Absence of spatial variation in rates of the common mental disorders
- Correspondence to: Professor S Weich Division of Health in the Community, Warwick Medical School, Medical School Building, University of Warwick, Coventry CV4 7AL, UK; s.weichwarwick.ac.uk
Place may still matter—but not in the ways that have been studied to date.
The intuitive importance of location as a determinant of life chances1,2 contrasts with growing evidence of little or no variation in the prevalence of the most common mental disorders (CMD), anxiety and depression, across small and mid-sized areas—particularly after adjusting for the characteristics of individual residents.3–9 The study by Henderson and her colleagues,10 based on an urban US sample confirms this. By contrast, larger area level effects are found for psychotic illnesses (such as schizophrenia) and more severe forms of depression.11–15 Should we conclude that place doesn’t matter for the most CMD, or are there alternative explanations for these negative findings?
ARE WE STUDYING THE WRONG SPATIAL SCALE?
The spatial scale at which contextual factors might have an impact on mental health remains unknown. Most studies have used data collected within administrative boundaries.2,16 Studies of large areas, such as UK regions (with hundreds of thousands of residents), are difficult to interpret.17,18,19 Recent studies have examined effects over smaller areas, ranging from Amsterdam boroughs (average population 33 000), postcode sectors and neighbourhoods (average population 8000–10 000),19 to UK electoral wards (average population 5500),3,4,7 and US census tracts (average population 4000).9 Effect sizes at these levels are small and rarely statistically significant—percentage of variance in symptoms of anxiety and depression ranges from 0.5% to 4% before adjusting for residents’ characteristics, to less than 1% after doing so.
Wards may be too large and heterogeneous to detect contextual effect, and variance in CMD may be greater over smaller areas.20 The significance of this modest trend remains unclear, and there have been few studies of very small areas. A study using postcode units (average population 150) in South Wales …
