The brains of identical twins are genetically identical. When only one of a pair of identical twins is a schizophreni...

Julie-V July 19, 2019

(C)

Hi LSAT Max, I wasn't sure which answer to pick, so I chose (C) thinking that it was bridging the gap between the twins used in the study and the conclusion. If someone could kindly explain why that wasn't the correct approach, I would greatly appreciate it!

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Irina July 19, 2019

@Julie,

This is a necessary assumption question, meaning we are looking for an implicit premise that is required for our argument to follow logically. One approach to these questions is to look at each of the answer choices and consider which one would make the argument fail if it were not true, i.e try to negate the answer choice and think if the argument still makes sense.

Let's briefly look at the argument.

Pr 1: Brains of identical twins are identical.
Pr 2: Brain areas of the schizophrenic twin are smaller than the corresponding areas of the healthy twin's brain.
Pr 3: No differences are observed when neither twin is schizophrenic.
C: Schizophrenia is caused by damage to the physical structure of the brain.

Let's consider each of the answer choices:

(A) is irrelevant, the argument only talks about certain areas of the brain being smaller not the overall brain size;

(B) is the correct answer choice. Let's think about it for a moment. What if this statement is not true? What if the relative smallness of certain parts of the brains is a result of schizophrenia or medication used in its treatment? The argument is based on the assumption that physical structure of the brain causes schizophrenia, but if the observed physical structure is a result of schizophrenia or medication - notice how cause and effect are reversed -, our conclusion no longer makes sense.

(C) is irrelevant, we are only interested in certain areas of the brain in people suffering from schizophrenia, it makes no differences how brain sizes of twins v. non-twins are correlated;

(D) is irrelevant, even if it is true, it does not explain what causes schizophrenia. It only supports our premise that brains of twins are identical, but we are looking for a statement that is necessary for our CONCLUSION;

(E) is irrelevant. It is entirely out of scope, we are not interested in likelihood of schizophrenia based on whether one is a twin or not, the conclusion only concerns the physical structure of the brain.

Let me know if this helps and if you have any other questions.