Logical reasoning PrepTest 135 · Section 2 · Question 19

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Theory - Drug overdoses killed the dinosaurs
Correlation - Plants with a drug showed up around the same time that the dinosaurs became extinct
Mammals vs. Dinosaurs:
(1) Mammals didn't eat the plant because it was bitter; Dinosaurs couldn't taste the bitterness
(2) Mammals can detoxify the drug with their livers; Dinosaurs couldn't
(3) Strongest support - Dinosaur fossils were found contorted

Answer Anticipation:
This is a great question. Not just because the content is crazy, but because it can teach a lot about several common concepts on the exam and how the writers of the test think about how to weaken an argument featuring them. So let's start unpacking it.

At the highest level, this argument has two common types of logic, both of which we should analyze.

The first? Almost all theories are causal in nature, and this one is no exception—drug overdoses are the "cause" of dinosaur extinction. Is that based on a correlation? It's easy to miss at first because the information establishing it seems like background, but it is—the plants containing the drugs that supposedly wiped the dinosaurs out "first appeared" when the dinosaurs became extinct. So we have a causal conclusion based on a correlation.

The second? The comparison with mammals. Let's start by figuring out why the argument compares dinosaurs with mammals. It talks a lot about the effects of the plants on dinosaurs and mammals, and how the mammals avoided the plants/drugs or wouldn't be affected as dinosaurs were. What's that have to do with the conclusion? Well, remember one way to strengthen a causal relationship—show a "control" group, or a situation that lacks the cause and the effect. Since the argument wants to show that the drugs caused the extinction, showing a class of animals that didn't eat or wouldn't be affected by the drugs that didn't go extinct reinforces the causality. This comparison is meant to strengthen the relationship by showing animals alive at the time that didn't have the cause and didn't have the effect.

That comparison, however, is incomplete in a key way—the "strongest" support for the theory is the contorted dinosaur fossils. The argument is stating that the contortions are caused by dying while "tripping" on these psychoactive substances. But the argument doesn't talk about how mammal fossils are found.

To bring the correlation/causation and incomplete comparison together, then, this argument would be weakened by finishing the comparison but with a difference instead of a similarity. If mammals are also found in the contorted position, that brings up a relevant similarity that weakens this argument that is based on mammals and dinosaurs being different. And this serves as a common weakener—since the argument assumes the drugs were the cause of the contortions, showing mammals who didn't die of drugs also have contorted fossils would be a counterexample (effect without cause).

Answer Explanation:
This answer establishes a similarity between mammals and dinosaurs that serves as a counterexample to the causality assumed in the "strongest" evidence for the conclusion, thus weakening the argument.

Key Takeaway:
There's a lot to be unpacked here about how correlation/causation and comparisons work on the LSAT. For that reason, this is one of our favorite questions. If you really understand everything going on in this question, you're rock solid on some of the trickiest concepts on the exam.

Additionally, while rare, the LSAT occasionally tells you that something is the "strongest" or "best" evidence. The correct answer has always addressed that evidence.

Incorrect. The amount of nutrition they provide is out of scope, as they also provide psychoactive substances, and that's the relevant consideration here.

Incorrect. This answer strengthens the argument by explaining how even dinosaurs that didn't eat the angiosperms could still die from them, by eating drug-filled herbivores.

Incorrect. But these ones do, so these other plants are out of scope.

Incorrect. This answer can, at first look, appear to serve as some type of counterexample. However, think about the conclusion—angiosperms wiped out the dinosaurs. If they also kill mammals with their taste buds and livers, that's just evidence of how deadly the angiosperms can be, strengthening the argument.