For a little over ten years it has been known that children who come from small families are more likely to have allergies, especially hayfever, and to be atopic than children from large families. This was first observed by Professor David Strachan in London.

We are very interested in this. Nobody knows exactly why it should be so but most people think it may be related to the number of infections children meet in early life. Those from small families, with few or no brothers and sisters, are less likely to meet common infections than those who come from big families. Such infections may be helpful in protecting children against allergies.

It may be that there are other explanations altogether and that this 'family size' effect is nothing to do with infections.

What have we found in Ashford?

The Ashford families most commonly have two children. The families of course may not yet be complete; 42% of the study children are first born.

How many children do the Ashford study families have now? number of families number of children

Children from the larger families tended to wheeze a little more often but they are also much less likely to be atopic by the time they are four:

Children from larger families are much less likely to be atopic - but a little more likely to wheeze percentage of Ashford study children number of children in the family

Among the Ashford parents, those from large families were much less likely to have a positive skin test; in other words much less likely to be 'atopic'. This finding was very clear and seemed to be related only to the number of brothers they had:

Among the Ashford parents, having more brothers (but not sisters) made them less likely to be atopic percentage of parents who were atopic number of brothers   number of sisters

We couldn't account for this pattern however by measuring the number or kind of infections during the first five years of each adults' life. This may be because general practice records from that time are not a very complete way of counting infections; but it may mean that the explanation for the 'large family' observation lies elsewhere altogether.

We are in fact looking at alternative explanations. We have discovered that among the Ashford mothers, those with more children are less likely themselves to be atopic. This may mean that mothers 'lose' their atopy when they have more children. To look at this possibility we have started to repeat the skin tests among all the Ashford mothers - some seven years after their first tests.

Already among the Ashford children we have observed that those from small families are more likely to have a positive skin tests (to be 'atopic') than those from larger families.

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