Q5 - The corpus callosum-the thick

[robowarren]

I was going back and forth between A and D, and actually had A selected but convinced myseld D was better. Would you mind explaining your reasoning for A?

[timmydoeslsat]

It is good that you kept answer A until the final decision.

It is so vital to remember that necessary assumption questions are asking what is an unstated premise that the argument needs to have a chance of being valid.

The stimulus boils down to this.

On average, the corpus callosum is larger in musicians than nonmusicians. That is a fact. We do not know of a cause though.

However, the argument concludes a cause! They conclude cause based on a correlation (relationship) between activities that musicians did since the age of 7 and those that did not.

The author believes this proves the cause. So we need to figure out how to "protect" this conclusion from alternative causes.

Answer A: The corpus callosum in musicians at the age of 7 do not tend to be larger than that of nonmusicians of the same age before training begins.

What would happen if we took that assumption away from the argument by negating it?

The corpus callosum in musicians at the age of 7 do tend to be larger than that of nonmusicians of the same age before training begins.

Well if they do tend to be larger than that of nonmusicians before training begins, how can we then conclude that it is the training that causes the larger corpus callosum.

This is hinting towards these children simply being born to love music and they have big corpus callosums already and the training is not causing anything.


Answer choice (D) is not necessary because we do not need all musicians to have larger corpus callosums than nonmusicians. The premise stated on average. It allows for some not to be.

[bbirdwell]

Good explanation, Timmy. I'll add this:

Remember to be sure that the answer you choose supports the precise wording of the conclusion. When down to two choices, it's always a good idea to re-affirm what exactly the conclusion is stating.

Here, the conclusion states that musical training (particularly at a young age) causes brain changes. Note that (D) is simply about "musicians," and not about "training." When you're unsure about two choices, a distinction such as this is good reason to lean away from (D) and towards (A).

[mitrakhanom1]

why would answer E be considered wrong?

[Dannyboy3D]

why would answer E be considered wrong?

I eliminated "E" because the conclusion is relying on an assumption about MUSICIANS...not non-musicians! Without directly saying "musicians", the conclusion claims that musical training causes brain changes. "E" is all about non-musicians, so just on that basis I would be cautious.

And as I read "E" I thought: If adult nonmusicians didn't have ANY growth of the "CC" (as stated in "E"), well isn't that directly going against the part in the stimulus where it says that the CC in musicians is ON AVERAGE larger than nonmusicians? To me, that statement in the stimulus leaves just enough wiggle room to infer that SOME nonmusicians have bigger CC's than musicians...and that goes against the claim in "E."

[christine.defenbaugh]

why would answer E be considered wrong?

I eliminated "E" because the conclusion is relying on an assumption about MUSICIANS...not non-musicians! Without directly saying "musicians", the conclusion claims that musical training causes brain changes. "E" is all about non-musicians, so just on that basis I would be cautious.

And as I read "E" I thought: If adult nonmusicians didn't have ANY growth of the "CC" (as stated in "E"), well isn't that directly going against the part in the stimulus where it says that the CC in musicians is ON AVERAGE larger than nonmusicians? To me, that statement in the stimulus leaves just enough wiggle room to infer that SOME nonmusicians have bigger CC's than musicians...and that goes against the claim in "E."

Dannyboy3D, I'm thrilled to see you flexing your LR muscles by helping lay out explanations! It's a great activity for pushing your own understanding to the next level.

I've got to warn you to be a little careful though, in your zeal to eliminate (E).

It is actually not a problem that (E) is about nonmusicians instead of musicians. Consider if (E) had said: Adult nonmusicians did not participate in activities when they were children that would have slowed the growth or reduced the size of the corpus callosum.

This answer, had it existed, would have absolutely been a valid necessary assumption. How can this be? The argument, on a very simple level, is this:

Premise: difference in size of X between two groups.
Conclusion: Must be because first group did thing to increase size.
Necessary Assumption: It wasn't because the second group did thing to decrease size.

Be very careful about assuming an answer choice is out of scope just because it uses a term or group not mentioned in the conclusion. "Nonmusicians" is entirely relevant to the argument!


