The relationship between breastfeeding duration and infant IQ development

Breastfeeding and IQ

Infants who are breastfeed tend to have higher IQ as they mature. This paper examines that phenomenon, which presents a complex set of cause-and-effect questions, including how long infants must be breastfed to receive any benefits associated with nursefeeding, whether there are important intercultural variations among children whose IQs are higher (for example, if there is a greater correlation in societies where most women breastfeed or, alternatively, where few breastfeed) and whether the rise in IQ results from the chemistry of breastmilk, the practice of breastfeeding itself, or from the fact that women who breastfeed are significantly different (on an a priori basis) from those who do not.

Introduction

For a number of years the scientific and medical communities have argued that breastfeeding provides a substantial benefit to infants, including primarily the fact that it promotes emotionally security and increases a child's overall health by transferring the mother's accumulated immunity to the infant. There have also been a number of reports that link breastfeeding to health benefits for the mother, primarily in terms of quicker weight loss after pregnancy. However, recent research suggests that there may be another important reason to breastfeed, which is that infants who are breastfed have a higher IQ when they are older. This paper examines this issue.

While it is tempting to assume that it is breastfeeding itself that produces the rise in IQ, the issue presents us with an almost classic case of the ways in which causation and correlation can become confused in terms of understanding how even a relatively straightforward phenomenon (http://www.fnri.dost.gov.ph/htm/philng.htm).There are a number of questions that must be answered to understand this phenomenon:

Methodology

Thus we are presented with two questions: Does breastfeeding cause a rise in IQ and if so why exactly does this occur. Studies have suggested that there is a causative link:

Regression adjustment for maternal and other factors associated with breastfeeding reduced the associations between breastfeeding and cognitive or educational outcomes. Nonetheless, in 10 of the 12 models, fitted duration of breastfeeding remained a significant predictor of later cognitive or educational outcomes. After adjustment for confounding factors, children who were breastfed for >= 8 months had mean test scores that were between 0.11 and 0.30 SD units higher than those not breastfed (http://www.breastfeeding.com/all_about/all_about_iq.html#FIRST).

However, other studies refute this finding - or at least call into question the overall importance (over the course of the entire lifespan) of breastfeeding in terms of increased IQ. While other research has correlated the findings of the New Zealand study that breastfeeding does provide an initial advantage to children, this advantage is reduced over time. Such a finding again underscores the complexity of attempting to determine what accounts for human intelligence (setting aside for the moment the fact that "intelligence" itself is a complex concept that has a number of different definitions, none of which works as well in all cultures). Certainly it is possible that any benefits vis-a-vis raised IQ that a child receives because of breastfeeding may well be wiped out later on if - for example - the child is severely abused.

Not all pediatric professionals or researchers agree with the findings of the three studies mentioned. An Australian study, concluding in early 1998, followed 375 children, assessing their cognitive development at ages 6 months and at 2, 4, 7 and 11 to 13 years. The results of this study, published in The Australian and New Zealand Journal of Public Health, indicated that breastfed children scored slightly higher than their formula-fed counterparts, but only at the younger ages. They began with a 5.5 point difference and declined to a 3.8 point difference in IQ points. Researchers concluded that the small benefit due to breastfeeding at an early age virtually disappears at later ages. The study states one explanation could be that a small cognitive benefit of breastfeeding may be overwhelmed at later ages by the effects of other social and environmental factors (http://breastfeed.com/resources/articles/breastfeediq.htm).

A corollary to this is the fact that the initial gains in IQ may be increased by later events.

Still, it remains the case that relatively few women breastfeed for long enough to have an effect on their chidlren's IQ:

Prevalence of breastfeeding at ages up to 9 months by country (1995 and 2000)

Age of baby

Percentage breastfeeding at each age

England & Wales

Scotland

Northern Ireland

UK Average

1995 2000 1995 2000 1995 2000 * based on a reduced number of cases excluding those babies who had not reached this age by stage 3.

Birth 68-71 55-63 45-54 66-69 1 Week 58-57 46-50 35-37 56-55 2 Weeks 54-44 47-32 34-53 52 6 Weeks 44-43 36-40 25-26 42 4 Months 28-29 24-30 12-14 27-28 6 Months 22-19 24 8-10-21 8 Months * 16-17 14-18 6-7-15-16 9 Months * 14-13 15 5-7-14-13 (http://www.nctpregnancyandbabycare.com/nct-online/bfaw2003/statistics.html)

This is true despite the fact that many women wished to continue to breastfeed but stopped for the variety of reasons listed below:

Reasons given by mothers for stopping breastfeeding, by duration of breastfeeding (United Kingdom, 2000)

Reason, by percent

Baby's age when breastfeeding ceased

1-2 Weeks

2-6 Weeks

Weeks - 4 Months

4-6 Months

6-9 Months code introduced at later waves of the survey.

Percentages don't always add up to 100% as some mothers gave more than one answer.

Baby rejected breast

Painful breasts / nipples

Insufficient milk

Too long / tiring

Mother was ill

Didn't like breastfeeding

Baby was ill

Domestic reasons

Baby couldn't be fed by others

Returned to work

Breastfed for as long as intended

Baby teething / biting *

Time was right to stop *

Base (http://www.nctpregnancyandbabycare.com/nct-online/bfaw2003/statistics.html)

Discussion

Even if we can assume that breastfeeding itself causes an increase in IQ (rather than simply being correlated with it) there remains the question of why breastfeeding should increase IQ. Current studies suggest that it is the chemistry of milk itself rather than the bonding that occurs during nursing:

As mentioned above, recent research places more value on the chemical makeup of milk rather than the bonding process. Dr. Dettwyler believes that, while human milk does provide the nutrients babies need, the bonding experience should not be overlooked. "Breastfeeding releases hormones in the mother which make her feel more affection toward her baby," she says. "Prior to modern times, the only time a woman would give birth and not breastfeed would be if the baby died. [Without breastfeeding], a mother's body assumes the baby has died and gears up for another attempt at reproduction. Yet she has a young baby, and the mother doesn't have the appropriate hormones" (http://breastfeed.com/resources/articles/breastfeediq.htm).

This finding deserves further study and might lead both to increased breastfeeding and healthier baby formulas.