Take a look at this question. I am posting only the answer choices relevant to this discussion on reverse causation.
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?
(C) People with high immune-system activity cannot develop mental illness·
(D) Mental illness does not cause people's immune-system activity to decrease.
Here, the conclusion is that the immune system protects against mental illness.
On what basis/evidence is this conclusion made? On the observation that people who have low levels of immune system activity tend to score lower on tests of mental health than do people with normal or high immune system activity.
The problem with option C is that it doesn't talk about people with normal immune system activity! Only about those with high ISA.
Let's try negating option D:
Mental illness causes people's immune system activity to decrease.
What does this say about the experiment? It is not low ISA that causes poor mental health (as manifested through low scores on the tests); on the other hand, the people already had mental health problems, which, in turn, lowered their immune system activity. This is a case of reverse causation.
We can see that negation of option D breaks the argument - hence, it is the right answer.
Essentially, when you observe some correlation between 2 events X and Y, check for the presence and direction of a cause-effect relationship.
i.e. X and Y are correlated, but is X causing Y or is Y causing X?
To conclude one way or the other, you need to assume that the reverse does not exist.
Hope this is clear.
