A population perspective

In terms of the final of the four questions about events listed earlier, 'neurotic' conditions that form the bulk of depressive disorders, even in patient series, need to be considered in terms of a population perspective. Figure.,2 summarizes the findings of six population studies of women aged between 18 and 65 carried out in a comparable manner, using the same semistructured interview-based measures as in the Islington survey, including the Present State Examination. The bottom half of

Fig 2 shows the rate of depressive caseness in a 12-month period. Between the two extreme populations there is a 10-fold difference—3 per cent in a rural

Basque-speaking population in Spain (75> and 30 per cent in a black urban population in Zimbabwe. (8,,Z6> In addition these rates are fairly closely paralleled by differences in the experience of severe events particularly likely to provoke depression (see top half of Fig. .2).

One of the implications of these results becomes clear in the context of a behavioural genetic perspective. A key point about the concept of heritability is that it is specific to a particular population. The index is based on consideration of individual discrepancies from a population mean. While it is certainly possible that heritability for depression in each of the six populations is positive, this would reflect only the contribution of genetic factors to individual variability in risk within each population. Even if large, they would be unlikely to be of relevance for explaining the actual population differences in rates of depression. The most plausible interpretation of these is that the differences are essentially the result of psychosocial factors.

Differences in the frequency of relevant genes would need to be extremely large to explain such differences and in addition it now seems likely that significant secular changes in rates of depression are possible.(7 78 and 79> There is, however, no inherent conflict between the two perspectives—they refer to different ways of looking at the 'variance' of a condition. The general point is that the study of individual variability within particular populations cannot rule out the possibility that the mean level of disorder is largely under environmental control; that it can be increased or decreased markedly by external changes quite uninfluenced by the genetic make-up of a population.

A population perspective is also concerned with variability in rates of disorder within populations in terms of social categories such as socio-economic status. Thus, the survey in Camberwell in South London in the early 1970s found that, while the rate of severe events was related to social class position, this explained comparatively little of the large class difference in prevalence of depression; of greater importance were background vulnerability factors such as an unsupportive marriageA") However, the picture has recently become more complex with the finding that severe events involving humiliation and entrapment are not only especially depressogenic but particularly common in high-risk populations (such as Harare) and within populations in high-risk subgroups (such as working-class women in London). In Islington such events were common among single mothers, a social category that has expanded dramatically in most Western populations in recent years and among whom there is a high risk of depression.(8!»

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Natural Depression Cures

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