Critical Reasoning

People with major depressive disorder are much more likely to have immune disorders such as lupus and chronic fatigue syndrome than people who are not depressed. This suggests strongly that major depressive disorder reduces the body's ability to fight infection, because primary infections are thought to cause many of these immune disorders.

Which of the following is an assumption required by the argument above?

1. People who are depressed do not tend to overstate the severity of their symptoms.
2. People with major depressive disorder are less careful than others about protecting themselves from infection.
3. Major depressive disorder does not usually have a very sudden onset.
4. Immune disorders do not cause major depressive disorders in many people.
5. Many immune disorders are known to have causes other than primary infections.

This is a classic basic CR question, one in which the argument is based on a correlation upon which could be based a relationship that is the reverse of the one being discussed.

The conclusion of the argument is that the evidence suggests strongly that major depressive disorder causes immune disorders. Let's see what's necessary for this argument to work.

A) People who are depressed do not tend to overstate the severity of their symptoms.

This is a trap answer. What this answer discusses may be a valid concern, but the argument is not about people saying they are experiencing symptoms. It's about people who actually are experiencing certain health issues.

B) People with major depressive disorder are less careful than others about protecting themselves from infection.

This is another trap answer in that it is related to what the argument is discussing, but rather than being an assumption required by the argument, this, if true is a reason to question the conclusion.

C) Major depressive disorder does not usually have a very sudden onset.

The argument is not about what goes on at the onset of major depressive disorder. It is about what goes on concurrently with experiencing major depressive disorder.

D) Immune disorders do not cause major depressive disorders in many people.

The conclusion of the argument, that the evidence strongly suggests that major depressive disorder causes immune disorders, can be in a way reversed. Based on the same evidence could be the conclusion that immune disorders cause major depressive disorder. So the argument in the prompt requires the assumption that the reverse cause and effect scenario is not what is going on.

Possibly you can see this more clearly using a negation test. If we reverse this answer choice we get the following.

Immune disorders cause major depressive disorder in many people.

That destroys the argument because it provides a completely different explanation for the correlation between immune disorders and major depressive disorder. When the reverse of an answer choice destroys an argument, that answer choice is an assumption required by the argument.

E) Many immune disorders are known to have causes other than primary infections.

Actually the argument is based on the idea that immune disorders are caused by primary infections. So this, rather than being an assumption required by the argument, weakens it.

So the correct answer is D.

Thanks Murray.

Got it cleared now.
I was actually fixating on the later which says "because primary infections are thought to cause many of these immune disorders", Which made me think of B as the right option. As primary infections may be a cause of immunity decrease thus becoming the root cause of the subsequent disorders.

That's what makes B a, classic, CR trap answer. The writers know that you will have noticed that and may be thinking that way.

That's why you have to be careful to not just pick answers that sound somehow right and to seek to see clearly the logic of CR prompts, questions and answer choices.