Background: Several studies show that children who were breastfed as babies gain higher scores on intelligence tests than those who were bottlefed. Although these findings suggest that breastfeeding in early life may promote cognitive development, their interpretation is complicated by the current association between breastfeeding and higher social class. We investigated the relation between method of feeding in infancy and adult intelligence in a setting where breastfeeding was not linked with socioeconomic advantage.
Methods: We followed up 994 men and women, born between 1920 and 1930 in Hertfordshire, UK, for whom information on infant feeding had been recorded by health visitors. Intelligence was measured by the AH4 IQ test, taken on a computer. Factors significantly linked with IQ were investigated by multivariate analysis.
Findings: Study participants who had been exclusively breastfed gained slightly higher scores on the IQ test than those who had been exclusively bottlefed, or fed with both breast and bottle. IQ was lower in participants who had used a dummy (pacifier) in infancy, in those whose fathers were in manual occupations at the time of their birth, and in those whose mothers were young at the time they were born. Scores on the IQ test fell as the number of older siblings increased. In multivariate analysis, after adjustment for the effect of all other variables, no association was found between adult intelligence and method of feeding. Dummy use in infancy, number of older siblings, maternal age at birth of the participant, and father's occupational class remained independent predictors of adult intelligence.
Interpretation: The mechanisms that link type of feeding in early life with later intelligence may have more to do with the child's social environment that with the nutritional qualities of the milk.