Logical ReasoningDifficulty: Hard

PT106 S1 Q21 Explanation

Psychologist: Some astrologers

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

TopicsNecessary Assumption

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Stimulus

Psychologist: Some astrologers claim that our horoscopes completely determine our personalities, but this claim is false. I concede that identical twins—who are, of course, born at practically the same time—often do have similar personalities. However, birth records were examined to find two individuals who were born 40 years ago on the same tests revealed that the personalities of these two individuals are in fact different.

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
21.

Which one of the following is an assumption on which the psychologist’s

Answer choices

  1. Irrelevant Distinction: subjected to rigor2% picked this

    Astrologers have not subjected their claims to

    When Necessary Assumption answers rule out an idea with "not / no", we should carefully consider them, since many correct answers are written that way. If we negate this and say, "Astrologers have subjected their claims to rigorous experimentation", does that weaken? No. It would weaken if we knew that the astrologers' claims were subjected to rigorous experimentation and were verified by that experimentation. But simply being subjected to rigorous experimentation doesn't weaken the argument.

  2. Opposite, if anything23% picked this

    The personality differences between the two individuals cannot be explained by the cultural differences between

    The author thinks that horoscopes don't determine our personalities. He probably thinks that other stuff determines our personalities -- our family members / our friends / our environment. So, it's very likely that the author thinks that cultural differences between Toronto and New York are part of what explains the personality differences between the two individuals. If we negated this answer and said, "the personality differences can be explained by the cultural differences between the two different cities", that would just strengthen the author's argument. That would show that personality differences were not completely explained by horoscopes.

  3. Correct70% picked this

    The geographical difference between Toronto and New York did not result in the two individuals

    Why this is right

    Again, we have the lovable ruling out "not / no". If we negate this, it's saying: The geographical difference did result in them having two different horoscopes. That unravels the whole argument. Our author can only prove that "horoscopes don't completely determine personality" by providing an example where people had the same horoscope, but different personalities. If these two individuals had different horoscopes, then of course they had different personalities! (the astrologers would say)

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

  4. Too Strong: complete1% picked this

    Complete birth records for the past 40 years were kept at

    We don't need for both hospitals to have kept records of 100% of babies over the past 40 years. If one of them only kept records for 99.9% of babies over the past 40 years, would that weaken this argument? No. As long as the birth records on these two individuals are correct (same date / same time), the author has all he needs from the hospital records.

  5. Irrelevant Distinction4% picked this

    Identical twins have identical genetic structures and usually have similar

    The remark about identical twins is a concession to astrologers. It plays no role in the author's argument, so the author isn't making any assumptions about identical twins. Would it matter if identical twins were only 99.9% genetically similar? No, that wouldn't do anything to the author's argument. Would it matter is identical twins usually have fairy distinct home environments? Nope.

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