PrepTest 110

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Study — Lab animals fed a reduced—calorie diet had longer lives than those fed normally

Conclusion 1 — Reduced—calorie diets extend lives

Rebuttal — Lab animals are generally overfed, and so restricting their calories just brings them back to normal, and thus brings their life expectancies back to normal
Main point — Conclusion 1 is not supported by the study

Answer Anticipation:
This argument is much more complicated than the normal argument we see based on a study. It has a study and a conclusion drawn from it, but the main point of the argument ends up saying that that conclusion isn't warranted.

Why isn't it warranted? To figure that out, we need to take a look at a common type of logic that shows up here—comparisons.

The Dietary Researcher's rebuttal focuses on differences between lab animals and animals in the wild. Namely, lab animals generally eat more than their wild counterparts, and thus the caloric restrictions in the study didn't decrease the normal amount of food they'd eat—it just brought it down to normal. As such, the study didn't actually show that restricting calories increases longevity—what it showed was that overfeeding might decrease it.

Interestingly, at this point, we need to consider the other comparison in the argument—that between lab animals and humans. The main point of the argument is that the conclusion that North American's can live longer by eating a reduced—calorie diet is wrong. How does that relate to what the author says about lab animals? It's a "matching" conclusion. She argues that we can't determine that reduced calorie diets make lab animals live longer because the diets in question aren't really reduced—calorie diets—rather they're normal diets. Since she reaches the same conclusion about humans as lab animals she's relying on these two groups being similar in relevant ways.

And since the relevant considerations she brought up about the lab animals vs. the wild animals is that the former were regularly eating more than what they would normally or should have been eating and the longer—living ones were eating what they should have been eating all along the argument assumes that humans match up with the longer—lived ones. Both of those groups wouldn't benefit from a calorie restricted diet thus justifying the conclusion. So we can weaken this argument by showing that most North Americans are eating more calories than they should thus making them more similar to the "regular" lab animals that are overfed and thus would live longer if they ate less.

Answer Explanation:
This answer makes the North Americans similar to the overfed lab rats—the ones that did benefit from eating fewer calories. As such it weakens the argument that the North Americans wouldn't live longer if they ate fewer calories by relating them to a group that did see that effect from a calorie—restricted diet.

Key Takeaway:
Comparisons can work in weird ways in arguments so while there are tendencies these tendencies break down at times. For example in general arguments that rely on similarities are strengthened by similarities and weakened by differences. Here however it was a little more complicated than that because there were multiple points of comparison.

What's true of all comparison—based arguments though is thinking through what elements are compared and what if anything is wrong with those comparisons will help you find the correct answer.

Incorrect. Since both groups are eating low—calorie diets, this answer can't speak to whether they'd live longer by switching to one. While this answer may establish fat as a relevant factor in life expectancy, it does nothing to speak to calories as a factor.

Incorrect. While an argument that calls into question the relevance of lab animals in drawing conclusions about humans could be correct here, it would need to be stronger than this answer that says, essentially, some studies aren't based on animal studies. We'd need an answer stating that studies on the diets of lab animals tend to provide results that don't translate to humans for an answer like this to affect the argument.

Incorrect. Such a weak correlation can't affect the argument. We'd need to know about more than "[s]ome" North Americans to affect an argument about population—wide tendencies.

Incorrect. "Diet" is too broad here as it doesn't speak specifically to calories. Also "some" species is too weak as it doesn't necessarily include humans.