Identify the Question Stem: The word assumptions indicates that this is a Find the Assumption question.
Deconstruct the Argument: A greater percentage of people in high-income countries is diagnosed with depression than in low-income countries. Some researchers think that this has to do with leisure time—specifically, that people in high-income countries have more leisure time and that, for some unstated reason, this results in more people becoming depressed.
Pause and State the Goal: On Assumption questions, your goal is to find something that the researchers must believe to be true in making their argument. For example, since they conclude that increased leisure time is the cause, they're assuming that there is not some other, different cause that is more likely. They're also assuming that there isn't some other reason to explain the data point—for example, perhaps more people in low-income countries are depressed but they don't have adequate access to the healthcare system to be diagnosed and treated.
Work from Wrong to Right:
(A) While it may be the case that there is a hereditary component to depression, the researchers' argument doesn't address whether this illness runs in families. Rather, the researchers hypothesize that the incidence of depression has something to do with the relative amounts of leisure time that different societies have.
(B) CORRECT. The researchers accept that there is actually a higher incidence of depression in high-income countries, not just that a greater percentage of the population manages to get diagnosed. In other words, the researchers are assuming that it is not the case that more people are depressed in the lower-income countries but are just not being formally diagnosed.
(C) This may be true but the researchers do not assume anything about how much money leisure activities might cost. They hypothesize only that people in high-income countries spend more time conducting leisure activities than do those in low-income countries.
(D) It's reasonable to think that this is true, but the argument concerns being diagnosed with depression in the first place. This choice is about being treated after someone is diagnosed; the researchers' argument doesn't address treatment or possible discrepancies in treatment.
(E) This answer is incorrect for the same reason as answer (D): The argument concerns being diagnosed with depression in the first place, but this choice is about being treated after someone is diagnosed. The researchers' argument doesn't address treatment or possible discrepancies in treatment.
The correct answer is (B).
