LSAT 91 - Section 2 - Question 20

This is an Inference question.

The question stem says “properly inferred.” This is a challenging question because there is so much information in the stimulus that is all connected. That can easily induce panic as you scramble to draw all the connections and valid inferences. Strategically, you shouldn't do that. The more connected information a stimulus contains, the more valid inferences there are to be drawn, the less you are able to anticipate the correct answer choice. In stimuli like those, POE is the better approach.

The first line in the stimulus about the study of primates being interesting is the only irrelevant fact. Everything else is fair game.

Only primates have opposable thumbs. Lemurs are lower primates (a subset of primates.) And lemurs are the only primates indigenous to Madagascar. Some species of lemurs are the only living lower primates that are diurnal. They go ahead and define diurnal for us but the answer choices never swapped out the term for its definition so we don't need to pay attention to it. Finally, all higher primates (a subset of primates) are thought to have evolved from a single diurnal species of lower primates.

Lots of information. Let’s POE.

The last piece of information is what sets up the trap in Answer Choice (A). It says that the chimpanzee, a higher primate, evolved from the lemur. This is not a proper inference. This is unsupported. All we can say is that the chimpanzee, being a higher primate, is thought to have evolved from a single diurnal species of lower primates. Which one, though? Must it be the lemur because the lemur is a diurnal species of lower primates and it is the only living one? No. Being alive isn’t required. We’re talking about evolution here. Most ancestor species are extinct. There may well have been other extinct, diurnal species of lower primates. One of those extinct species may well be the evolutionary starting point of all higher primates.

Correct Answer Choice (B) says no primates indigenous to Madagascar are diurnal higher primates. We can transform this into the following logically equivalent claim: all primates indigenous to Madagascar are not diurnal higher primates. This must be true. The stimulus says that the only primates indigenous to Madagascar are lemurs and that lemurs are all lower primates. It is implied that lower primates cannot be higher primates, diurnal or otherwise. This is the conditional chain: prim-indig-M → lemur → low-prim

Answer Choice (C) says no higher primate is nocturnal. This is unsupported. We simply have no idea if higher primates are nocturnal or diurnal or anything else. The only piece of information we have about higher primates is that they are thought to have evolved from a single diurnal species of lower primates.

Answer Choice (D) says there are some lemurs without opposable thumbs. This is unsupported. The stimulus says only primates have opposable thumbs. That means if something is not a primate then it doesn't have opposable thumbs. But lemurs are primates. Sufficient condition failed, rule goes away.

Answer Choice (E) says there are no nocturnal lemurs. This is unsupported. The stimulus tells us that some species of lemurs are diurnal. Maybe all species of lemurs are diurnal, maybe not.