LSAT PrepTest 43 Section 3 Question 17 Explanation | Logical Reasoning

I didn't write the following blog post. It was already on the blog when I took over the URL. The following blog post may contain mistakes. -Steve

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Here's a Logical Reasoning question from the June 2004 LSAT.

Here's a short chain of phrases (don't write it down on actual tests):

Mutations favored only if help survival --> Good affects must outweigh bad ones.

We cannot really pre-phrase here, so we'll just go through the choices, keeping in mind that we're looking for a choice which is not possible given the argument's statements:

A) This is out of scope and so it may be true, and we can eliminate it. It doesn't address what the argument's talking about; the argument talks only about what it takes for a mutation to be favored, not about traits in general.

B) Out of the argument's scope and so it may be true. The argument says only that good effects must outweigh the bad ones for a mutation to be selected for...it says nothing about whether all the effects can be good. So, this choice may be true, since the argument doesn't rule it out.

C) Wrong for the same reason as A; the argument's talking about what it takes for a mutation to be selected for. It doesn't say anything about whether traits in general.

D) Outside the argument's scope and thus possible, so we can eliminate it. The argument says only that negative effects must be outweighed by positive one. It never specifies how many negative effects a mutation can have.

E) This says exactly the opposite of what the argument does, so it cannot be true, and so it's actually the right answer in this case (since we're looking for a choice that can't be true). The argument says that positive effects of a mutation must outweigh the negative ones (in other words, there must be a net benefit) for the mutation to be selected for. If all the traits are neutral, there is no net benefit, so that mutation can't be selected for.

Take-home points:

1) Use a chain of phrases to understand how the argument works.

2) Eliminate choices that are out of scope in this case because if they aren't directly addressed by the argument, they might be true (since the argument's statements don't rule them out). In this case, because of the "except" in the question, a choice that is the opposite of what the argument says is correct, since it directly contradicts the argument and cannot be true, which is what we're looking for.