The Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK
SirsObel et al.1 report that their data fail to confirm the suggestion of Fukuda et al.2 that periconceptional smoking of either parent tends to lower the associated offspring sex ratio (proportion male). There are at least a dozen other studies of this sort, and some confirm the propositions and others do not. It would be possible to perform a Mantel-Haenszel test across these data sets and thus derive an overall meta-analytic assessment. However, I suggest instead that such a procedure would be invalid because all of these data are bedevilled by the possibility of confounding.
A potential for confounding arises when some forms of behaviour (e.g. smoking) are tested as risk factors for outcomes which are dependent on gonadal hormone levels.3 The potential arises because both the behaviour and the outcome are independently associated with the same variables viz. parental hormone concentrations. There can be no reasonable doubt that when the smoking habit is initiated, smokers have higher testosterone and oestrogen levels than non-smokers46 on the average. So cross-sectional studies would be expected to yield the spurious suggestion that smoking protects against pathologies which are actually caused by low levels of gonadal hormones. (Epidemiologists should always be aware that such pathologies, e.g. Parkinson's disease, may in fact be at least partially caused by low levels of gonadal hormones.) These points are relevant for the following reasons.
References
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