Birth order

One finding that has emerged consistently from epidemiological studies as a potentially important risk factor for allergic disease is birth order; the more older siblings a child has, the lower the risk of allergic disease. This finding was first reported by David Strachan in an analysis of a 1958 British birth cohort in which he observed that the risks of hay fever at age 11 and at age 23 and the risk of eczema in the first year of life were reduced in relation to the number of older siblings in the family. He suggested that “allergic diseases were prevented by infection in early childhood, transmitted by unhygienic contact with older siblings.” He went on to propose, “Over the past century, declining family size, improvements in household amenities, and higher standards of personal cleanliness have reduced the opportunity for cross-infection in young families.”.