From the Wall Street Journal.

Excerpt:

Breast-feeding longer can make children smarter. That’s the conclusion of a study published Monday in JAMA Pediatrics, a journal of the American Medical Association.

[…]The latest study examined and rated each child’s environment based on factors such as how many books are available, and gave each mother an IQ test. They also asked detailed questions about factors that might influence IQ, such as child care, income and parental education. They then subtracted those factors using a statistical model. Dr. Belfort said she hopes that “what we have left is the true connection” with nursing and IQ.

[…]Researchers at Boston Children’s hospital followed 1,312 babies and mothers from 1999 to 2010. They found out how many of those children were still consuming their mothers’ milk at their first birthday, and then tested the children’s intelligence at ages 3 and 7.

[…]Children who were still nursing after a year had higher receptive language scores at age 3, which means they understood what was being said to them better than their formula-fed peers. At age 7, the breast-fed children scored higher on verbal and nonverbal intelligence tests.

In 3 year olds, every month of breast-feeding raised cognition scores by an average of .21 point. Each month of breast-feeding was associated with a .35 more verbal IQ point and a .29 more nonverbal point in the 7 year olds. A full year of nursing would boost a child’s IQ by about 4 points over a child who didn’t nurse, said Dr. Belfort, a significant bump considering that IQs average around 100. That is for children getting some breast milk in their diets; those consuming only breast milk before starting to eat solid foods around six months of age saw even greater advantages.

[…]The possible link between breast milk and brain development is only starting to be teased out. Some theories suggest that it isn’t the content of the milk but the bond between mother and child developed while nursing that accounts for some of the boost. Other ideas hinge on nutrients found in breast milk such as DHA and ARA, which are fatty acids linked to brain development. Some formula companies put DHA and ARA in their offerings.

[…]One difficulty in studying breast milk is that every feeding can vary based on the mother and what she has eaten. So the Boston researchers also examined a component in mothers’ diets that might be responsible for children’s brain development: fish, which contains DHA.

The authors found that more than two or more servings of fish per week seemed to confer IQ benefits, but that boost in children’s cognition wasn’t statistically significant.