Logical reasoning PrepTest 143 · Section 3 · Question 6

Incorrect. Without knowing if these varieties differ in their indioside D levels, this answer doesn't affect the argument.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen

Stimulus Summary:
Phenomenon Ð Hornworms that first eat nightshade eat nothing else; if they first eat anything else they'll eat a variety of plants.
Hypothesis Ð Taste receptors get habituated to a nightshade chemical

Answer Anticipation:
This argument falls into a common pattern on the LSAT—a phenomenon is noted, and an explanation is concluded/hypothesized. Since scientific hypotheses are generally causal in nature, these arguments almost always feature a correlation/causation flaw—as this one does. A certain chemical is correlated with certain behavior in hornworms, and it's concluded that this chemical (and the taste buds it comes into contact with) are the cause of the behavior.

This is flawed for the normal correlation/causation reasons, and so, in a Strengthen question, we have common correct answers:

(1) Eliminate an alternative cause (here, an alternative explanation)
(2) Find more examples of the relationship (other animals that behave similarly when eating nightshade)
(3) Find a control group (the cause is absent and so is the effect)
(4) Explain how the causal relationship works

Any of these can serve as a correct answer, though alternative explanations tend to be featured more heavily in phenomenon/explanation arguments.

Answer Explanation:
This answer creates a "control" of a sort—removing what's hypothesized to be driving the behavior to see if the behavior stops. If it does, then there's a good chance that that thing is, in fact, driving that behavior. Here, the taste buds are what are hypothesized to be driving the behavior, so removing them and seeing the behavior stop is a good sign that the hypothesis is correct.

Key Takeaway:
Phenomenon/explanation passages are one of the most common structures on the LSAT. They're generally causal, and so you can usually use the strategies for correlation/causation flaws when dealing with them. The correct answers tend to bring up alternative causes/explanations more than other types of arguments, but, as we saw here, that isn't universal!

Incorrect. While this answer choice may establish that there's a strong likelihood that a hornworm will eat nightshade first, it doesn't establish a relationship between a chemical in the plant/taste receptors and the behavior of only eating nightshade, so it doesn't strengthen this argument.

Incorrect. If anything, this weakens the argument by suggesting an alternative cause of the behavior—one of these other chemicals might be the real culprit.

Incorrect. First, this answer doesn't establish that these reactions can lead to a hornworm preferring only that plant. Second, it doesn't establish that indioside D is one of these chemicals. Third, in raising other chemicals that would have this effect, this answer, if anything, is raising alternative explanations.