Another tool for Logical Reasoning: the “assumption” question

LSAT

One of the trickiest types of questions in the Logical Reasoning section of the LSAT is what I call the “assumption” question. An “assumption” question gives you an argument, and then asks you which of the following choices is an assumption on which the argument depends. Although this seems like a straightforward thing to ask, students often have trouble differentiating between choices on which the argument depends, and choices that merely support the argument. 

If you ever find yourself struggling to make this distinction, don’t panic! Take a deep breath, and calmly and methodically go through the following steps: 

  1. Negate whatever that choice says.
  2. Pretend the negated statement is true, and ask yourself, “Is the argument’s conclusion still valid?”
  3. If you answer “yes”, then eliminate that choice because it’s not an assumption on which the argument depends. If you answer “no”, then it is an assumption and you have your answer! 

To help illustrate how this works in practice, let’s apply these steps to an example question that I’ve taken from the October 2002 LSAT:

In humans, ingested protein is broken down into amino acids, all of which must compete to enter the brain. Subsequent ingestion of sugars leads to the production of insulin, a hormone that breaks down the sugars and also rids the bloodstream of residual amino acids, except for tryptophan. Tryptophan then slips into the brain uncontested and is transformed into the chemical serotonin, increasing the brain’s serotonin level. Thus, sugars can play a major role in mood elevation, helping one to feel relaxed and anxiety-free.

Which one of the following is an assumption on which the argument depends?

(A) Elevation of mood and freedom from anxiety require increasing the level of serotonin in the brain.

(B) Failure to consume foods rich in sugars results in anxiety and a lowering of mood.

(C) Serotonin can be produced naturally only if tryptophan is present in the bloodstream.

(D) Increasing the level of serotonin in the brain promotes relaxation and freedom from anxiety.

(E) The consumption of protein-rich foods results in anxiety and a lowering of mood.

(A)

First, we negate the choice. The negation of this statement would be: “Elevation of mood and freedom from anxiety do not require increasing the level of serotonin in the brain.” This essentially means that there could be other ways, aside from increasing the level of serotonin in the brain, to achieve elevation of mood and freedom from anxiety. 

If we assume that this statement is true, is the argument’s conclusion still valid? Yes, it is: the argument concludes that sugars, by increasing serotonin levels, can play a role in mood elevation. But it does not say that this is the only way to achieve mood elevation. There might very well be multiple ways of achieving mood elevation; as long as one of these ways is the consumption of sugars and subsequent increase in serotonin levels, the conclusion of the argument is valid.

So we can eliminate this choice because it is not an assumption on which the argument depends.

(B)

The negation of this statement would be: “Failure to consume foods rich in sugars does not result in anxiety and a lowering of mood.” 

If we assume that this statement is true, the argument’s conclusion is still valid: the argument only concludes that the consumption of sugars can help to elevate mood and reduce anxiety. It doesn’t say anything about the effects of failing to consume sugars. For example, you could imagine that failing to consume sugars just keeps your mood at a steady state, without raising or lowering it. 

So we can eliminate this choice because it’s not an assumption on which the argument depends.

(C)

The negation of this statement is a little trickier than the last two; we can’t simply add a “not”. Instead, we have to consider the underlying meaning of the statement; its negation will be the opposite of that. This statement means that the only way to produce serotonin naturally is through the presence of tryptophan in the bloodstream, or, in other words, the presence of tryptophan in the bloodstream is required for the production of serotonin. Thus, this is actually quite similar to choice (A), so we can use a similar negation: “The natural production of serotonin does not require the presence of tryptophan in the bloodstream.”

If we assume that this statement is true, the argument’s conclusion is still valid. The reasoning is again similar to that which we used for choice (A). The argument is merely claiming that the presence of tryptophan can produce serotonin, not that it is the only way to do so. 

So we can eliminate this choice because it is not an assumption on which the argument depends.

(D)

The negation of this statement would be: “Increasing the level of serotonin in the brain does not promote relaxation and freedom from anxiety.”

If we assume that this statement is true, the argument’s conclusion is no longer valid. If increased serotonin levels don’t promote relaxation and freedom from anxiety, then there’s no evidence that sugars play a major role in promoting relaxation and freedom from anxiety. This is because, according to the argument, the main effect that sugars have is to increase the brain’s serotonin level. 

So then we have our answer, because this choice is an assumption on which the argument depends. If you were taking the test, I would encourage you to quickly check the final choice, just to make sure you can eliminate it as well. But hopefully going through the first four choices has sufficiently illustrated this method for you.

Final Thoughts

I know that probably seemed like a lot of work just to answer one question. But I am not suggesting that you apply this method to every “assumption” question that you encounter. Many times, you might be able to quickly pick out the answer upon reading the question. You should think of this method as just one of many tools in your toolbox for tackling logical reasoning questions, pulling it out only when you think it might be helpful. 

Sarah studied Physics at Harvard College before earning her Master's in Applied Mathematics from Columbia. She's now pursuing her JD at Harvard Law, where her interest is in intellectual property law.

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