Incidence of schizophrenia does not vary with economic status of the country
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- Saha, S., Welham, J., Chant, D. et al. Soc Psychiat Epidemiol (2006) 41: 338. doi:10.1007/s00127-006-0041-7
A recent systematic review found that the prevalence of schizophrenia was lower in developing nations compared to developed nations. However, there is a lack of information about the association between economic status and the incidence of schizophrenia. The aim of this study was to examine the association between economic status and the underlying incidence of schizophrenia based on a recently published systematic review of the incidence of schizophrenia.
The analyses were based on 167 discrete incidence rates from 52 studies. Nations were divided into three categories according to per capita gross national product. Based on these categories, we compared the incidence rates for schizophrenia when adjusted for within-study variation.
The median (and 10–90% quantiles) incidence rates per 100,000 persons for Least Developed Countries (three studies), Emerging Economies (nine studies), and Developed Countries (42 studies) were 20.0 (0.4–35.0), 11.0 (5.0–26.0) and 16.0 (8.0–48.0) respectively. There was no significant difference in incidence rates between these groups.
While there is a lack of information on the incidence of schizophrenia in the developing world, there is no evidence to suggest that the incidence of schizophrenia varies by economic status. In light of the evidence that the prevalence of schizophrenia is higher in developed countries, more research is warranted focused on the interaction between economic measures, and the incidence, prevalence and course of schizophrenia.
Keywordsincidenceprevalenceschizophreniasystematic revieweconomic statusdeveloping countries
In a recent systematic review of the prevalence of schizophrenia , we reported that the prevalence of schizophrenia was significantly higher in developed nations compared to developing nations (based on 136 prevalence estimates derived from 85 studies). This finding is congruent with the belief that the outcome of schizophrenia is more favourable in poorer, developing nations in contrast to richer, developed nations [2, 3]. However, it remains unclear whether the underlying incidence of schizophrenia varies between nations with different economic status. Curiously, the WHO Ten Nation Study  found that the incidence of schizophrenia (defined according to broad ICD-9 criteria) was highest in the two sites from the developing world (rural and urban Chandigarh). To better understand how economic status interacts with the dynamic matrix of incidence, course of illness and prevalence, we need to examine the incidence of schizophrenia across economic gradients. The aim of this paper is to compare the distributions of incidence rates identified in a recent systematic review when the sites were categorized by economic status. In the absence of clear evidence on the matter, we postulated that the incidence rate distributions would not differ when divided by economic status (the null hypothesis).
Detailed search methodology for the review was published in our earlier paper on incidence . In brief, a broad search string ((schizo* OR psych*) AND (incidence OR prevalence)) was used in MEDLINE, PsychINFO, EMBASE and LILACS to identify all potentially relevant papers (in all languages) published between January 1965 and December 2001. To locate any missed or unpublished information, we also scrutinized all relevant bibliographies and wrote to authors.
We extracted data from non-overlapping ‘core’ incidence studies. A set of predefined selection rules was applied to identify discrete incidence estimates (see ). Detailed descriptions of these studies can be found as supplementary material attached to the systematic review, which are available in a full free-text on-line journal .
Following the World Bank Classification , we divided incidence studies into three levels of ‘economic status’ according to the per capita gross national product (as defined in 2004) : (a) Least Developed Countries (LDC) = mean income of less than US$2995; (b) Emerging Economy Countries (EEC) = mean incomes between US$2995 to $9266; and (c) Developed Countries (DC) = mean incomes of greater than US$9266.
We used Proc Mixed (SAS, version 9.1) to test for differences between the distributions of the three economic groups. This analysis took into account (a) within-study variation (rates drawn from the same study tend to be more alike than those from different studies), and (b) skewness of the distributions (using log transformation). We restricted our analysis to persons because of the relative lack of sex-specific rates in LDC and EEC.
We identified 167 discrete rates from 52 studies; three from LDC (India and Pakistan), nine from EEC (China, Jamaica, Russia and Brazil) and 42 from DC (Barbados, Canada, Croatia, Denmark, Finland, Iceland, Ireland, Italy, Germany, New Zealand, Norway, Singapore, Spain, Sweden, The Netherlands, Trinidad, UK and USA).
Discussion and conclusion
There is no evidence to suggest that the incidence of schizophrenia differs between nations when divided by economic status. Combined with the evidence that the prevalence of schizophrenia is higher in developed nations , our findings lend weight to the belief that the course of the illness is less favourable in developed nations compared to developing nations.
There are important limitations in the interpretation of our data. First, very few of the incidence rates used in the study were age-adjusted (only 7 out of 52 studies). As the age population pyramid in the developing world tends to have greater proportions of the young, unadjusted rates may overestimate incidence rates in developing nations compared to developed nations. Second, there is also a lack of consensus on how best to define the economic development of a nation. The use of a single economic variable is a crude way to assess the complex and multidimensional economic structure of a nation. In addition, ecological studies can only address variables at the population level, and thus may miss associations operating at the individual level. Finally, because of the relatively higher costs of conducting incidence studies in the developing nations, there is a marked paucity of information on the incidence of schizophrenia from these sites. This reinforces the urgent need for more epidemiological studies from developing countries .
Nevertheless, the study presents the largest survey of the relationship between economic measures and the incidence of schizophrenia. While strong causal inferences cannot be drawn from ecological analyses, we hope that the results of this study will provide further impetus to examine hypotheses about the interaction between economic measures versus incidence, prevalence and course of schizophrenia.
The Stanley Medical Research Institute supported this project.