Breastfeeding has been associated with higher IQs and other measures of cognitive and developmental abilities, but the studies were often dismissed because breastfeeding and IQs also parallel social, cultural and occupational differences. In other words, who's to say that breastfeeding made Johnny smarter when it could have been that his parents were also better educated and wealthier than Jimmy's and therefore they could have provided other advantages to increase his scholastic performance.
As pointed out in What to Feed Your Baby,* those confounding factors were able to be eliminated in a study of over 1000 children in Christchurch, New Zealand, where the children stayed in the area most of their lives and could be compared at different time points from infancy through their school years. And as you may have suspected, those that breastfed consistently performed better. Similarly, and interestingly, Danish infants who were breastfed for four to six months performed better when they entered the army at 18.7 years of age (though the confounding influences may not have been eliminated in quite the same way.
the reason for bringing all of this up is that a French study** tracked 1199 children to 3 years of age, testing their skills at 2 and 3 years. After adjusting for other variables, they did find that infants who breastfed for at least some period scored 3.7 points higher in their language ability at 2 years of age and their developmental abilities were 6.2 points higher at 3 years of age when compared to never-breastfed infants. Yes, the breastfeeding moms had more education, higher socio-economic status, smoked less and had more frequent activities with their children, but even when those factors were controlled for, these results remained.
What's also interesting is that a dose response relationship emerged. For every month of exclusive breastfeeding, there was an interest of 0.75 points at 2 years and 1.0 points at 3 years. Another point is that girls did substantially better on these tests than boys, with gender making more of a difference than breastfeeding, which may say more about the tests that were used (favoring girls) than about gender or breastfeeding.
BOTTOM LINE: While these may not have been the best tests to use (as shown by the gender difference), breastfeeding longer has a direct effect on brain function. If you're not breast feeding, offer formulas with higher DHA levels, since that's one factor that's been demonstrated to be important in the developing brain. Remember that brain development continues more actively through 2 years of age so continue to offer foods (fish and supplemented eggs, for example, or toddler formula) that have DHA available for brain and neuron growth and activation. And whether you breast or bottle feed, make sure you have frequent activities with your growing infants and children for their intellectual and emotional development.
*Horwood and Furgusson, Pediatrics 1998 and Mortensen et al JAMA 2002 in What to Feed Your Baby
**Bernard et al, J Pediatrics July 2013