A study comparing a group of chronically depressed individuals with an otherwise matched group of individuals free from depression found significantly more disorders of the immune system among the depressed group. According to the researchers, these results strongly support the hypothesis that mental states influence the body’s vulnerability to infection.

Which of the following, if true, casts the most serious doubt on the researchers’ interpretation of their findings?

  1. The researchers’ view does little more than echo a familiar theme in folklore and literature.
  2. Chronically depressed individuals are no less careful than others to avoid exposure to infections.
  3. Disorders of the immune system cause many of those individuals who have them to become chronically depressed.
  4. Individuals who have previously been free from depression can become depressed quite suddenly.
  5. A high frequency of infections can stem from an unusually high level of exposure rather than from any disorder of the immune system.

Highlight to see answer: C

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