October 1991 LSAT Section 2 Question 18

Learning how to build a nest plays an important part in the breeding success of birds. For example, Dr. Snow has reco...

Aley on August 27, 2019

Explanation of Answer/Correct My Thinking

I interpreted the stimulus as following: Birds nesting for the first time are less successful than older birds. Birds nesting for the second time are less successful at breeding than the first time they breed. It is unclear what qualifies as an older bird - but I think we can say third time onward. Size and Strength is not a reason that affects breeding ability because they are full grown (assuming full grown = size and strength) when they leave the nest. The conclusion is that through nesting experience leads to a higher probability of breeding. The question is looking for a weaken argument so: A) Not sure if this is relevant. B) Laying viable eggs increases probability of breeding with each trial. This agrees with the conclusion so I am not sure why this is the answer. C) The probability of mating the second time is greater than the first. This directly disagrees with first part of my interpretation of the answer and disqualifies that you benefit each time from experience as second time you would be more experienced than the first. D) Smaller and weaker birds breed just as well. The stimulus suggests that this not the answer and thus this Strengthens it. E) Not really relevant as the stimulus pertains to the first nest onward and not pre first nest. Can you please explain why the answer is B and where I went wrong in my line of thinking in interpreting the answer/stimulus? Thanks ahead of time.

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Irina on August 27, 2019

@Aley,

The stimulus is saying that birds nesting for the first time are less successful in breeding than birds nesting for the second time. It looks like you might have misinterpreted this sentence "and also less successful than they themselves are a year later" to mean that birds are less successful in their second year, but this part refers to "the birds nesting for the first time" part of the sentence, meaning the birds nesting for the first time are less successful than the same pair in their second year. The stimulus then concludes that because breeding success appears to correlate with the nesting experience, nesting experience must improve blackbirds' breeding success.

This is a weaken question, meaning the correct answer choice provides an alternative explanation or questions one of the premises to undermine the conclusion.

The author argues that A causes B, i.e. nesting experience causes breeding success. The correct answer choice (B) demonstrates that there is a confounding variable - blackbird's age, the older the bird is, the higher their reproductive rate/ breeding success is and the more skilled they are at building nests. Since the bird's age explains the breeding success and the nesting experience, we can no longer definitively conclude that A causes B.

Let's look at the other choices:

(A) is irrelevant. We are only interested in blackbirds, not other species;

(C) is wrong because it strengthens the argument;

(D) is irrelevant because the author argues his conclusion is true even when controlling for size and strengths. -"[t]his cannot be a mere matter of size and strength," meaning we are not interested in the breeding success of weak v. strong blackbirds but rather birds with more nesting v. less nesting experience;

(E) is irrelevant and has no impact on the validity of the conclusion.

Let me know if you have any further questions.

ajcaviness on June 1, 2022

It seems to me that B strengthens the argument because wouldn't each successive trial during the first few years imply that the bird is getting more nesting experience, thus agreeing with the argument?

Emil-Kunkin on June 7, 2022

Hi Ajcaviness,

(B) Suggests an alternative cause for the observed phenomenon. The author thinks that nesting is a major part of breeding success, and supports this claim on the grounds that breeding success becomes more likely as birds age, so the greater experience with building a nest must be the cause.

B suggests that birds simply lay "better"eggs as they age. This has nothing to do with nesting but it would explain the fact that breeding success increases with age- because older birds lay better eggs.