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Commentary: Alcohol, child development and harm to others: a 'hard' problem.

In epidemiology and public health, as in any other area of science, not all interesting and potentially important questions are equally easy to address. Some are tough and refractory, resisting repeated attempts to answer them convincingly. The impact of breastfeeding on child growth and development is at the hard end of this spectrum. Observational studies are fraught with difficulties, as breastfeeding in many countries is highly socially patterned and strongly linked to a range of maternal characteristics, all of which could confound any of the observed associations. In addition, the risk of reverse causality is substantial (a sick infant will be fed differently). Nevertheless, there remains considerable scientific, policy and public interest in the potential benefits of breastfeeding on reduced risk of obesity, improved neurocognitive performance and cardiovascular risk profile in childhood and later life, to name but a few outcomes. One of the most important recent contributions to this research area has been the PROBIT trial set up in 1996–97 in Belarus. 1 Taking advantage of the newly found openness resulting from the then recent break-up of the Soviet Union (1991–92), this study used an ingenious and pragmatic clusterrandomized design, in which maternity units were allocated to provide either enhanced breastfeeding support or usual practice. During the past few years, this large trial has provided some of the most convincing evidence to date that extended breastfeeding is associated with increased childhood cognition, 2 but appears not to increase stature or reduce blood pressure or adiposity or obesity in childhood. 3,4 This study is now being exploited to see what it can tell us about the impact of parental behaviours and family circumstances on child development. In this issue of the IJE, Yang and Kramer report the results of their analyses of parental drinking and family break-up on childhood cognition and behaviour at 6.5 years of age. 5 Like the impact of breastfeeding,