Several mechanisms have been proposed Due to a
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. One proposed explanation is parental attention. A truncated birth interval has been linked to several adverse outcomes with regard to child, maternal, and even sibling morbidity and mortality. These outcomes might have ramifications for adult performance in the outcome measures observed.
Mothers who breastfeed tend to have smaller families, and children from small NSC59984 tend to out-perform those from large families on IQ testing. 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. 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. 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. These findings support the resource dilution and confluence models as well as my current sibling interval postulate. In the study by Victora and colleagues, 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. Breastfeeding has undoubted benefits and should be singularly encouraged and promoted. 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.
Colin Cooper argues that the positive association between breastfeeding and adult intelligence might be due to residual confounding by maternal intelligence quotient (IQ). 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; in our cohort participants, the correlation coefficient between IQ and years of schooling was 0·64. Had maternal IQ been a confounder in our study, our adjustment for proxy variables (maternal and paternal education, family income, and household assets) would weaken the association between breastfeeding and IQ, rather than strengthen it, as described in our paper. Furthermore, findings from a meta-analysis showed that, even in studies that controlled for maternal IQ, breastfeeding was associated with increased IQ (mean pooled difference 2·19, 95% CI 0·89–3·50). Finally, the only randomised study on this issue (the PROBIT trial) reported a 7·5-point increase in IQ among Belarusian children born in maternity hospitals in which breastfeeding was promoted. Zeljka Buturovic and Suzana Ignatovic argue that factors related to the child can also confound the association. Adjustment for the child\'s sex, birthweight, and gestational age did not affect our results. We did not measure the “baby\'s ability and willingness to nurse”, which they suggest would lead to reverse causation—that is, infants who learn to suckle more quickly would breastfeed for longer. This is a novel hypothesis in our view, but it would not explain the results from the PROBIT trial, nor would it make much sense in evolutionary terms since extended breastfeeding was universal in early mankind and lack of breastfeeding would greatly increase the risk of death. We hope that this hypothesis can be tested in future studies by obtaining detailed information on suckling intensity, although it might be necessary to wait for another 30 years to verify whether this variable can explain findings such as ours.