In 1989 Strachan published a paper observing that the prevalence of hay fever varied with family size.1 He proposed that allergic diseases may be prevented by infection in early childhood, transmitted by unhygienic contact with older siblings. This protection from atopy is also related to order of birth; younger siblings are less likely to be atopic.2 To explain this observation Strachan suggested that scientists should be seeking other influences of development, lifestyle, or environment that vary strongly with birth order.2

With this in mind, I (the author), as the father of three young boys, was concerned that, when it came to playing football, the youngest was always going to be nominated the goalkeeper and the eldest the striker. This led to my primary hypothesis that perhaps not only atopy varied with birth …