Background: Several studies have reported an association between family size and atopic conditions, but few have found an association between asthma and family size.
Objective: This study was designed to assess the effect of familial aggregation and sibling number on asthma in children.
Methods: Children from 5 to 11 years of age in England and Scotland participated in a study in 1990 or 1991 in which responses to questions on asthma and wheezing in children and atopic conditions in parents were ascertained. Data on 11,924 complete family sets of child, mother, and father in three samples (the "representative" samples of England and Scotland and the English "inner city" sample) were analyzed.
Results: The following factors were highly associated with child's asthma or wheezing: mother's or father's asthma or wheezing (with the same odds ratio [OR], 2.4; 95% confidence interval [CI], 2.0 to 2.9), other atopic condition in mothers and fathers (OR, 1.5; 95% CI, 1.3 to 1.8 and OR, 1.4; 95% CI, 1.2 to 1.7, respectively), and three or more siblings compared with no siblings (OR, 0.5; 95% CI, 0.4 to 0.6).
Conclusion: The negative relation between family size and wheezing or asthma indicates a protective effect of number of children sharing an environment at a young age. The level of association of asthma or wheezing in the parents and the child and the association between the child's asthma or wheezing and other atopic conditions in parents suggest that a general susceptibility and a lung-specific susceptibility are important in the etiology of asthma.