GMAT Critical Reasoning: Weaken Questions

The flipside of the “strengthen” question coin on the GMAT is the “weaken” question. The same set of evidence and conclusion can easily be used for either question type, so it’s always a good idea to start by reading the question itself first, to determine the task before you go to the argument. After that, your approach is almost identical to the way that you would approach a strengthen question. Take this question, for example:

You should always read the question first, and here the commonly-used phrase “calls into question” means that this question is asking for a weakener.
Looking at the argument, we can see from the clue word “therefore” that the conclusion is the last sentence of the argument: “if Healica remains in production, the banananut tree will eventually become extinct.” Now our job is to find a way to attack the argument, and the easiest way to do that is to identify an existing weakness and exploit it.
Here, as in many arguments on the GMAT, the argument has an unstated assumption. The evidence states that the banananut tree is “rare in New Zealand,” and concludes that extinction will occur because of that. But for something to be extinct, it must not exist anywhere in the world, and we only have evidence about the tree’s growth in one country. The unstated assumptions are that the tree doesn’t grow anywhere else, and that growth can’t keep up with the demand for the trees. A great way to weaken the argument, then, is to attack one of those assumptions.

a) The company that holds the patent to Healica has exclusive rights to produce the drug for another 10 years.
This choice would require another assumption in order to weaken the conclusion: that the one company producing Healica would not produce enough of it to kill off the banananut tree. Weakeners, like strengtheners, shouldn’t require so much work to fit into the argument.
b) Healica is expensive, and is not currently covered by most major insurance plans.
This choice, like choice a), is not closely enough related to the argument.
c) Banananut leaves are considered a gourmet delicacy in many parts of the world.
This choice makes it more likely that the trees will be in demand and will face extinction. The argument is strengthened, not weakened, by this choice.
d) The banananut tree, although native to New Zealand, can easily be grown in other parts of the world.
This is the correct answer, since it exploits the unstated assumptions by explicitly disproving that one of them.
e) Producing Healica is time-consuming and expensive for the drug manufacturer.
This choice has the same problem as choices a) and b): it takes extra assumptions to fit it into the argument, and that’s not what we want.
If you find yourself taking several extra logical steps to explain how an answer choice relates back to the argument and makes the conclusion less likely to follow from the evidence, then the answer choice is probably not close enough to the text of the argument as written. The test is written so that each question belongs in its own little world, and outside knowledge is rarely relevant to finding the correct answer.
 

Remember

Stay close to the internal logic of the argument, and don’t bring in specialized outside knowledge!

 
Remember that the correct answer just has to make the conclusion less likely to follow from the evidence; it doesn’t need to completely disprove the conclusion, although sometimes it will. So make sure that you’re not looking for unnecessarily extreme answers.