find the assumption

[sachindia]

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.

[Kasia@EconomistGMAT]

What we need to look at when dealing with this question is the connection between mental illness and the immune system. The author argues that the low level of immune-system activity might be the REASON for mental illness. Now we need to find an assumption that will support this claim.
A - this is not an assumption because this answer basically repeats the claim stated in the conclusion - if you reverse this sentence, you will have: Low immune-system activity provides worse protection against mental illness than normal immune-system activity does
B - out of scope, not mentioned
C - again, this is the repetition of the paraphrased information from the conclusion
E - out of scope
D - correct, since this answer implies that the level of immune-system activity is the same before and after somebody starts to suffer from a mental disorder, meaning that the low immunity could have been the CAUSE and not the EFFECT

[hey_thr67]

For cause and effect type of reasoning there are some well defined rules.

(1) if Cause ------> effect then effect ----X---> cause

(2) if Cause ------> effect then NOT cause ------> NOT effect

(3) if Cause 1 -----> effect then any other Cause CAN NOT HAVE effect

The choice D is the example of (1)