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Difficulty: HardFlaw in the Reasoning

Cognitive Neuroscientist: A comprehensive study demonstrated that individuals who consistently obtain at least eight hours of sleep per night exhibit significantly higher rates of hippocampal neurogenesis than those sleeping fewer than six hours. Furthermore, elevated hippocampal neurogenesis is a prerequisite for achieving optimal memory consolidation. Therefore, if a student wants to guarantee that they achieve optimal memory consolidation, they need only maintain a consistent routine of eight or more hours of sleep each night.

Which one of the following most accurately describes a flaw in the cognitive neuroscientist's reasoning?

  1. A
    relies on a sample of research subjects that may be unrepresentative of the broader student population
  2. B
    presumes without justification that obtaining fewer than six hours of sleep makes hippocampal neurogenesis entirely impossible
  3. treats a factor that is necessary for achieving a desired outcome as though it were sufficient to guarantee that outcomeAnswer
  4. D
    rejects a potential explanation solely by disputing the credibility of the underlying observational data
  5. E
    infers that because inadequate sleep fails to stimulate neurogenesis, any increase in neurogenesis must be caused solely by sleep

Answer

The argument is flawed because it treats a factor that is necessary for achieving a desired outcome as though it were sufficient to guarantee that outcome.
The neuroscientist states that elevated hippocampal neurogenesis is a 'prerequisite' for achieving optimal memory consolidation, meaning it is a necessary condition (Memory Consolidation → Elevated Neurogenesis). However, the conclusion claims that obtaining eight hours of sleep (which provides elevated neurogenesis) is all one needs to 'guarantee' optimal memory consolidation (Sleep/Neurogenesis → Memory Consolidation). This improperly treats a necessary requirement as a sufficient condition.

Step-by-Step Solution

1
Analyze the stimulus argument structure
Premise 1: Consistently getting 8+ hours of sleep produces significantly higher hippocampal neurogenesis than <6 hours. Premise 2: Elevated neurogenesis is a prerequisite (necessary condition) for optimal memory consolidation. Conclusion: Sleeping 8+ hours is all that is required to guarantee (sufficient condition) optimal memory consolidation.
Deconstructing the argument into premises and conclusion is essential to map the logical relationship between the conditions.
2
Evaluate the logical connection between the premises and the conclusion
The author establishes that elevated neurogenesis is required (prerequisite) for optimal memory consolidation. However, having a necessary condition met does not mean all other requirements for memory consolidation are satisfied or that the outcome is assured.
Identifying the gap between necessity ('prerequisite') and sufficiency ('guarantee', 'need only') exposes the core formal fallacy.
3
Match the identified logical defect to the corresponding option
The statement describing the confusion of a necessary condition with a sufficient condition precisely matches the analytical vulnerability.
Standard LSAT flaw questions require isolating the exact description of the erroneous inferential leap.

Key Concept

Confusion of Necessary and Sufficient Conditions
Estimated Time:1m 45s
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Topics

Flaw in the Reasoning

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