BMJ  2006;333:929-930 (4 November), doi:10.1136/bmj.39020.473322.80

Editorial

Breast feeding and intelligence in children

Mediated by mother's intelligence rather than better nutrition

Many studies report that children who were breast fed perform better in tests of intellectual competence than those who were not.^1-3 This association is seen for full term infants and more strongly for those who are preterm or of low birth weight. The IQ advantage for full term infants is small, roughly three to four points, but it is remarkably consistent across studies. It is unclear, however, whether this difference reflects a direct nutritional advantage or a difference in socioenvironmental factors that are more favourable in women who breast feed.

Women who breast feed are more likely to provide their child with a more enriched and cognitively stimulating environment than those who do not breast feed, which could contribute to their children's better cognitive performance. Several studies have reported a positive relation between mother's IQ and the decision to breast feed.¹ A study in this week's BMJ by Der and colleagues assesses the impact of controlling for maternal intelligence and other confounding factors on this association.¹

To distinguish between the contributions of nutrition and socioeconomic factors to childhood IQ, previous studies have used socioeconomic status and maternal education to measure the quality of the child rearing environment.² Studies that show a positive effect of breast feeding on childhood IQ after controlling for socioeconomic status and mother's education conclude that breast feeding has a benefit above and beyond that of a better child rearing environment. In our cohort of 323 children from Michigan, we confirmed previous findings of a positive association between breast feeding and higher childhood IQ after adjustment for socioeconomic status and mother's education.⁴ However, when we also controlled for more direct indicators of parental cognitive stimulation—maternal vocabulary IQ and the home observation for measurement of the environment scale (HOME)—the association between breast feeding and childhood IQ was no longer evident. HOME is a semistructured interview and behavioural observation measure that assesses the quality of intellectual stimulation and emotional support provided by parents. Despite these findings, many recent studies that have considered socioenvironmental factors have underplayed the role of maternal IQ in the association between breast feeding and cognitive function.

Der and colleagues' new study demonstrates the importance of controlling for maternal IQ by showing that the association of breast feeding with cognitive performance in children drops from 4.7 to 0.5 points after adjusting for mother's cognitive competence and other socioenvironmental measures in a large cohort of 5475 children from the national longitudinal survey of youth.¹ Moreover, the study identifies maternal IQ as the main variable that accounts for the association between breast feeding and childhood IQ. Of the nine indicators included in their regression analysis, maternal IQ was the strongest predictor. The authors suggest that the association between breast feeding and childhood IQ results from the relation between cognitive competence in mother and child—attributable at least in part to heritability—and the fact that mothers with a higher IQ are more likely to breast feed.

The strength of the association between the outcome and a given predictor in multiple regression analysis depends, in part, on the reliability with which that predictor is measured. The short form of HOME was administered in the national longitudinal survey of youth, and Der and colleagues included only two of the subscales in their analysis. We used the complete HOME assessment in our research, and maternal vocabulary IQ and HOME contributed equally to the prediction of IQ score at 11 years of age.⁵ These data suggest that heritability and rearing environment play an equally important part in the association between breast feeding and childhood IQ, and that these factors should be considered in evaluating the impact of breast feeding on cognitive ability in children. In contrast to IQ and quality of child rearing, maternal depression, anxiety, and stressful events were not related to duration of exclusive breast feeding.^{5 6} These data support Der and colleagues' finding that maternal intellectual enrichment and genetic endowment, as measured by maternal IQ, are crucial potential confounders of the effect of breast feeding on cognitive competence in children.

The US Department of Health and Human Services Office on Women's Health concluded that "the observation... that... cognitive function of [breast-fed] children is greater than in non-breastfed children... has not been conclusively proven."⁷ The inconsistency in the literature is due, at least in part, to the use of global indirect indicators of the social environment. Der and colleagues' meta-analysis shows that, when the relevant confounding variables are included, the data do not support a direct nutritional benefit for childhood IQ in full term infants.

Other benefits of breast feeding—such as the effect of colostrum on the immune system, emotional benefits that may promote more secure infant-mother attachment, and the effects of omega 3 fatty acids on cognitive and visual development in preterm and low birthweight infants—are well established. But Der and colleagues' results may help doctors when giving advice on breast feeding to mothers with particular problems—for example, women who take drugs to prevent seizures or depression—which may affect the infant when transferred via breast milk—or women who want or need to go back to work. These data indicate that one consideration—an adverse long term effect on cognitive function—need not be of concern.

Department of Psychiatry and Behavioral Neurosciences, Wayne State University, School of Medicine, Detroit, Michigan 48207, USA
(Sandra.Jacobson{at}wayne.edu)

Department of Psychiatry and Behavioral Neurosciences, Wayne State University, School of Medicine, Detroit, Michigan 48207, USA

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Competing interests: None declared.

Research p 945

References

1. Der G, Batty GD, Deary IJ. Effect of breast feeding on intelligence in children: prospective study, sibling pairs analysis, and meta-analysis. BMJ 10.1136/bmj.38978.699583.55[Abstract/Free Full Text]
2. Lucas A, Morley R, Cole TJ, Lister G, Lesson-Payne C. Breast milk and subsequent intelligence quotient in children born preterm. Lancet 1992;339: 261-4.[CrossRef][ISI][Medline]
3. Anderson JW, Johnstone BM, Remley DT. Breast-feeding and cognitive development: a meta-analysis. Am J Clin Nutr 1999; 70: 525-35.[Abstract/Free Full Text]
4. Jacobson SW, Jacobson J. Breastfeeding and intelligence. Lancet 1992;339: 926.[CrossRef][ISI][Medline]
5. Jacobson SW, Chiodo LM, Jacobson JL. Breastfeeding effects on intelligence quotient in 4- and 11-year-old children. Pediatrics 1999;103: e71.[Abstract/Free Full Text]
6. Rao MR, Hediger ML, Levine RJ, Naficy AB, Vik T. Effect of breast-feeding on cognitive development of small for gestational age infants. Acta Paediatr 2002; 91: 267-74.[CrossRef][ISI][Medline]
7. US Department of Health and Human Services, Office on Women's Health. Health and human services blueprint for action on breastfeeding. Washington, DC: US DHHS, 2000.

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Relevant Article

Effect of breast feeding on intelligence in children: prospective study, sibling pairs analysis, and meta-analysis

Geoff Der, G David Batty, and Ian J Deary
BMJ 2006 333: 945. [Abstract] [Full Text] [PDF]