Background: Declining family size is one factor that has been proposed to contribute to increasing asthma and hay fever prevalence, but its relative importance has not been quantified.
Objective: Our purpose was to determine the change in asthma and hay fever prevalence that would be expected from the reduction in family size that has occurred in England/Wales and New Zealand over recent decades.
Methods: The relative change in family size between 1961 and 1991 in England/Wales and New Zealand was determined from census data for these years. Summary weighted odds ratios were calculated for the associations among birth order, family size, and asthma and hay fever prevalence. The expected increase in the prevalence of asthma and hay fever between 1961 and 1991 resulting from changes in family size was then calculated.
Results: The expected relative increase in the prevalence of asthma between 1961 and 1991 as a result of the smaller family size was 1% and 5% for England/Wales and New Zealand, respectively; smaller family size would be expected to increase the prevalence of hay fever prevalence in England/Wales by 4%.
Conclusions: Changes in family size over the last 30 years do not appear to explain much of the reported increase in asthma or hay fever prevalence. The contribution that other risk factors have made to these increases could be assessed with use of a similar approach.