The prevailing consensus from large-scale epidemiological studies is that children who were breastfed perform, on average, higher on tests of IQ and cognitive functioning than do children who were exclusively formula-fed, even when factors such as birth-weight, gestation duration, and maternal education and socioeconomic status (SES) are accounted for.
The primary hypothesized substrate for these developmental advantages is the rich compliment of long-chain fatty acids found in breast milk, specifically docosahexaenoic (DHA) and arachidonic (AA) acids. Together, DHA and AA comprise approximately 20% of the fatty acid content of the brain and are involved in early neurodevelopment by promoting healthy neuronal growth, repair, and myelination.
... and from the abstract
Positive relationships between white matter microstructure and breastfeeding duration are also exhibited in several brain regions, that are anatomically consistent with observed improvements in cognitive and behavioral performance measures.
They had 3 groups: exclusively breastfed, exclusively formula, and some of both. There are a few minor oddities with the exclusively formula group: a tighter distribution of gestation times (rms of 12 rather than 34 or 40--is this enriched in c-sections?),slightly lower birth weight with a bigger rms, and what has to be a typo on the Mullen visual reception score (rms of 1 rather than 10-12). But nothing really jumps out at me.
They looked at the infants while they slept in a quiet MRI. I didn't know such a thing existed.
The raw curves for the amount of white matter vs age for the different groups look quite similar. The plot of difference vs mean is clear enough, but the individual variation is huge. It looks like a real effect.
The "receptive language" score differences (Receptive language: breastfed=41.1 ± 3.3, formula-fed=34.5 ± 5.6, p-value that these are the same distribution=0.0019) look rather different (most of the rest are not so dramatic, but overall breast-fed score higher).
I wonder about that "receptive language" difference. Do nursing mothers sing more to their babies?
I also wonder if the nutrition is the entire story. Being in such close contact with the mother for long periods should have some effect too. It might be interesting to look at whether there are brain changes (well before and after) when someone gets married and starts to spend more time touching someone else: Most of us don't do a lot of touching outside the immediate family. POSSLQ's probably muddy the possible change. Longitudinal study is the ticket here.