Of course, the epidemiology of depression still remains a mystery, but how could it be otherwise without a good theoretically based diagnostic and classification system? The situation is akin to trying to study astronomy with nothing but a dirty milk bottle. Nevertheless, despite the considerable inadequacies in the measuring tools, there are still some surprises in the epidemiology that no theory of depression has yet been able to cope with. That is, the lifespan and gender data on depression (see Chapter 1 and Chapter 5) seem to show that the approximate 2:1 ratio of women to men for depression appears sometime around puberty and then disappears sometime in later life; children show an approximately equal rate, and this is also true in older adults. In addition, these are clearly both cultural effects in that cultures in which women have been traditionally devalued show higher rates of depression in women, but there are also generational cohort effects in that some Western cultures seem to be showing an increase in depression and suicide rates in young men (Chapter 18). The good news for depression research is that no simplistic model can account for the epidemi-ological data. The bad news is that there is as yet no complex model that can account for it either.
The present and immediate future for epidemiology is showing increased use of genetic epidemiology methods to study both unipolar and bipolar depressions through the use of large-scale twin studies (e.g., Kendler & Prescott, 1999) and large-scale family studies that can look at, for example, obligate carriers (see Chapter 11). A recent such study by Sullivan et al. (2002) found that for unipolar depression there was 37% heritability and no significant shared environment effects, but only individual-specific environmental factors (such as specific individual life events or traumas) in their analysis of the presence and absence of depression in a large family study. These genetic epidemiology studies show great promise and can begin to disentangle genetic, gender, age, and cultural effects in their contributions to both unipolar and bipolar disorders in the next decade. However, large-scale, prospective, high-risk studies, in which adolescents and young adults who are at increased risk because of affected family members and other factors, must also be carried out to examine exactly how genetic and psychosocial factors interact to produce or protect the individual from depression.
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