Logical reasoning PrepTest 152 · Section 1 · Question 8

Tempting answer. One might think that their inability to detect the parasites undermines the author's conclusion that migrating allows them to avoid the parasites. However, if the monarchs accidentally avoid the parasites through migrating, that works just as well for the argument as if they intentionally flee the areas with parasites.

Since the argument doesn't mention any length of migration, or connect the parasites to migration length, this answer choice is outside the scope of the argument.

The premise about relative infection rates deals with just that—rates. The number of butterflies in the populations doesn't impact the rate of infection. This jump between percent and overall amount is a common trap the LSAT will lay for you!

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Monarch butterflies have to deal with some gnarly parasites that stop them from migrating. Comparing non-migrating to migrating populations, we see that 95% of non-migrating monarchs are infected, compared to 15% of migrating monarchs. The author concludes that migrating allows monarch to avoid infection.

Strategy Overview:
Once created for a question type, we can just reuse that for future questions.

Answer Anticipation:
Any argument that relies on comparing two groups needs a deeper analysis of these two groups. Here, the author tells us that the groups are different—one has never migrated, the other migrates. So it seems as if migrating has something to do with the infection rate (though that's not guaranteed). But does it have to be what the author states?

Since this is an Errors in Reasoning question, no! There must be another explanation as to why we see this difference in parasite infection between the non-migrating and migrating group.

Knowing that is enough of an anticipation, but you could go a step further and think about some alternatives. One is that the author has it backwards—migrating doesn't cause safety from infection; infection causes the butterflies to not migrate. This makes sense since we've been told the parasites hurt the butterflies' ability to fly.

Answer Explanation:
This answer gets at the reversal ignored by the author. It's not that migrating prevents infection, but rather that infection prevents migration.

Key Takeaway:
When a conclusion settles on an explanation, consider alternative explanations. When a conclusion deals with causality, consider if the reverse makes sense. When an argument brings up percent, be careful of answers that deal with numbers or overall size.

Unless you're a lepidopterist (which may or may not be the correct term for butterfly scientist), you can't say whether the parasites are related to a food source. This answer choice suggests an alternative reason for the monarchs not to migrate, but it doesn't connect that to parasitical infection, and so it's outside the scope of the argument.