Background: Several studies have reported an association between a child's risk of atopic disorders and family size. However, the inverse association might not be the same in populations with a different genetic disposition for atopic disorders.
Objective: This longitudinal study was designed to assess risk factors of atopy.
Methods: Lifetime prevalence of asthma, hay fever and eczema of 1440 families including 3165 offspring was ascertained by means of standardized questionnaires.
Results: After possible confounders had been controlled for, an inverse association between atopic disorders and the number of older siblings was found only in the offspring of atopic fathers (trend for older siblings: chi2 = 13.38, degrees of freedom [d.f.] = 1, P= 0.0002; odds ratio 'no older sibling'= 2.87 (95% confidence interval 2.18-3.78); '1 older sibling' = 2.11 [1.52-2.92], '2 older siblings' = 1.29 [0.74-2.23]; '3 or more older siblings' = 0. 15 [0.02-0.981). No such relationship was found for children without a history of paternal atopy (trend for older siblings: chi2 = 1.5 1, d.f. = 1, P = 0.22; odds ratio 'no older sibling' = 1 [reference]; '1 older sibling' =0.82 [0.63-1.06]; '2 older siblings' = 0.97 [0.67-1.40]; '3 or more older siblings' = 0.64 [0.31-1.33]). The trend for older siblings in the case of paternal atopy was significantly different from the trend for older siblings without a history of paternal atopy (chi2 = 8.68, d.f. = 1, P = 0.003). The number of younger siblings was not related to child's risk of atopy (trend for younger siblings: chi2 = 0.001, d.f. = 1, P = 0.97).
Conclusions: Data from this study suggest a protective effect of sibship size only in children with a history of paternal atopy and if older siblings are present. The reason for this combined effect remains unclear. Thus, further investigations are needed to interpret the biological cause of the so called 'sibling effect'.