Myths and plain truths about schizophrenia epidemiology - the NAPE lecture 2004

Objective: Science needs to constantly match research models against the data. With respect to the epidemiology of schizophrenia, the widely held belief that the incidence of schizophrenia shows little variation may no longer be supported by the data. The aims of this paper are (i) to explore data-vs.-belief mismatch with respect to the incidence of schizophrenia, and (ii) to speculate on the causes and consequences of such discrepancies. Method: Based on a recently published systematic review of the incidence of schizophrenia, the distribution of incidence rates around the world was examined. In order to examine if the incidence of schizophrenia differed by sex, male vs. female risk ratios were generated. Results: The distribution of incidence rates for schizophrenia is asymmetrical with many high rates skewing the distribution. Based on the central 80% of rates, the incidence of schizophrenia varies in a five-fold range (between 7.7 and 43.0 per 100 000). Males have a significantly higher incidence of schizophrenia compared with females (median male to female risk ratio = 1.4), and this difference could not be accounted for by diagnostic criteria or age range. Conclusion: The beliefs that (i) the incidence of schizophrenia does not vary between sites and (ii) males and females are equally affected, may have persisted because of an unspoken deeper belief that schizophrenia is an egalitarian and exceptional disorder. Our ability to generate productive hypotheses about the aetiology of schizophrenia rests on an accurate appraisal of the data. Beliefs not supported by data should be identified and relabelled as myths.

A systematic review and meta-analysis of Northern Hemisphere season of birth studies in schizophrenia

Based on the epidemiological finding that individuals with schizophrenia tend to be born in winter/spring when compared to the general population, we examined (1) the strength and timing of this effect in Northern Hemisphere sites, and (2) the correlation between the season of birth effect size and latitude. Studies were located via electronic data sources, published citations, and letters to authors. Inclusion criteria were that studies specify the diagnostic criteria used, that studies specify the counts of schizophrenia and general population births for each month, and that subjects and the general population be drawn from the same birth years and catchment area. We extracted data from eight studies based on 126,196 patients with schizophrenia and 86,605,807 general population births and drawn from 27 Northern Hemisphere sites. Comparing winter/spring versus summer/autumn births, we found a significant excess for winter/spring births (pooled odds ratio = 1.07; 95% confidence interval 1.05, 1.08; population attributable risk = 3.3%). There was a small but significant positive correlation between the odds ratios for the season of birth comparison and latitude (r = 0.271, p < 0.005). Furthermore, the shape of the seasonality in schizophrenia births varied by latitude band. These variations may encourage researchers to generate candidate seasonally fluctuating exposures.

Season of birth is associated with anthropometric and neurocognitive outcomes during infancy and childhood in a general population birth cohort

The 'season of birth' effect is one of the most consistently replicated associations in schizophrenia epidemiology. In contrast, the association between season of birth and development in the general Population is relatively poorly understood. The aim of this study was to explore the impact of season of birth on various anthropometric and neurocognitive variables from birth to age seven in a large, community-based birth cohort. A sample of white singleton infants born after 37 weeks gestation (n =22,123) was drawn from the US Collaborative Perinatal Project. Anthropometric variables (weight, head circumference, length/height) and various measures of neurocognitive development, were assessed at birth, 8 months, 4 and 7 years of age. Compared to surnmer/autumn born infants, winter/spring born infants were significantly longer at birth, and at age seven were significantly heavier, taller and had larger head circumference. Winter/spring born infants were achieving significantly higher scores on the Bayley Motor Score at 8 months, the Graham-Ernhart Block Test at age 4, the Wechsler Intelligence Performance and Full Scale scores at age 7, but had significantly lower scores on the Bender-Gestalt Test at age 7 years. Winter/spring birth, while associated with an increased risk of schizophrenia, is generally associated with superior outcomes with respect to physical and cognitive development. (c) 2005 Elsevier B.V. All rights reserved.

Vitamin D supplementation during the first year of life and risk of schizophrenia: a Finnish birth cohort study

Objective: Based on clues from epidemiology and animal experiments, low vitamin D during early life has been proposed as a risk factor for schizophrenia. The aim of this study was to explore the association between the use of vitamin D supplements during the first year of life and risk of developing schizophrenia. Method: Subjects were drawn from the Northern Finland 1966 Birth Cohort (n = 9 114). During the first year of life, data were collected about the frequency and dose of vitamin D supplementation. Our primary outcome measures were schizophrenia, psychotic disorders other than schizophrenia, and nonpsychotic disorders as diagnosed by age 31 years. Males and females were examined separately. Results: In males, the use of either irregular or regular vitamin D supplements was associated with a reduced risk of schizophrenia (Risk ratio (RR) = 0.08, 95% CI 0.01-0.95; RR = 0.12, 95% CI 0.02-0.90, respectively) compared with no supplementation. In males, the use of at least 2000 IU of vitamin D was associated with a reduced risk of schizophrenia (RR = 0.23, 95% CI 0.06-0.95) compared to those on lower doses. There were no significant associations between either the frequency or dose of vitamin D supplements and (a) schizophrenia in females, nor with (b) nonpsychotic disorder or psychotic disorders other than schizophrenia in either males or females. Conclusion: Vitamin D supplementation during the first year of life is associated with a reduced risk of schizophrenia in males. Preventing hypovitaminosis D during early life may reduce the incidence of schizophrenia. (C) 2003 Elsevier B.V. All rights reserved.

Variations in the incidence of schizophrenia: Data versus dogma

The schizophrenia research community has shared a belief that the incidence of schizophrenia shows little variation. This belief is related to the dogma that schizophrenia affects all individuals equally, regardless of sex, race, or nationality. However, there is now robust evidence that the incidence of schizophrenia is characterized by substantial variability. There is prominent variation in the incidence of schizophrenia between sites. The incidence of schizophrenia is significantly higher in males than in females (male:female ratio = 1.4). Migrants and those living in urban areas have a higher incidence of schizophrenia. The incidence of schizophrenia has fluctuations across time. In addition, the prevalence of schizophrenia is also characterized by prominent variation. The realization that schizophrenia is characterized by rich and informative gradients will serve as a catalyst for future research.