Question

All intelligent people are nearsighted. I am very nearsighted. So I must be a genius. Which one of the following exhibits both of the logical flaws exhibited in the argument above? (A) I must be stupid because all intelligent people are nearsighted and I have perfect eyesight. (B) All chickens have beaks. This bird has a beak. So this bird must be a chicken. (C) All pigs have four legs, but this spider has eight legs. So this spider must be twice as big as any pig. (D) John is extremely happy, so he must be extremely tall because all tall people are happy. (E) All geniuses are very nearsighted. I must be very nearsighted since I am a genius.

Short answer

Option D

Step-by-step solution

Identifying Logical Flaws

The argument contains two logical flaws: (1) assuming that because one shares a trait common to a group, one must belong to that group. (2) incorrect causation, implying that nearsightedness causes genius.

Analyzing Option A

Option A states that all intelligent people are nearsighted and concludes one must be stupid if not nearsighted. This implies causation and assumes a shared trait implies group membership, similar flaws to the original.

Analyzing Option B

Option B suggests that since all chickens have beaks and a bird has a beak, the bird must be a chicken. This mirrors the flaw by assuming a shared trait indicates group membership, but lacks the flaw of incorrect causation.

Analyzing Option C

Option C states all pigs have four legs and a spider has eight legs, leading to the conclusion that the spider is bigger. This does not match the logical flaws of membership by shared trait or incorrect causation.

Analyzing Option D

Option D concludes someone is tall because they are happy, based on the premise that all tall people are happy. This matches both logical flaws: assuming a shared trait implies group membership, and suggesting causation from happiness to tallness.

Analyzing Option E

Option E states all geniuses are nearsighted and implies someone must be nearsighted if a genius. This reflects a causation flaw but assumes correct membership, unlike the original argument.

Conclusion

Option D is the only one exhibiting both logical flaws: attributing a group membership based on a trait and implying incorrect causation.

Key concepts

Logical Flaws

Logical flaws in arguments are errors or missteps in reasoning that undermine the logic of the conclusion. They can make an argument seem convincing when it is not. Understanding these flaws is crucial for LSAT logical reasoning questions.

A common type of logical flaw is assuming that because someone has a trait, they necessarily belong to a group that shares this trait. For example, just because all intelligent people are nearsighted, it doesn't follow that someone who is nearsighted is intelligent. This misstep reverses the relationship and fails to recognize that many groups may share a trait without being identical.

• Example: Thinking all swans are white does not mean all that is white must be a swan.
• Another Example: Since basketball players are tall, anyone tall must be a basketball player is a flawed logic.

This logic is faulty as it ignores the possibility of shared characteristics not indicating exclusive group membership.

Recognizing logical flaws helps to critically assess the strength of an argument and effectively answer questions on the LSAT logical reasoning section.

Incorrect Causation

Incorrect causation is another common logical flaw encountered in LSAT logical reasoning. It involves wrongly assuming that because two events occur together, one must cause the other. Such reasoning can lead to incorrect conclusions.

The classic fallacy here is correlation does not imply causation. Just because nearsightedness and intelligence might co-occur doesn't mean one causes the other. To clear this up, it's vital to test if there's a real causal link between the two, which often requires more information. Here are some helpful considerations:

• Consider possibility of a third factor causing both events.
• Question whether it's plausible for one event to directly cause the other.

Improperly attributing causation not only leads to incorrect reasoning in test answers but can also perpetuate misconceptions in everyday thinking.

Group Membership Assumption

Group membership assumption is a common logical misstep where it's believed that because a member of a group shares a characteristic, anyone with the characteristic must belong to that group. This assumption blurs the line between correlation and definitive group inclusion.

For example, you might think that if all geniuses are nearsighted, then being nearsighted means you are a genius. This misses the fact that many others could share this trait and not belong to the specific group. Here are some tips to tackle group membership assumptions:

• Check if the characteristic is exclusive only to the group.
• Consider other potential reasons for the shared trait.

Using these strategies, you can identify when assumptions about group membership are unfounded. This critical thinking is particularly valuable in solving LSAT logical reasoning problems, ensuring you understand the core of each argument correctly.