(E) also does not contradict the premises. You're absolutely correct in seeing that the 'on average' leaves some possible wiggle room - but it is only possible wiggle room. It is possible, under the premise, that some adult nonmusician might have a larger CC than some adult musician - as long as the 'average' for nonmusicians is lower. But that doesn't mean there MUST be someone who fits that description!

So, (E) could theoretically be true. The real issue is that we don't need it to be true.

The most effective way to assess this is to try to negate it, and see if we can destroy the argument! The Negation Test is a powerful tool for just this circumstance. Negating (E) leaves us with:

Adult nonmusicians DID participate in activities as kids. that stimulated CC growth.

Does this destroy the conclusion? No! The conclusion is that musical training causes brain changes, but it doesn't claim that it's the only possible cause of brain changes. So an additional cause (whatever these nonmusicans would be doing) doesn't damage that claim at all! After all, even with whatever that activity was, the musicians' CC is still larger on average.

Since this answer negated does not destroy the conclusion, it cannot be a necessary assumption!

Please continue stretching yourself to explain why wrong answers are wrong! It's some of the best practice you can get!

[Dannyboy3D]

I agree with the above post, and in my mind I was doing ALL of that (believe me).

My response was meant to be relatively concise and to the point. I'm all for explanations; but I've read one-too-many posts on some of these threads that go on and on, and get enormously confusing. As such, I intend to be concise, but thoughtful in my explanations.

It's all a work in progress, of course.

[christine.defenbaugh]

I agree with the above post, and in my mind I was doing ALL of that (believe me).

My response was meant to be relatively concise and to the point. I'm all for explanations; but I've read one-too-many posts on some of these threads that go on and on, and get enormously confusing. As such, I intend to be concise, but thoughtful in my explanations.

It's all a work in progress, of course.

Concision is always appreciated when it is accompanied by accuracy! Unfortunately, you cannot validly eliminate (E) for the reasons you stated in your earlier post.

Keep testing your reasoning skills out here though, and soon you'll be throwing down concise and well-reasoned explanations right and left!

[WaltGrace1983]

Rehashed this old problem (loving the 4th Ed. "10 Real LSATs Grouped" by the way) and thought I would do some analysis on this kind of question.

On the LSAT, we see this kind of problem all too much. The argument assumes causation from mere correlation. In other words, we are given two things that are connected in some way (corpus callosum size and musicianship) and are told that one thing causes the other (that musicianship causes the corpus callosum size). The most important word in this question is "causes" and this is the key to unlocking the problem.

In a necessary assumption question, we have only one ways (I think) of understanding what is necessary in a correlation/causation flaw. We must:

Show that no other causes are possible. When asserting A causes B, this means...

(1) Show that B does not cause A
(2) Show that there is NOT something else that is causing both A & B
(3) Show that there is NOT something else that causes B

To my knowledge (and someone correct me if I am wrong) there is really no other way to do it. On strengthen/weaken questions, correlation/causation is the main flaw they test on so get really comfortable with some of these ideas above. For this question, let's focus on doing (1), (2), or (3)

(A) This seems to satisfy (3). Why? Because it shows that that there is NOT some other cause of the corpus callosum's size in musicians. (A) is saying that it is NOT mere genetics (i.e. that these corpora callosa were not just naturally bigger). If we negate (A), we get this: "The corpora callosa if musicians, before they started training, DO tend to be larger than those of non musicians." If we accept this as true, we cannot accept that it is musical training that causes the expanded corpus callosum size. Why? Because we see that the bigger size happened before musical training ever began! This looks pretty good so let's move on.

(B) We are talking about musical training early in life, first of all. Second of all, maybe it DOES cause anatomical changes to the brain but this doesn't have much to do with the corpus callosum unless we assume so.

(C) They do not have to be the "same size." We are talking about average sizes and thus we don't need to get so specific and, plus, this is incredibly narrow and altogether kind of ridiculous.

(D) We don't need to assume "ALL." Once again, we are talking about "averages" and so even the negation of this "all" (it would be "not all" by the way) would still fall right within line of our argument and wouldn't hurt it.

(E) We don't need to assume that it couldn't have stimulated ANY growth.

Hope the breakdown was helpful!