Logical ReasoningDifficulty: Medium

PT4 S4 Q23 Explanation

The brains of identical twins

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsNecessary Assumption

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Stimulus

The brains of identical twins are genetically identical. When only one of a pair of identical twins is a schizophrenic, certain areas of the affected twin’s brain are smaller than corresponding areas in the brain of the unaffected twin. No such differences are found when neither twin is schizophrenic. caused by damage to the physical structure of the brain.

What this question is testing

Necessary Assumption

Your task

Find the assumption the argument requires in order for its conclusion to hold.

Common trap

Answers that would help the argument but aren't strictly required (sufficient, not necessary).

Winning move

Negate each choice — the right one breaks the argument when negated.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
23.

Which one of the following is an assumption required by

Answer choices

  1. Trap8% picked this

    The brain of a person suffering from schizophrenia is smaller than the brain of anyone

  2. Correct76% picked this

    The relative smallness of certain parts of the brains of schizophrenics is not the result of schizophrenia or of

    Why this is right

    Answer B is correct.

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

  3. Trap6% picked this

    The brain of a person with an identical twin is no smaller, on average, than the brain of a person

  4. Trap9% picked this

    When a pair of identical twins both suffer from schizophrenia, their brains are

  5. Trap2% picked this

    People who have an identical twin are no more likely to suffer from schizophrenia than

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