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A systematic review of the incidence of schizophrenia. John McGrath Sukanta Saha Joy Welham David Chant.

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Schizophrenia - the most common form of psychosis – is characterised by disorders of cognition (eg paranoia), affect (mood), communication (thought disorder) and perception (eg hallucinations) – leading to a loss of contact with reality along with various forms of impaired behavior.

Because schizophrenia (a) has an early onset and (b) has a relapsing or chronic course, it is a significant public health problem. It ranks in the top 10 leading causes of disability, with a burden of disease comparable to cancer and greater than heart disease.

Previously the incidence of schizophrenia has been thought to be relatively uniform across time and place. The identification of variations in the incidence of schizophrenia is important in unravelling the causes of the disorder


To systematically review studies of the incidence of schizophrenia

To examine variation in time, place and person by examining the distribution of these rates

To explore a limited number of a priori hypotheses

  • Research questions
  • Methods & Systematic reviews
  • Key results
  • Caveats and Conclusions
the incidence of schizophrenia hypotheses
The incidence of schizophrenia:Hypotheses
  • Sex difference

Males > females

  • Migrant status

Migrants > native born

  • Urbanicity

Urban born > rural born

types of incidence studies
Types of incidence studies
  • ‘Core studies’

Sentinel surveys

Register based studies

  • Migrant studies
  • Cohort studies
  • Other special groups
methods systematic review
Methods: systematic review
  • Electronic data search
    • Medline, PsychoInfo, Embase, LILAC
  • 1965-2001 inclusive
  • (schizo* OR psycho*) AND

incidence OR prevalence)


  • Review article bibliography
  • Wrote to authors

Screen abstract and reviewed papers to cull irrelevant citations

rate items and discrete data
Rate items and discrete data
  • Non-overlapping

Sex Male, Female

  • Overlapping

Age eg all ages or age 15-54

Diagnosis eg Catego S+ or Catego SPO + clinical

Diagnostic categories

eg DSMIV Schizophrenia or Schizophrenia + Delusional disorder

Site overlap eg Denmark or Copenhagen

Epoch overlap eg 1990-92 or 1989-91

  • Electronic search = 834 potential papers
  • Manual checking = 249 potential papers
  • Letters from 52 authors, who provided an additional 41 references

Of potentially relevant papers, 74% were identified from electronic sources

results 2
Results (2)
  • Rates based on 176,056 potentially overlapping incident cases

After review

  • 158 were included from 32 countries
  • Types of studies
    • Core studies = 100
    • Migrant studies = 24
    • Cohort studies = 23
    • Other special groups = 14
cohort studies
Cohort studies
  • Australia (n=2)
  • Denmark (n=3)
  • Finland (n=5)
  • Israel (n=2)
  • Italy (n=1)
  • Sweden (n=2)
  • The Netherlands (n=2)
  • USA (n=3)
  • United Kingdom (n=3)
other special groups
Other special groups
  • over age 65
  • twins
  • various ethnic and/or religious subgroups
  • students
  • deaf individuals
  • workers in a radiation contamination zone
key findings
Key findings
  • Most of the distributions are ‘data rich’
  • Variation
    • Asymmetrical
    • Long upper tail (>25% rates)
    • Median 15.2 (10-90% 7.7- 43.0) per 100,000
    • Five-fold range within the 10-90% quantiles
key findings 2
Key findings (2)
  • Males > females
  • Migrants > native born
  • Urban > mixed urban/rural
  • Schizophrenia has a varied and detailed epidemiological landscape
  • Comparisons in systematic reviews should be planned, based on directional hypotheses & limited to a reasonable number
  • Systematic reviews are best suited to hypothesis-generation
  • Geographical boundaries are administrative
  • Mostly ‘treated’ incidence

There is a wealth of data available on the incidence of schizophrenia. Studies come from many countries, with many different methodological features, and conducted over several decades.

The width and skew of the rate distribution, and the significant impact of sex, urbanicity and migrant status on these distributions, indicate substantial variations in the incidence of schizophrenia.

Thus these data may provide leads for further research into risk factors