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For eczema, 9 of 11 studies reported an inverse relation with number of siblings; for asthma and wheezing, 21 of 31 reported the inverse association; for hay fever, all 17 studies showed the effect; for allergic sensitisation or immunoglobulin E reactivity 14 of 16 studies supported the \u201cprotective\u201d effect of a higher number of siblings. The studies emphasise a \u201ctheory\u201d that is based exclusively on epidemiological associations.<\/jats:p>\n               <jats:p>Conclusions: Research has not yet answered the question of which causal factors explain the sibling effect. Causal factors must meet two criteria; they must vary with sibship size and they must protect against atopic manifestations. The prevailing \u201chygiene hypothesis\u201d failed to explain the findings adequately. Alternative explanations include in utero programming or endocrine explanatory models. The epidemiology research into siblings and atopic disorders has entered an intellectually challenging phase. Possessing sufficient knowledge about the causal factors might prevent at least 30% of all cases of asthma, eczema, and hay fever.<\/jats:p>","DOI":"10.1136\/jech.56.3.209","type":"journal-article","created":{"date-parts":[[2002,7,27]],"date-time":"2002-07-27T06:29:57Z","timestamp":1027751397000},"page":"209-217","source":"Crossref","is-referenced-by-count":250,"title":["Does a higher number of siblings protect against the development of allergy and asthma? 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