title: Does the association of education with breast cancer replicate within twin pairs? A register-based study on Danish female twins.
abstract: BACKGROUND
  A positive association between socio-economic position and breast cancer has been widely observed, but not hitherto within twin pairs, where shared familial factors were adjusted for.
  
  
  METHODS
  We used data on education and other factors from the Danish Twin Registry, The Danish Cancer Registry, and official registers on a total of 16 310 twins. Unpaired and intrapair Cox regression analyses were compared.
  
  
  RESULTS
  In the unpaired analysis, an educational gradient in breast cancer risk was found. Similar results were seen in the intrapair analyses of all twins, although no longer statistically significant. When intrapair analyses were stratified on zygosity, the effect of education was attenuated in the monozygotic twins.
  
  
  CONCLUSIONS
  The main findings support an effect of education beyond shared familial factors.
authors.affiliation: Unit of Epidemiology, University of Southern Denmark, The Danish Twin Registry, DARC, University of Southern Denmark. mimadsen@health.sdu.dk
authors.forename: M
authors.initials: M
authors.lastname: Madsen
authors.forename: P K
authors.initials: PK
authors.lastname: Andersen
authors.forename: M
authors.initials: M
authors.lastname: Gerster
authors.forename: A-M
authors.initials: AM
authors.lastname: Nybo Andersen
authors.forename: K
authors.initials: K
authors.lastname: Christensen
authors.forename: M
authors.initials: M
authors.lastname: Osler
mesh_terms: D000328:Adult; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000235:genetics; D003718:Denmark / Q000453:epidemiology; D004200:Diseases in Twins / Q000453:epidemiology*; D004493:Education / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D016016:Proportional Hazards Models; D012042:Registries; D012307:Risk Factors; D012959:Socioeconomic Factors