CR Weaken: Why We Need to be Flexible

With Weaken questions, we need some flexibility with the answer choices, since there are several ways to weaken an argument:

-Provide an explanation
-Prove the assumption is wrong
-Undermine a piece of evidence

I find the easiest “default” for a Weaken prediction is to identify the Evidence, Conclusion, and Assumption of the argument, and then simply reverse the assumption.

The hard part is trying to do all of this in your head, so I HIGHLY suggest you do all CR work on your scratch pad. No need to write the entire argument down. You can easily summarize it with abbreviations and symbols. Let’s examine a question:

Microbiologist: A lethal strain of salmonella recently showed up in a European country, causing an outbreak of illness that killed two people and infected twenty-seven others. Investigators blame the severity of the outbreak on the overuse of antibiotics, since the salmonella bacteria tested were shown to be drug-resistant. But this is unlikely because patients in the country where the outbreak occurred cannot obtain antibiotics to treat illness without a prescription, and the country’s doctors prescribe antibiotics less readily than do doctors in any other European country.

Which of the following, if true, would most weaken the microbiologist’s reasoning?

A) Physicians in the country where the outbreak occurred have become hesitant to prescribe antibiotics since they are frequently in short supply.
B) People in the country where the outbreak occurred often consume foods produced from animals that eat antibiotics-laden livestock feed.
C) Use of antibiotics in two countries that neighbor the country where the outbreak occurred has risen over the past decade.
D) Drug-resistant strains of salmonella have not been found in countries in which antibiotics are not generally available.
E) Salmonella has been shown to spread easily along the distribution chains of certain vegetables, such as raw tomatoes.

Here, the evidence tells us a prescription is hard to get, so antibiotics aren’t to blame for the salmonella, even though the salmonella were drug-resistant.

The bad logic here is that the author is assuming there isn’t ANOTHER way that antibiotics could have caused the salmonella, just because doctors don’t give out prescriptions.

So, to weaken, let’s reverse the assumption: what if there was ANOTHER way that the antibiotics contributed to the salmonella that didn’t involve doctors???

Notice that once we’re crystal-clear on the argument, the correct answer is much easier to spot.

B) is saying the salmonella could have come from people eating animals who eat antibiotics. Maybe it transferred?

This is a solid option, and a good reversal of the assumption.

Many students, however, will eliminate (B) right off the bat, because they will think, “huh, I didn’t read anything about “animals that eat antibiotics-laden livestock feed.” But remember, Weaken questions can be exceptionally difficult, and since there is more than one way to weaken, we have to be open to the idea that the correct answer may introduce a detail that at first seems totally irrelevant, but after a few seconds reveals itself as something that destroys the conclusion.

The author said it is “unlikely” that antibiotics could be blamed BECAUSE of the doctors not giving prescriptions.

BUT, if the people are getting antibiotics from a totally different source, then yes, the antibiotics could still be blamed.

Here is how our analysis could have looked for this question on our scratch pad:


Remember to make a prediction for a “Weaken” question on your scratch pad, but ALSO be open to the idea that the correct answer might weaken the argument in an unexpected way!