — more riders means more cars off roads. (C) notes population growth, which might have worsened congestion, making the reduction more impressive — this could strengthen. (D) strengthens the transit-congestion link. (E) is a general principle, not specific evidence about this city.

Question 8 of 10 GMAT Verbal

A researcher argues that children who read for at least 30 minutes per day develop significantly stronger vocabulary than those who do not, and concludes that reading causes vocabulary improvement. Which of the following most weakens this reasoning?

Correct Answer: (B)

(B) is correct. This introduces reverse causation: vocabulary skill may drive reading behavior, not the other way around. Children who already have strong vocabularies find reading easier and more enjoyable, so they read more. The correlation could run the opposite direction from what the researcher claims. (A) limits generalizability but doesn't address causation. (C) about demographics — doesn't affect causal interpretation. (D) is about correlation with achievement — irrelevant to the reading-vocabulary causation question. (E) justifies threshold choice — doesn't weaken causation.

Question 9 of 10 GMAT Verbal

A healthcare company argues that its new telemedicine platform will reduce healthcare costs by allowing more frequent, shorter consultations instead of fewer, longer in-person visits. Which of the following most weakens this argument?

Correct Answer: (B)

(B) is correct. The argument assumes that shorter + more frequent = lower total cost. But if frequency increases dramatically while each visit still has administrative costs, total costs could rise. This directly attacks the cost-reduction assumption. (A) addresses patient preference — doesn't address cost structure. (C) is about development process — irrelevant. (D) is about access, not cost. (E) is about integration — irrelevant to cost.

Question 10 of 10 GMAT Verbal

A real estate company argues that housing prices in Westville will continue to rise because demand from high-income buyers is increasing while the housing supply remains constrained. Which of the following most weakens this prediction?

Correct Answer: (B)

(B) is correct. The argument's foundation is constrained supply + rising demand = rising prices. A zoning change that will add 5,000 new units directly undermines the "constrained supply" premise, the structural basis of the prediction. (A) confirms high-income demand but doesn't address future prices. (C) supports the demand side of the argument. (D) provides national context that may even support the prediction. (E) gives a quality signal that supports high demand.

Key Takeaways: Weaken Questions

1. Attack the assumption, not just the evidence

The most impactful weakeners break the logical bridge, not individual facts.

2. Alternative causes are powerful

A different explanation for the same evidence undermines the causal conclusion.

3. Pre-phrase vulnerabilities

Before reading choices, ask: "Where could this argument fail?" This guides you to the right answer.

4. Compare by degree of damage

The correct answer delivers the greatest damage to the specific conclusion stated.