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The Importance of Breastfeeding and Its Relation to the Intelligence

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Breastfeeding in connection with intelligence has long been a study of scientists in psychological professions in the years succeeding a 1929 study on the subject. Argumentation has gone back and forth, with some arguing that breastfeeding affects children’s general intelligence—intelligence as measured by a variety of intelligence tests. Others, however, contradict this stance by observing that if one controls for the parents’ IQs, that supposed causal relationship dissipates. Several studies appear to have laid previous groundwork that this researcher wished to expand upon and use for further research and analysis. In this study, the relationship between breastfeeding and intelligence is put to the test and analyzed.

Goal of the Research

In response to the contradicting opinions on the connection between breastfeeding and child intelligence, the author of this study states that he is attempting to study the effect of breastfeeding on children’s intelligence while establishing a control for the mothers’ IQ’s. In his introduction to the study, he makes it clear that he sees several studies that may present a challenge to his ideas, but he believes that he may have a new approach to an old question. In the past, little statistical significance was found after controlling for parental IQs, but the author of this study hopes that a longitudinal study and multiple IQ tests (previous studies only used one test) could result in more reliable and valid results.


Satoshi Kanazawa’s hypothesis appears to be that, in general, when women breastfeed their babies, they are more likely to grow into children with higher IQs. Isolating mothers’ IQs and other outside factors, Kanazawa believes that this will still hold true.


In studying the first generation of participants, the respondents operated within the confines of a dependent variable of general intelligence and an independent variable of breastfeeding. The child respondents took several tests throughout their childhood years—ages 7, 11, and 16. These tests were testing their general intelligence through a variety of intelligence tests, and their names were listed as “Copying Designs Test, Draw-a-Man Test, Southgate Group Reading Test, and Problem Arithmetic Test.” Using those scores, the psychologist calculated the IQ scores of the children at those three ages and used those scores as his primary numbers for the dependent variables. He then asked the mothers of the participants whether they had breastfed their children while they were infants, and had them rank the amount they breastfed their children. These scores were ranked from 0-2, representing whether the child was fed not at all, less than a month, or for more than a month. These factors were all controlled according to the intelligence of their parental figures. These factors were controlled by the total year of education of the parents (ranked on a 1-10 scale) and by the style of occupation of the fathers in the families, ranging from no father to white collared and professional positions. Outside of those main factors, Kanazawa also controlled for father’s and mother’s ages at birth, as well as birth weight in ounces. After having controlled for those factors, Kanazawa proceeded with his studies.

Results and Conclusions

After having collected his data, Kanazawa performed regression analysis and found that breastfeeding was closely associated with intelligence in every set of age data he analyzed. After having analyzed that raw data, he found that the results, while less obvious, were still statistically significant when the data was controlled for the education of the parents, as well as the social class of the families. Even further, Kanazawa still saw statistical significance after calculating for birth weight as well! He found the interesting evidence that the individual intelligence tests could not be used to develop a correlation between IQ and breastfeeding, but when the tests were combined and analyzed together, the result was clearly significant. Using this evidence, Kanazawa argues that previous studies may have found a lack of evidence because they only used one intelligence test. He argues that because of his use of multiple tests, he was able to uncover a correlation that was previously less noticeable.


With Kanazawa’s recent discoveries in this area of connection between breastfeeding and intelligence, much research potential comes to light. In regards to a theoretical application, this research builds on an overlying assumption in the relationship between nature and nurture that psychologists have long known to be true—that parental nurturing affects the intelligence of children. With this specific question related to breastfeeding, this assumption can be further solidified in the minds of psychologists. It continues to prove that parents are fundamentally important in the intelligence of their children. At a more practical level, Kanazawa’s research reignites the idea that breastfeeding your children may make them smarter. On the surface, his evidence appears to suggest as much.

Future Research

Sparked by the previously mentioned overlying questions concerning nature vs. nurture, it seems to me that this study could be expanded upon in a few different ways. First, more study along the lines of the effects of breastfeeding could be performed. I would be curious if the study would produce the same results in other countries or when taking into account another generation or using different IQ tests. In a broader sense, the study could lead us to ask more questions about the influence of breastfeeding on children. How does breastfeeding affect the ability of children to develop stable relationships with their parents? How about in relationship to their peers? Does it have any effect? At another level, how does breastfeeding affect a child’s future health? Is there a correlation between breastfeeding and childhood BMI? Is there a correlation between breastfeeding and a child’s desire for good health as a child? Those would all be questions that could be explored with further research.

In my case, I would be particularly interested in exploring whether the relationship between maternal breastfeeding and children’s general intelligence is truly causation or whether it is simply a loose correlation. I understand that the study was controlled for social status and parental general intelligence, but I do not believe that those controls are enough to adequately isolate the effect of breastfeeding. Those controls may remove parental privilege from the picture, but they do not control for parental motivation. If parents breastfeed their children, it would seem to me that they would also be more likely to stay closely invested in their children’s education, resulting in smarter children by correlation, rather than breastfeeding actually causing a higher IQ. Personally, I know that my father’s education level is not impressive even if it is controlled for, which could lead researchers to assume that my intelligence could be associated with other factors, but my father truly is one of the most important elements of me developing my intelligence as an individual. While he had very little formal education, he had endless motivation and a deeply inquisitive mind. He was my hero, and his motivation and inquisition inspired me to work hard and develop a mind that really likes to think deeply. Because of that, I question whether the controls that Kanazawa used were truly effective in isolating parental influence. If I were to produce further research, I would develop a more intricate method of controlling for parental influence, perhaps looking at twins—one of whom was breastfed while the other was not. While this data set may be harder to find, I believe it would be far more effective in isolating parental involvement.

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