Sibship size does not increase the risk of developing rheumatoid arthritis.

Although the cause of rheumatoid arthritis (RA) is unknown, one hypothesis is that an infectious episode may trigger the disease and this may occur in childhood. Observational studies performed at least 25 years ago have suggested that the incidence of RA is increased in individuals from large families. We therefore tested this hypothesis using data from a case-control study of 218 females with RA aged 35-70 (mean 58.9 years) and 210 similar aged osteoarthritis (OA) females. Information was obtained by postal questionnaire on sibship size, position in family and sex ratio of siblings. No significant differences were found between the cases and controls for any of these variables. This study did not support the hypothesis that early childhood infection as a consequence of overcrowding is an important factor in the development of RA.