Necessary Assumption Question: The Rattlesnake Folktale

LSAT Blog Necessary Assumption Question Rattlesnake FolktaleLSAT Logical Reasoning Question Types: A New Approach started off this series. This is the second part, where I go into a particular example.

Let's look at one of my favorite Logical Reasoning questions: the Rattlesnake Folktale question. It's PrepTest 30 - December 1999 LSAT, Section 2, Question 22 - page 60 in Next 10.

We know this is a necessary assumption question because it says "which one of the following is an assumption the argument requires?"

I can't publish the full text of the question due to copyright law, but I can give a general summary of the argument and correct answer choice.

The argument describes a rattlesnake age folktale. According to this folktale, you can determine a rattlesnake's age by counting the number of sections on its rattle. This is because the rattlesnake forms a new section on its rattle each time it molts. The argument claims this rattlesnake age folktale doesn't work only because rattle sections break off due to their brittleness. It then concludes that if the rattles were not so brittle, the rattlesnake age folktale would work fine.

The question then asks for a necessary assumption. The correct answer tells us that food availability does not affect the molting rate. If food availability did affect the molting rate, then you could have two rattlesnakes, one that's had a lot of food in its life, one that's had very little food in its life, and they'd appear to be different ages.

As such, the claim that the rattles' brittleness is the only thing stopping the rattlesnake age folktale from being valid is making a large assumption. The argument assumes nothing else also needs to be true in order for the rattlesnake age folktale to be valid.

Therefore, the argument depends upon this assumption in order to be valid.

Must Be True
This answer choice could have also served as the correct answer choice to a Must Be True question. It needs to be true that food availability doesn't affect the molting rate in order for the argument to be valid (more on Necessary Assumption and Must Be True questions).

Must Be False
Because this large assumption must be true in order for the argument to work, the negation of this answer choice (the denial of this assumption) cannot be true for the argument to be valid, so it must be false that food availability affects the molting rate.

The argument as it stands is not airtight, so it's possible to strengthen it. Again, it's claiming that all we need to do to make this rattlesnake age folktale work is remove brittleness as a factor. If we view the answer choices through the lens of providing new information (Which one of the following, if true, would most strengthen the argument?), what we previously viewed as a Must Be True can now be viewed as a strengthener.

The correct answer to what was originally a necessary assumption question also serves to strengthen the argument by dismissing the possibility that food availability affects the molting rate.

Of course, choice A also serves to strengthen the argument. In fact, it fully justifies the conclusion and serves as a sufficient assumption. It just didn't NEED to be true.)

An answer choice that strengthens the argument often does so by dismissing potential problems, alternative causes, or alternative explanations.

This is the case with our strengtheners above. If we negate an answer choice that would strengthen the argument, we are then weakening the argument.

As such, the negation of these answer choices would serve to weaken the argument.

Meaning that if we learned that food availability did affect the molting rate, that would weaken the argument. In fact, it would destroy the argument entirely. Similarly, if we learned that rattlesnakes did not molt exactly once a year, that would weaken the argument, but only a tiny bit

Evaluate the Argument
This is when we take a major strengthener or weakener and phrase it as a question or as a "what if?".

Question: Which one of the following would be most important to know in evaluating the conclusion drawn above?

Answer: Whether food availability affects the molting rate

If food availability affected the molting rate, that would weaken the argument
If food availability did not affect the molting rate, that would strengthen the argument

Resolve the Paradox / Discrepancy
Let's rephrase the stimulus by keeping the evidence the same but saying the conclusion did not logically follow. Something like:

"We genetically engineered rattlesnakes to remove brittleness as a factor, yet our top-secret Pentagon-funded rattlesnake age folktale still didn't reliably determine a rattlesnake's age."

How is this possible?

Well, if we learned that food availability affected the molting rate, that would explain why the rattlesnake age folktale still wasn't working.


Now go through the exact same process with another Logical Reasoning question. I propose PrepTest 30 - December 1999, Section 2, Question 15 (page 58 in Next 10).

Photo by zackbittner


  1. I'm loving the multi-question type analysis of the stimulus. I don't have book on hand at the moment, but I'm assuming you added the ONLY to make the point more transparent.

  2. Excellent analysis!

  3. Thanks for this blog -- it's been a lot of help preparing for the LSAT. But I'm having a hard time understanding why A isn't necessary. If snakes don't molt exactly once a year - let's say they molt 363-366 days apart - then how can we draw the conclusion that the sections are a reliable indicator of age? Wouldn't they necessarily be unreliable on occasion, even if only for a few days? To push it further, if we grant that the molting doesn't have to happen exactly once a year, why couldn't the variation in days between molting depend on food availability? In that case, neither A nor E would be necessary. What am I missing?

  4. Glad you're enjoying the blog!

    I explain this question's choices here:

  5. Thanks for the quick reply. I take your point about A on the other page to be that an approximately annual molting is good enough to call it a reliable index. But if we get some flexibility in recurrence, why can't the difference in molting caused by food variation fall within that range? For example, what if scarcity of food causes the snakes to molt every 366 days, whereas a glut of food causes them to molt every 363 days? We still have a reliable index of age without E being true. Which is to say, E is not a necessary assumption. Thoughts?

  6. Sorry, I retract my question. I didn't understand your implication that molting monthly or bi-monthly or whatever would do just as well as annually.

    Thanks again for the great blog.

  7. Think about it this way - how do we know the snakes have only a feast or a famine?

    Sure, food availability can be either of those two, but it can also be anything in between.

    Aside from that, when we find a snake in the wild, we don't know its food history. If food availability affects molting rate, but we don't know what amount of food has been available to the snake, we can't determine its age based simply on the number of rattles.

    Hope that helps.

  8. Hey Steve, this was great. I also did wonder why it wasn't A, but as soon as I saw your explanation, I immediately realized the interval between molting wasn't as crucial as their regularity not being affected by food availability.

    Just a quick note though - I think it's worth posting on blog that people have to do lots of questions 'to get a feel of right answer.' LG answers are straightforward, and for most LR Qs also. However, many RC and some LR Qs are, while completely logical, quite subtle (like this Q), and the only way one can really master them is to do many questions, all the while actively analyzing answers. Many people think that, like a common mistake with math or physics, knowing the fundamentals will automatically lead them to excel in Qs.

    Just a thought!

  9. This is gold! Thank you, Steve! :)