Of the 10 plagues visited on the biblical Egyptians, the last was the most terrible; after the rain of frogs, the plague of boils, and the hailstorms came the indiscriminate slaughter of all firstborn animals including children. Infanticide of this degree is thankfully rare—but is it possible that the author(s) of Exodus were expressing a subtler truth?

ASSOCIATIONS BETWEEN BIRTH ORDER/SIBSHIP AND DISEASE

Studies of birth order—or sibship size—as a risk factor have a long history and have examined a wide variety of diseases. Thus, for example, the rates of Hodgkin’s lymphoma in young adults,^1,2 HBsAg+ hepatocellular cancer,³ acute lymphoblastic leukaemia,⁴ and type I diabetes mellitus^5,6 all appear to fall with increasing birth order. In each case the pattern has been assumed to reflect the relatively late age at which children of low birth order (or their mothers during pregnancy) acquire common infections. A similar (but opposite) reasoning has been applied to the observations that children of low birth order are at reduced risks of non-Hodgkin’s lymphoma,⁷ schizophrenia,⁸ gastric carcinoma and ulcer,⁹ acute myeloblastic leukaemia,¹⁰ and some congenital heart defects.¹¹ Children of low birth order are also more likely to have infantile pyloric stenosis, to be taller,¹² to be right handed¹³ and, if they are male, to be heterosexual;¹⁴ these are less easily attributed to patterns of early infection.

Nowhere, however, are the patterns of birth order/sibship clearer than with the childhood respiratory allergies. First observed by Butler and Golding in 1986,¹⁵ reductions in the risks of hay fever, eczema, atopy and, less consistently, asthma with increasing birth order or sibling numbers have been reported in at least 30 studies and usefully reviewed by Karmaus …