HORMONAL changes in women over successive pregnancies could partly explain why firstborns are more likely to develop allergies than their younger brothers and sisters.
Wilfried Karmaus of Michigan State University in East Lansing and his colleagues have found that firstborn babies have higher levels of a key immune protein associated with allergies.
Epidemiologist David Strachan, now at St George's Hospital Medical School in London, was the first to notice that the bigger a family is, the less likely its youngest members are to have allergies such as asthma, hay fever and eczema. Other studies have confirmed this. "I don't think there's any doubt that there's some sort of a relationship," he says.
In 1989, to explain the sibling effect, Strachan proposed the hygiene hypothesisthe idea that children's immune systems go haywire if they aren't exposed to enough challenges early on (
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