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  1. Alex J MacGregor, Arthritis Research Campaign senior fellow (alex.macgregor@kcl.ac.uk),
  2. Jerry Lanchbury, reader in molecular immunogenetics,
  3. Alan S Rigby, senior lecturer in statistics and epidemiology,
  4. Jaakko Kaprio, professor of genetic epidemiology,
  5. Harold Snieder, genetic epidemiologist
  1. Twin Research and Genetic Epidemiology Unit, St Thomas's Hospital, London SE1 7EH
  2. Guy's, King's and St Thomas's School of Medicine, Guy's Hospital, London SE1 9RT
  3. University of Sheffield School of Medicine, Sheffield Children's Hospital, Sheffield S10 2TH
  4. Department of Public Health, PO Box 41, FIN-00014 University of Helsinki, Finland
  5. Georgia Prevention Institute, Medical College of Georgia, Augusta, GA 30912-3715, USA

    EDITOR—The cover of the BMJ on 2 February poses the question “Rheumatoid arthritis: is it genetic?” and answers “Probably not.” Neither the question nor the response is valid.

    The Danish twin study on which this conclusion is based identified no concordant monozygotic twin pairs and only two concordant dizygotic pairs from a sample of 37 338.1 By modelling the reported numbers of confirmed cases of rheumatoid arthritis, we calculate that the study had 80% power to detect a …

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