title: Geographical variation in dementia: examining the role of environmental factors in Sweden and Scotland.
abstract: BACKGROUND
  This study aimed to estimate the magnitude of geographical variation in dementia rates and suggest explanations for this variation. Small-area studies are scarce, and none has adequately investigated the relative contribution of genetic and environmental factors to the distribution of dementia.
  
  
  METHODS
  We present 2 complementary small-area hierarchical Bayesian disease-mapping studies using the comprehensive Swedish Twin Registry (n = 27,680) and the 1932 Scottish Mental Survey cohort (n = 37,597). The twin study allowed us to examine the effect of unshared environmental factors. The Scottish Mental Survey study allowed us to examine various epochs in the life course-approximately age 11 years and adulthood.
  
  
  RESULTS
  We found a 2- to 3-fold geographical variation in dementia odds in Sweden, after twin random effects-likely to capture genetic and shared environmental variance-were removed. In Scotland, we found no variation in dementia odds in childhood but substantial variation, following a broadly similar pattern to Sweden, by adulthood.
  
  
  CONCLUSIONS
  There is geographical variation in dementia rates. Most of this variation is likely to result from unshared environmental factors that have their effect in adolescence or later. Further work is required to confirm these findings and identify any potentially modifiable socioenvironmental risk factors for dementia responsible for this geographical variation in risk. However, if these factors do exist and could be optimized in the whole population, our results suggest that dementia rates could be halved.
authors.affiliation: From the aAlzheimer Scotland Dementia Research Centre, University of Edinburgh, Edinburgh, United Kingdom; bScottish Dementia Clinical Research Network, NHS Scotland, Edinburgh, United Kingdom; cCentre for Cognitive Ageing & Cognitive Epidemiology, University of Edinburgh, Edinburgh, United Kingdom; dDivision of Psychiatry, University of Edinburgh, Edinburgh, United Kingdom; eDepartment of Psychology, University of Southern California, Los Angeles, CA; fDepartment of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden; gGreater Glasgow & Clyde Nursing Homes Medical Practice, NHS Greater Glasgow & Clyde, Glasgow, United Kingdom; hPublic Health and Intelligence, NHS National Services Scotland, Edinburgh, United Kingdom; and iDepartment of Epidemiology and Public Health, UCL, London, United Kingdom.
authors.forename: Tom C
authors.initials: TC
authors.lastname: Russ
authors.forename: Margaret
authors.initials: M
authors.lastname: Gatz
authors.forename: Nancy L
authors.initials: NL
authors.lastname: Pedersen
authors.forename: Jean
authors.initials: J
authors.lastname: Hannah
authors.forename: Grant
authors.initials: G
authors.lastname: Wyper
authors.forename: G David
authors.initials: GD
authors.lastname: Batty
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Deary
authors.forename: John M
authors.initials: JM
authors.lastname: Starr
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001499:Bayes Theorem; D002648:Child; D015331:Cohort Studies; D003704:Dementia / Q000453:epidemiology* / Q000209:etiology / Q000235:genetics; D004200:Diseases in Twins / Q000453:epidemiology* / Q000209:etiology / Q000235:genetics; D004777:Environment; D005260:Female; D020022:Genetic Predisposition to Disease; D006306:Health Surveys; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D012606:Scotland / Q000453:epidemiology; D013548:Sweden / Q000453:epidemiology