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 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.

The correct answer is (A) I was not convinced with the explanation in Verbal Review. Is it the case to eliminate the alternate models of causation?

Thanks
GMAT 2007

The answer is D on this one, not A. I'm going to assume you know it's D and that was just a typo.

Yes, this one's about reverse causation. Specifically, the author notes a correlation between levels of immune system activity and scores on test of mental health, and then uses the mere correlation to conclude a cause-effect relationship. Just because milk and cereal are correlated doesn't mean that one causes the other. :)

So, the argument concludes that specifically, if you have a good immune system, then you'll have protection against mental illness. The reverse causation could be equally true though: if you have a mental illness, then it could negatively affect your immune system. Answer D negates this possibility - which is the form a "reverse causation" assumption is always going to take. The author must assume that the reverse possibility is NOT the case in order to assume that his cause-effect conclusion IS the case.