GMAT Critical Reasoning: Spotting Logical Fallacies
Many self-study forum members ask, “What should I do if I can’t grasp the core of CR questions?” However, I believe the “Seven Deadly Sins” that everyone memorizes for the AWA should be freed from their AWA constraints. Memorizing them last minute misses their logical value. After two years of organizing case studies and researching questions, I’ve found that many CR approaches actually stem from these Seven Deadly Sins. So, if you’re struggling to pinpoint the core of CR questions, start by reviewing this note: “Where are the Seven Deadly Sins Hiding in CR Questions?”
Below is a note I’ve compiled based on my recollection and understanding to help self-study forum members. Some questions are from case studies, some are official GMAT questions, and some are self-created based on similar concepts. This might not be entirely comprehensive, but my aim is to give self-study candidates a direction for their CR critical thinking.
1. Temporal Sequence Does Not Imply Causation / Oversimplification
1-1 Often seen in weaken questions for surveys and experiments (usually looking for alternative causes or reversing cause and effect):
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Ex. People who eat apples generally live longer. Conclusion: Eating apples causes people to live longer.
Weaken Answer:
– People who eat apples tend to live in Washington state, where the mild climate contributes to good health. (Alternative cause)
– Wealthy people can afford apples, and wealthy people have access to better medical care. (Alternative cause)
– People tend to eat apples when they are old. (Reverse causation)
Ex. After the indoor smoking ban was implemented in 1993 with the Tobacco Hazards Prevention Act, the public smoking rate decreased in 1994.
Conclusion: The Tobacco Hazards Prevention Act effectively reduced smoking rates.
Weaken Answer:
– Between 1993 and 1994, tobacco companies significantly increased the price of cigarettes by 20%. (Alternative cause)
– The decrease in smoking rates in 1994 led the government to enact the Tobacco Hazards Prevention Act in 1993. (This is highly unlikely, so reversing cause and effect doesn’t fit the question’s timeline.)
2. Hasty Generalization (the reverse, generalizing from the whole to a part, is also tested):
2-1 In survey and experiment questions: Attacking sampling bias
Ex. A survey found a decrease in average lifespan. Conclusion: The average lifespan in this country is declining.
Weaken Answer:
– The survey primarily sampled residents of City A, where the sanitation environment is poor. (Sampling bias)
2-2 In statistics questions about populations and individuals: Differences between average/individual, sample distribution
China’s per capita income is about $10,000 USD. Based on China’s average consumption level, $5,000 is considered enough to live comfortably. Conclusion: The Chinese people generally live comfortably.
Weaken Answer:
– Per capita income is unevenly distributed. The extremely wealthy skew the average, while most people earn only $2,000 USD.
– The per capita income in Shanghai is $50,000 USD, while in other provinces it’s $500 USD.
– The cost of living varies by city. In Beijing and Shanghai, where $20,000 is needed to live comfortably, the per capita income is only $10,000. In Guizhou, where $3,000 is enough to live comfortably, the per capita income is also $10,000.
2-3 Generalizing from the whole to a part: Also attacking uneven distribution
Ex.
Roland: The alarming fact is that 90 percent of the people in this country now report that they know someone who is unemployed.
Sharon: But a normal, moderate level of unemployment is 5 percent, with one out of 20 workers unemployed. So at any given time if a person knows approx. 50 workers, one or more will very likely be unemployed.
Sharon’s argument relies on the assumption that _________ .
b. unemployment is not normally concentrated in geographically isolated segments of populations
3. Faulty Analogy
3-1 Appearing in question phrasing like “Example A implies Example B”: Breaking the analogy
Ex. South Korea signed a free trade agreement with China and prospered. Therefore, if we sign a similar agreement, we will also prosper!
Weaken Answer:
– South Korea’s economy is highly autonomous and not easily suppressed by China, whereas ours is vulnerable. (Breaking the analogy)
– The content of the proposed agreement is different from the China-South Korea FTA. (Breaking the analogy)
Ex.
Although the discount stores in Goreville’s central shopping district are expected to close within five years as a result of competition from a SpendLess discount department store that just opened, those locations will not stay vacant for long. In the five years since the opening of Colson’s, a nondiscount department store, a new store has opened at the location of every store in the shopping district that closed because it could not compete with Colson’s.
Which of the following, if true, most seriously weakens the argument?
B. Increasingly, the stores that have opened in the central shopping district since Colson’s opened have been discount stores.
