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I came across a CR question whose answer explanations, I couldnt understand.
Q-1) 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.
My query: the researcher is saying that low level of immune system activity means lower on tests of mental health. So why is it not true that if there is mental illness there is a chance that immune system is low. thats why he is scoring low on mental health.
Conclusion : Immune system protects against mental illness. X = immune system Y = protection against mental illness The most important assumption in the causal argument is the preclusion from reversal of causality. i.e. Y does not leads to X. D does that.
D. Mental illness does not cause people’s immune-system activity to decrease. ---> Y does NOT leads to X.
This is the classic problem of Defender Assumption.
It usually happens when two events A and B happen at the same time and Researcher then conclude that one event lead to the event say A leads to B. Of course then the assumption has to be that B doesn't lead to A. If B leads to A, how can you say A leads to B.
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Choice A, C and E are out of scope. Choice B, it is not relevant, the conclusion do not discuss how effects two kinds of ailment on body systems. Choice D is correct, It strengthens the conclusion, mental illness not cause immune-system decrease, it means that the immune-system still protect body from physical disease. Negate it, mental illness decrease immune-system, immune-system will not protect against physical disease (weaken the conclusion).
the portion you yourself mentioned that "why is it not true that if there is mental illness there is a chance that immune system is low. thats why he is scoring low on mental health." is an assumption he assumes that this fact is not true and option D states the mental illness is not the cause of low immune levels but the other way round