LSAT 4, Logical Reasoning II, Q23, LSATHacks

LR Question 23 Explanation, by LSATHacks

QUESTION TEXT: The brains of identical twins are genetically identical…

QUESTION TYPE: Necessary Assumption

CONCLUSION: This fact about twins’ brains provides definitive evidence that schizophrenia is caused by damage to the physical structure of the brain.

REASONING: The brains of identical twins are generally identical except when one twin has schizophrenia. Then certain areas of the schizophrenic twins’ brain are smaller.

ANALYSIS: Is being smaller the same thing as physical damage? And does the smallness cause schizophrenia or does schizophrenia cause the smallness?

___________

1. Not necessary. This is a very broad statement “the brain of anyone (even the tiniest human in the world) not suffering from schizophrenia.” There could be people with small brains who don’t have schizophrenia.
2. CORRECT. If schizophrenia or the medications cause the brain smallness then it is no longer possible to say that the smallness caused schizophrenia. So the argument proves nothing.
3. It doesn’t matter if they’re smaller. What matters is whether the brains of identical twins are the same as compared to each other.
4. Not necessary. Their brains could shrink in different ways that still cause schizophrenia in each twin.
5. We’re concerned with what causes schizophrenia, not how likely people are to have it.

Recap: The question begins with “The brains of identical twins are genetically identical”. It is a Necessary Assumption question. Learn how to master LSAT Necessary questions on the LSAT Logical Reasoning question types page.

Hi, I'm Graeme Blake

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