The "famous" Lucas study was done on premature infants given mother's milk
in a tube vs. formula in a tube, so you could separate out the effects of
"breast milk" from "breastfeeding."  ALL the other IQ studies, and there
have been more than 10 (see Jack Newman's post, or my web page) have been
done on full-term infants, so this IS NOT just a premature baby issue.  And
the Rogan and Gladen study looked at IQ differences and even school grades
out to age 8 years (3rd grade in the US) and found small but statistically
significant deficits in IQ for those not breastfed, and also found that the
highest difference was between those who had breastfed the longest (up to 2
years was all, though) and those bottle-fed from birth.
Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University