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.
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:
1) Is breastfeeding correlated with increased IQ?
This does seem to be supported by research, such as the following results that followed a group of children from birth to 18 years and focused on the relationship between duration of breastfeeding as infants and later broad-based measures of intelligence (Lucas, 1992, p. 261).
Increasing duration of breastfeeding was associated with consistent and statistically significant increases in 1) intelligence quotient assessed at ages 8 and 9 years; 2) reading comprehension, mathematical ability, and scholastic ability assessed during the period from 10 to 13 years; 3) teacher ratings of reading and mathematics assessed at 8 and 12 years; and 4) higher levels of attainment in school leaving examinations. Children who were breastfed for >= 8 months had mean test scores that were between 0.35 and 0.59 SD units higher than children who were bottle-fed (http://www.breastfeeding.com/all_about/all_about_iq.html#FIRST).
2. If we can then assume that it is true that breastfeeding is correlated with increased IQ, does it cause that increase?
This is not a simple question for while breastfeeding is something that is natural in the sense that nearly every human (as well as every other mammal) can nurse infants, within the context of human society it is far more a question of culture than of nature (Bauer, 1991). The profile of women who breastfeed is different from those who do not, and this suggests that those infants who are breastfed have both a different genetic inheritance as well as a different micro-cultural environment that they grow up in (http://www.vhihealthe.com/news/n041002c.html).Nevertheless, an assessment of intelligence across cultures suggests that there are real links between intelligence and breastfeeding:
Katherine Dettwyler, Ph.D., member of the anthropology department at Texas A & M. University, has researched the role of breastfeeding rituals and their results in primitive cultures as well as in today's society. "I have carefully read most of these studies, and find them to be carefully constructed and carried out," she says. "The ones that include duration of breastfeeding show that the longer the child is breastfed, up to study limits of 24 months, the greater their IQ scores and school performance. The human child's brain is growing most rapidly during the first two years of life. Since we know that some of the ingredients in breastmilk are critical to brain growth and development, the results are not surprising" (http://breastfeed.com/resources/articles/breastfeediq.htm).
It is impossible definitively to sort out differences in IQ in those who are breastfed and those who are not given the range of factors that contribute to intelligence:
Mothers who elected to breastfeed tended to be older; better educated; from upper socioeconomic status families; were in a two-parent family; did not smoke during pregnancy; and experienced above average income and living standards. Additionally, rates of breastfeeding increased with increasing birth weight, and first-born children were more likely to be breastfed (www.breastfeeding.com/all_about/all_about_iq.html).
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
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.