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Introduction
Since 1929 many studies have examined the link between breast feeding and intelligence, most finding higher IQ scores among children who were breast fed. 1 The few randomised controlled trials were confined to preterm infants, 2 â[euro]" 4 whereas studies of individuals born at term were observational in design. Inference from the observational studies is hampered by confounding: there are fundamental differences between mothers who choose to breast feed and those who do not. Studies that do not control adequately for confounders may mistake residual confounding for a real effect of breast feeding.
Many potential confounding variables have been identified, including duration of breast feeding, sex, maternal history of smoking, maternal age, maternal intelligence, maternal education, maternal training, paternal education, race or ethnicity, socioeconomic status, family size, birth order, birth weight, gestational age, and childhood experiences. 5 Those singled out as particularly important include socioeconomic status, maternal education, and birth weight 6 or socioeconomic status/parental education and stimulation of the child. 7 In contrast, maternal intelligence is relatively overlooked as a potential confounder. This is surprising given the heritability of intelligence 8 and the known association of maternal intelligence with both the initiation and duration of breast feeding. 9
We examined the relation between breast feeding and intelligence and assessed the role of maternal IQ and other covariates in generating the association. We took both a conventional approach to control for confounders and an alternative approach using sibling comparison analysis. Used recently to assess the benefits of breast feeding, 10 this approach has the considerable advantage of controlling for many confounding factors without having to measure them. Any factor that is the same for both members of a pair of siblings is automatically and fully controlled for. Thus, the method implicitly controls for parental intelligence.
Methods
We used data from the US national longitudinal survey of youth 1979 (NLSY79). 11 This is a population based sample of 12 686 young people (6283 female) aged 14 to 22 when first interviewed in 1979 who were then interviewed annually until 1994 and biennially thereafter. Since 1986 the children of the women in the survey have also been assessed biennially. The database for the children is referred to as the NLSY79 child and young adult sample ("young...