In the previous two posts, we saw how Weaken-Type Critical Reasoning questions based on correlation-causation passages can be solved in under a minute.

Just to summarize, X and Y are correlated does not mean X is causing Y since

• there is no evidence to prove that the direction of causation is from X to Y, it can also be from Y to X

So based on arguments that conclude that X is causing Y since X and Y are correlated, Assumption and Strengthen questions can also be asked.

Assumption Type: The assumption is that Y is not causing X.

A researcher discovered that people who have low levels of immune-system activity tend to score much lower on tests of mental health than do people with normal or high immune-system activity. The researcher concluded from this experiment that the immune system protects against mental illness as well as against physical disease.

The researcher’s conclusion depends on which of the following assumptions?

(A) High immune-system activity protects against mental illness better than normal immune-system activity does
(B) Mental illness is similar to physical disease in its effects on body systems.
(C) People with high immune-system activity cannot develop mental illness.
(D) Mental illness does not cause people’s immune-system activity to decrease.
(E) Psychological treatment of mental illness is not as effective as is medical treatment.

In this particular study, researchers have found that immune-system activity (X) is closely related (correlated) with mental health (Y). Based on this they concluded that X is causing Y.

In doing so that the researchers have assumed that Y is not causing X. So, all one needs to do is identify the option that shows the same, in this case option D; the rest of the options can be eliminated straightaway.

Suppose we have to strengthen the researchers’ claim, we again have to choose the option which says that Y is not causing X.

So for both Assumption and Strengthen question based on correlation-causation we have to choose that option that says there is no reverse causation or in other words Y does not cause X.

All the questions discussed on these posts are from the GMAT Official Guides and the GMAT® Prep Software.