4. All Things Being Equal: The present is not the past (objective conditions have changed)
4-1 Appearing in question types with a time gap: The past does not represent the present, or the present does not represent the past. Also commonly seen in explain questions.
Ex. Two years ago, SONY, based on market research preferences, found that consumers preferred more features over smaller size. Consequently, they launched a 17-inch laptop with diverse features but a larger size than other laptops. However, sales were dismal.
Explain Answer:
– Consumer habits changed over the two years, with a preference for slim and compact electronic devices over feature-rich ones. (All things not being equal)
Ex. (Case study question) Someone wants to apply to Malaysia Airlines. However, in previous years, Malaysia Airlines only had 10 openings annually, with nearly 1,000 applicants, resulting in an average acceptance rate of only 1%. Therefore, they believe their chances are low and decide not to apply this year.
Weaken Answer:
– In the past year, Malaysia Airlines’ reputation has significantly declined, and job seekers are no longer as interested in applying to them.
4-2 Appearing in proposal questions (especially those involving money): Dynamic equilibrium (supply and demand, cost and revenue, quantity and price)
Talking about cost savings -> consider potential loss of revenue.
Talking about increased sales volume -> consider potential decrease in unit price.
Talking about increased supply -> demand might also increase simultaneously.
The reverse is also true.
Ex. (Case study question) 2,000 gold coins were recently unearthed. At current market prices, each coin can be sold for $100 USD. Therefore, selling all of them will yield $200,000 USD.
Weaken Answer:
– A large increase in supply will cause market prices to fall. (Objective conditions have changed)
Appearing in proposal questions (especially those involving money): Dynamic equilibrium (supply and demand, cost and revenue, quantity and price)
Talking about cost savings -> consider potential loss of revenue.
Talking about increased sales volume -> consider potential decrease in unit price.
Talking about decreased supply -> demand might also decrease simultaneously.
Ex. (Cost savings -> potential loss of revenue)
Switching product packaging from plastic to paper can save costs and increase profits. Therefore, the company has decided to make this packaging change.
Weaken Answer:
– Consumers dislike purchasing products packaged in paper. (Objective conditions have changed)
Ex. (Decreased supply -> demand might also decrease)
A product that was previously popular, selling 5,000 units per month, is now being limited by the company to 1,000 units per month. It is expected that after this limitation, the market price of the product will rise.
Weaken Answer:
– Consumer interest in this product has waned; only 1/10 of the original number of customers are still interested.
5. False Dichotomy
5-1 Personal impression, more likely to appear in flaw in the reasoning/Boldface questions involving two-person dialogues
Ex. Commentator A: According to surveys, the public supports environmental protection. Candidate X’s policy is to boost the economy, therefore Candidate X is going against public opinion.
Commentator B: Your argument is flawed because boosting the economy does not necessarily mean environmental protection will be neglected.
6. Data Reliability
More commonly seen in questions discussing history or archaeological dating.
6-1 In strengthen questions: The data is accurate and error-free
Sviatovian is a medieval Moringian text whose author and exact date of composition are unknown. However, the events in the life of Prince Sviatov that the text describes occurred in 1165, and in the diagram of Sviatov’s family that accompanies the text his father, who died in 1167, is identified as still living. Thus Sviatovin must have been written between 1165 and 1167, assuming that___________
(C) the diagram accurately represents the composition of Sviatov’s family at the time Sviatovin was written
6-2 In assumption/weaken questions: The evidence is not flawed
Scientists discovered human skeletons and iron tools unearthed in a certain area. Radiocarbon dating suggests the human skeletons are approximately 50,000 years old. Therefore, scientists conclude that humans were crafting iron tools 50,000 years ago.
Assumption Answer:
– The iron tools were not left behind by later inhabitants of the area.
Weaken Answer:
– The iron tools might have been left behind by later inhabitants of the area.
7. Faulty Premise
I recall this rarely appearing in single-narrator questions (as we cannot question the facts presented in the question). However, it is common in two-person dialogue “flaw in the reasoning” questions, where one person attacks the other’s assertion.
A: People prioritize environmental protection over economic development. Candidate A has proposed a comprehensive environmental policy, therefore we should vote for them.
B: According to polls, 60% of people prioritize the economy over the environment, while 40% only care about the environment and not the economy. (Attacking A’s premise)
B: Candidate A’s environmental policy is not actually comprehensive. (Attacking A’s premise)
