Consistent data are available to suggest that children who have been breast fed are, on average, intellectually more able than their formula-fed contemporaries. This has been shown in eight of 10 population studies and all three studies of low birthweight infants. In general, the longer the child has been breast fed the more pronounced the effect. There is evidence that breast milk that has been pasteurised before feeding does not have this effect, but that fresh breast milk is effective whether the milk is delivered by tube or by the breast. However no studies have been able to have both sufficient statistical power and the ability to allow for other confounders such as parental ability, parental IQ and other factors that might explain these findings. Additional data from studies of visual acuity show an association between breast feeding and enhanced vision which is hypothesised to be due to the unique fatty acid composition of breast milk. The differences in intellectual development might also be related to these fatty acids. Alternative explanations for the effect on intellectual development concern the possible consequences of early infections, particularly gastroenteritis, which are more common in bottle-fed babies. In contrast with the many publications on cognitive function and breast feeding, there was only one on neurological dysfunction (showing a protective effect of breast feeding) and one on childhood behaviour (using the teacher's assessment no relationship with breast feeding was found). Further research is needed in both areas.