The publication in the Lancet of the European Concerted Action on sudden infant death syndrome (SIDS) (ECAS study)¹ resulted in front page headlines such as “Don’t sleep with your baby” (Daily Telegraph: D Derbyshire, Science Correspondent, 16 January 2004). Yet the ECAS study said nothing new about bed-sharing and cot death: both the CESDI study (Confidential Enquiry into Stillbirth and Death in Infancy),² data from New Zealand,³ and work from Ireland⁴ have superficially come to similar conclusions. Is the quality of evidence such that paediatricians, midwives, and health visitors should reasonably dissuade mothers from bed-sharing or co-sleeping, or is there more to it than that?

First, we must question the validity of extrapolating health messages from case controlled data sets. Bradford-Hill suggested robust criteria (temporal relationship, specificity, biological plausibility, coherence; others would add dose response) for inferring causality from associative data when prospective randomised trials are impossible.⁵ It took some time before these criteria were satisfied to such an extent that the successful “back to sleep” campaign could be accepted as public policy. That success should not seduce us into accepting a lower standard of evidence of causality for some “new” hypothetical risk factor. Arguably, now that we have good reason to promote supine sleeping, an appropriate thermal environment, and the avoidance of cigarette smoke, the benefit of any further message on reducing the risk of SIDS is likely to be marginal at best.

Second, there is a general lack of understanding about the heterogeneity of bed-sharing in particular, and infant sleep environments in general, in the data collection and analyses of case control studies. These …