IJE Advance Access published online on July 20, 2007
International Journal of Epidemiology, doi:10.1093/ije/dym141
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Commentary: Socioeconomic position and common mental disorders: what do we need to know?
1Academic Unit of Psychiatry, University of Bristol, Cotham House, Cotham Hill, Bristol BS6 6JL, UK.
2Department of Psychiatry, University of Ioannina School of Medicine, Greece.
E-mail: p.skapinakis{at}bristol.ac.uk
Accepted 11 June 2007
Studies of the association between socioeconomic status and mental disorders have a long history and one early example is the 1939 Chicago study conducted by Faris and Dunham.1 These researchers used aggregate data and reported an association between admission for schizophrenia and living in a deprived neighbourhood. Later studies on the association between severe mental disorders and socioeconomic status generally confirmed these early observations.2 The controversy remained on the explanation of this finding with two competing explanations (social causation vs social selection/social drift). There are arguments in favour of both2, although recent epidemiological research has challenged the more traditional social drift hypothesis.3
Common mental disorders is a term mainly used in Britain to denote mild forms of neurotic disorder composed from symptoms of depression and anxiety.4 These are distinguished from the more severe mental disorders. The concept of common mental disorders has proved useful in epidemiological research and is often measured with simple self-reported questionnaires like the 12 item general health questionnaire (GHQ-12) or the mental health index of the short form health survey (SF-36). More specific psychiatric syndromes included in the concept of common mental disorders are major depression and specific anxiety disorders such as panic disorder, social phobia or obsessive compulsive disorder. Operational diagnostic criteria for these more specific diagnoses have been published from WHO (ICD-10) or the American Psychiatric Association (DSM-IV) and tested in various epidemiological surveys around the world.5,6 Although there is a high correlation between general measures of psychological distress and more specific psychiatric syndromes, one should not assume that associations elicited with simple scales should apply to more specific syndromes or vice versa.
A paper in this issue of the International Journal of Epidemiology7 investigates the association between common mental disorders, assessed with the GHQ-12, and various socioeconomic indicators in two similar samples of middle-aged public sector employees from the UK and Finland. The UK sample is from the British Whitehall II study while the Finnish sample is from the Helsinki Health study. This paper failed to find a consistent association between income, social class and education and GHQ-12 in both cohorts. A more subjective question on current or past financial difficulties was significant.
Is there a difference in the social patterning between severe mental disorders and the more common mental disorders as this paper implies? A recent meta-analysis of the association between depression and socioeconomic status concluded that there is evidence for a socioeconomic gradient in depression with an odds ratio for depression of 1.81 in the lowest socioeconomic status group compared to the highest (P < 0.001)8. This meta-analysis included papers that assessed depression or common mental disorders either with a crude measure (such as the GHQ-12) or with more complex instruments. A more selective look on high quality recent general population surveys that used detailed diagnostic interviews to assess common mental disorders also supports the results of this meta-analysis: the two large surveys in America,9 the Epidemiologic Catchment Area study and the National Comorbidity Survey, the British Psychiatric Morbidity Survey10 and the Dutch Nemesis survey,11 all showed a significant association with most socioeconomic indicators and the common mental disorders after adjustment for age and sex. Only education has failed to show a consistent association and there is evidence that this variable may be less important in western countries compared with less developed countries.12
The current article is certainly not the only one that failed to find an association between traditional socioeconomic indicators and common mental disorders. Several points may explain these inconsistencies in the literature:
- Measurement of common mental disorders: as discussed earlier, the GHQ-12 or other simple self-reported scales may be inadequate to show the socioeconomic gradient of common mental disorders. Studies that used more detailed instruments have generally shown positive results. In our own analysis of a large British survey, we looked at specific disorders that are usually included in the concept of common mental disorders.13 We noticed a difference between operationally defined depression (inverse association with socioeconomic status) and non-specific psychological distress (no association with socioeconomic status). Being a case on the GHQ-12 is more similar to non-specific psychiatric morbidity and this may explain the reported negative association in the Laaksonen et al. study.7
- Assessment of prevalent cases: even in studies that reported an association between socioeconomic status and common mental disorders, it is not known whether this is due to incident cases or chronic/persistent cases. Several longitudinal studies have generally shown that chronicity is more likely influenced by low socioeconomic status compared with a new episode of disorder.8 I would like to note however, that common mental disorders differ from severe mental disorders or chronic medical conditions in that they have an episodic form, a good outcome in each episode but an increased likelihood of recurrence. The few studies that used incident cases of common mental disorders did not show consistent results with socioeconomic status but studies with first onset of depression or anxiety are rare. In the single study conducted in America (but with the limitation of the retrospective recall of first onset) poverty was significantly associated with first onset of depression even after adjustment for previous subclinical depressive symptoms.14
- Selection: the authors point out in their discussion that the use of employees may have influenced their results. It is known that unemployment is a strong risk factor for common mental disorders and is correlated with socioeconomic indicators.15 More importantly, job insecurity is also a potential risk factor for common mental disorders.15 It is expected that job insecurity in the public sector of these two western affluent countries should be relatively small in the period the research has been conducted. If there is any health-related selection, its effect is more likely to be towards the null value.
Despite the few inconsistencies, I believe that there is evidence in favour of a socioeconomic gradient in common mental disorders, at least when one looks at representative samples of the general population. What we do not know yet and we need to clarify in the future is the issue of social causation or social selection. This is not a theoretical question since it may have public health implications as well. To answer this question, there is a need for longitudinal research that will focus on operationally defined syndromes of common mental disorders and on first onsets rather than new episodes or recurrences. In addition, we need to look further from the individual level and extend our research to multi-level studies as Silver and colleagues16 did in their revision of the Faris and Dunham hypothesis. There is a complex interaction between individual and contextual (e.g. neighbourhood) socioeconomic status and future longitudinal studies should be able to better inform us about the pathways that lead people of low socioeconomic position to depression and other common mental disorders.
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