Breastfeeding and adult intelligence
Authors: Ossie Ferdinand Uzoigwe
Journal: The Lancet Global Health (2015)

Cesar Victora and colleagues1Victora 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 (389) Google Scholar use a robust method to show an association between the duration of breastfeeding and adult intelligence, educational attainment, and professional success. However, some potentially important confounders are overlooked. These confounders include the birth interval between children and family size. It is well established that breastfeeding increases the interval between siblings.2Brown RE Breast-feeding and family planning: a review of the relationships between breast-feeding and family planning.Am J Clin Nutr. 1982; 35: 162-171PubMed Scopus (16) Google Scholar, 3Saadeh R Benbouzid D Breast-feeding and child-spacing: importance of information collection for public health policy.Bull World Health Organ. 1990; 68: 625-631PubMed Google Scholar, 4Howie PW McNeilly AS Effect of breast-feeding patterns on human birth intervals.J Reprod Fertil. 1982; 65: 545-557Crossref PubMed Scopus (135) Google Scholar Direct evidence from the UK National Child Development Study,5Kanazawa S Intelligence, birth order, and family size.Pers Soc Psychol Bull. 2012; 38: 1157-1164Crossref PubMed Scopus (30) Google Scholar which included 17 419 participants, showed that increasing birth interval was associated with increasing childhood intelligence as measured by intelligence quotient (IQ). This association was independent of birth order, number of siblings, social class, and maternal or paternal education level. Indeed, the older the children the more demonstrable the effect.5Kanazawa S Intelligence, birth order, and family size.Pers Soc Psychol Bull. 2012; 38: 1157-1164Crossref PubMed Scopus (30) Google Scholar Hence, increasing birth interval had a much stronger positive effect on intelligence at age 16 years than in early childhood. This lends support to the notion that it is the effect of birth interval not lactation per se that affects adult outcomes. Wagner and colleagues6Wagner ME Schubert HJ Schubert DS Effects of sibling spacing on intelligence, interfamilial relations, psychosocial characteristics, and mental and physical health.Adv Child Dev Behav. 1985; 19: 149-206Crossref PubMed Scopus (12) Google Scholar in their early review in 1985 also found short sibling spacing to have a deleterious effect on intelligence. Several mechanisms have been proposed. Due to a greater birth interval, children who are breastfed will enjoy a greater period of exclusive parental attention as either the only child or youngest child. This exclusivity might have an effect on intelligence. It has been consistently observed that the eldest child in a family tends to have the highest IQ when compared with his or her siblings.7Kristensen P Bjerkedal T Explaining the relation between birth order and intelligence.Science. 2007; 316: 1717Crossref PubMed Scopus (159) Google Scholar One proposed explanation is parental attention.7Kristensen P Bjerkedal T Explaining the relation between birth order and intelligence.Science. 2007; 316: 1717Crossref PubMed Scopus (159) Google Scholar A truncated birth interval has been linked to several adverse outcomes with regard to child, maternal, and even sibling morbidity and mortality.2Brown RE Breast-feeding and family planning: a review of the relationships between breast-feeding and family planning.Am J Clin Nutr. 1982; 35: 162-171PubMed Scopus (16) Google Scholar, 3Saadeh R Benbouzid D Breast-feeding and child-spacing: importance of information collection for public health policy.Bull World Health Organ. 1990; 68: 625-631PubMed Google Scholar, 6Wagner ME Schubert HJ Schubert DS Effects of sibling spacing on intelligence, interfamilial relations, psychosocial characteristics, and mental and physical health.Adv Child Dev Behav. 1985; 19: 149-206Crossref PubMed Scopus (12) Google Scholar These outcomes might have ramifications for adult performance in the outcome measures observed. Mothers who breastfeed tend to have smaller families,2Brown RE Breast-feeding and family planning: a review of the relationships between breast-feeding and family planning.Am J Clin Nutr. 1982; 35: 162-171PubMed Scopus (16) Google Scholar, 3Saadeh R Benbouzid D Breast-feeding and child-spacing: importance of information collection for public health policy.Bull World Health Organ. 1990; 68: 625-631PubMed Google Scholar, 4Howie PW McNeilly AS Effect of breast-feeding patterns on human birth intervals.J Reprod Fertil. 1982; 65: 545-557Crossref PubMed Scopus (135) Google Scholar and children from small families tend to out-perform those from large families on IQ testing.8Belmont L Marolla FA Birth order, family size, and intelligence.Science. 1973; 182: 1096-1101Crossref PubMed Scopus (233) Google Scholar, 9Rodgers L Cleveland HH van den Oord E Rowe DC Resolving the debate over birth order, family size, and intelligence.Am Psychol. 2000; 55: 599-612Crossref PubMed Scopus (170) Google Scholar This family size effect is strong. The superior intelligence of the eldest child observed has been suggested to be merely an artefact of the effect of family size.5Kanazawa S Intelligence, birth order, and family size.Pers Soc Psychol Bull. 2012; 38: 1157-1164Crossref PubMed Scopus (30) Google Scholar, 9Rodgers L Cleveland HH van den Oord E Rowe DC Resolving the debate over birth order, family size, and intelligence.Am Psychol. 2000; 55: 599-612Crossref PubMed Scopus (170) Google Scholar One hypothesis posits that parents have finite resources, as children all vie for this valuable commodity. The greater the family size, the less is distributed to each child, which might influence adult intelligence. This is the resource dilution hypothesis.5Kanazawa S Intelligence, birth order, and family size.Pers Soc Psychol Bull. 2012; 38: 1157-1164Crossref PubMed Scopus (30) Google Scholar A short birth interval would tend to exaggerate this phenomenon. The confluence theory claims that in large families, with a greater number of children, there are a greater number of immature interactions, leading effectively to a so-called intellectual dilution. Twin studies are intriguing. Meta-analyses and national studies consistently show that twins tend to have lower IQs than singleton siblings.10Voracek M Haubner T Twin-singleton differences in intelligence: a meta-analysis.Psychol Rep. 2008; 102: 951-962Crossref PubMed Google Scholar, 11Eriksen W Sundet JM Tambs K Twin-singleton differences in intelligence: a register-based birth cohort study of Norwegian males.Twin Res Hum Genet. 2012; 15: 649-655Crossref PubMed Scopus (8) Google Scholar These findings support the resource dilution and confluence models as well as my current sibling interval postulate. In the study by Victora and colleagues,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 (389) Google Scholar the effects noted might be the result of breastfed childhood receiving special and exclusive attention, as the youngest and most altricial member of the family, for a greater period before the arrival of the next child. Indeed it is generally accepted that the purpose of the contraceptive effect of lactation is to preserve an adequate birth interval.3Saadeh R Benbouzid D Breast-feeding and child-spacing: importance of information collection for public health policy.Bull World Health Organ. 1990; 68: 625-631PubMed Google Scholar Breastfeeding has undoubted benefits and should be singularly encouraged and promoted.12Cleminson J Oddie S Renfrew MJ McGuire W Being baby friendly: evidence-based breastfeeding support.Arch Dis Child Fetal Neonatal Ed. 2015; 100: F173-F178Crossref PubMed Scopus (34) Google Scholar It is part of a nexus of factors that might contribute to adult intelligence. However, the mechanism by which this contribution occurs might not be a direct effect of lactation or the contents of breastmilk. 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