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Difficulty: MediumParallel Flaw

Sociologist: Most of the residents in this neighborhood commute by train. Furthermore, most of the residents in this neighborhood regularly vote in local elections. Consequently, it must be the case that most of the residents in this neighborhood who commute by train regularly vote in local elections.

The flawed pattern of reasoning in the sociologist's argument is most closely paralleled by which one of the following?

  1. A
    Most of the residents in this neighborhood who commute by train vote in local elections. Furthermore, most of the residents in this neighborhood who vote in local elections are homeowners. Thus, most of the residents in this neighborhood who commute by train must be homeowners.
  2. B
    Some of the restaurants in the city are expensive. Some of the restaurants in the city are highly rated. Therefore, some of the expensive restaurants in the city must be highly rated.
  3. Most of the novels on the bestseller list are thrillers. Additionally, most of the novels on the bestseller list are written by debut authors. Hence, most of the thrillers on the bestseller list must be written by debut authors.Answer
  4. D
    All of the birds in the sanctuary are tropical. Most of the birds in the sanctuary are colorful. Therefore, most of the tropical birds in the sanctuary must be colorful.
  5. E
    Most of the paintings in the gallery are landscapes. All of the landscapes in the gallery are oil paintings. Thus, all of the paintings in the gallery must be oil paintings.

Answer

The correct answer is the option stating that most of the novels on the bestseller list are thrillers and most are written by debut authors, concluding that most of the thrillers on the list must be written by debut authors.
The correct answer exactly replicates the flawed logical structure of the stimulus: 'Most X are Y, and most X are Z. Therefore, most X that are Y must also be Z.' In the stimulus, X is residents, Y is commuting by train, and Z is voting. In this option, X is novels on the bestseller list, Y is thrillers, and Z is written by debut authors. Both arguments erroneously assume that the intersection of two majorities must itself constitute a majority of the first subgroup.

Step-by-Step Solution

1
Analyze the premises and conclusion of the original argument.
Premise 1: Most A are B. Premise 2: Most A are C. Conclusion: Most of the A that are B, are C.
To find a parallel flaw, we must first abstract the exact structural pattern of the argument.
2
Evaluate the validity of the abstracted structure to identify the flaw.
The argument is flawed because two 'most' statements about a single group do not guarantee that a majority of the first subgroup belongs to the second subgroup.
For example, if 51% of group A are B, and 51% of group A are C, they might only overlap by 2%. Thus, we cannot logically deduce that 'most' of B are C.
3
Examine the answer choices to find the one that matches this specific flawed structure.
The option about novels and thrillers perfectly matches the 'Most A are B, Most A are C, therefore Most of A that are B are C' structure.
Parallel flaw questions require matching both the logical mechanics and the specific type of error committed, ignoring surface-level subject matter traps.

Key Concept

Identifying and matching flawed quantifier logic (specifically involving overlapping 'most' statements).
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Topics

Parallel Flaw

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