Breastfeeding and adult intelligence
Authors: Colin Cooper
Journal: The Lancet Global Health (2015)

Cesar Victora and colleagues (April, 2015)1Victora CG Lessa Horta B Loret de Mola C et al.Association between breastfeeding and intelligence, educational attainment, and income at 30 years of age: a prospective birth cohort study from Brazil.Lancet Glob Health. 2015; 3: e199-e205Summary Full Text Full Text PDF PubMed Scopus (368) Google Scholar report that breastfeeding is associated with children developing higher intelligence quotients (IQ) in later life. Victora and colleagues viewed this association as being causal. However, their experimental design ignored two important facts: first, that the children were biologically related to their mothers; and second, that adult IQ is strongly heritable (narrow heritability of at least 0·5, meaning that additive genetic effects account for half the variation in intelligence from person to person in Western populations).2Plomin R Petrill SA Genetics and intelligence: what's new?.Intelligence. 1997; 24: 53-77Crossref Google Scholar These genetic effects also interact with socioeconomic status.3Bates TC Lewis GJ Weiss A Childhood socioeconomic status amplifies genetic effects on adult intelligence.Psychol Sci. 2013; 24: 2111-2116Crossref PubMed Scopus (55) Google Scholar Mothers with high IQs might be more likely to breastfeed their children than mothers with lower IQs. The increased IQ of their children would then be a consequence of simple genetic transmission. Breastfeeding might have little direct effect on the IQ of the offspring. Evidence suggests that this is the case. Mothers with higher IQs are far more likely to breastfeed than are mothers with lower IQs;4Der G Batty GD Deary IJ Effect of breast feeding on intelligence in children: prospective study, sibling pairs analysis, and meta-analysis.BMJ. 2006; 333: 945Crossref PubMed Scopus (309) Google Scholar one SD difference in IQ more than doubled the odds of a mother breastfeeding. Furthermore, controlling for maternal IQ halved4Der G Batty GD Deary IJ Effect of breast feeding on intelligence in children: prospective study, sibling pairs analysis, and meta-analysis.BMJ. 2006; 333: 945Crossref PubMed Scopus (309) Google Scholar or—with the inclusion of socioeconomic status—eliminated5Jacobson SW Jacobson JL Breast feeding and intelligence in children.BMJ. 2006; 333: 929-930Crossref PubMed Scopus (15) Google Scholar any association between breastfeeding and adult IQ compared with bottle feeding. These studies4Der G Batty GD Deary IJ Effect of breast feeding on intelligence in children: prospective study, sibling pairs analysis, and meta-analysis.BMJ. 2006; 333: 945Crossref PubMed Scopus (309) Google Scholar, 5Jacobson SW Jacobson JL Breast feeding and intelligence in children.BMJ. 2006; 333: 929-930Crossref PubMed Scopus (15) Google Scholar also showed that maternal education is not an effective proxy for maternal IQ. Thus, although Victora and colleagues controlled for maternal education, this probably did not adequately control for maternal IQ. Whenever a highly heritable trait or behaviour such as IQ is linked to parental behaviour, it is crucial to control for parental levels of the trait or to develop alternative designs that remove confounding genetic effects. Otherwise, there is a real risk that public health interventions based on such research will be far less effective than might be expected. I declare no competing interests. Association between breastfeeding and intelligence, educational attainment, and income at 30 years of age: a prospective birth cohort study from BrazilBreastfeeding is associated with improved performance in intelligence tests 30 years later, and might have an important effect in real life, by increasing educational attainment and income in adulthood. Full-Text PDF Open AccessBreastfeeding and adult intelligence – Authors' replyColin Cooper argues that the positive association between breastfeeding and adult intelligence might be due to residual confounding by maternal intelligence quotient (IQ).1 This is indeed an important issue in high-income countries, but not in low-income or middle-income settings. Although we did not measure maternal IQ in 1982, we found no associations between breastfeeding duration and strong correlates for IQ, including schooling and family wealth;2 in our cohort participants, the correlation coefficient between IQ and years of schooling was 0·64. Full-Text PDF Open Access