The evidence that breastfeeding improves cognitive development is based generally on observational studies and therefore may be affected by differences in the breastfeeding mother's behavior or her interaction with the infant. The PROBIT study followed up children from the original cohort at 6.5 years of age to assess whether prolonged and exclusive breastfeeding improves children's cognitive ability. A total of 17,046 healthy breastfeeding infants were enrolled, of whom 13,889 (81.5%) were followed up at age 6.5 years. The researchers measured IQ scores on the Wechsler Abbreviated Scales of Intelligence and teacher evaluations of academic performance in reading, writing, mathematics, and other subjects. The experimental intervention led to a large increase in exclusive breastfeeding at age three months and a significantly higher prevalence of any breastfeeding at all ages up to and including 12 months. The experimental group had higher mean scores on all of the Wechsler Abbreviated Scales of Intelligence measures for IQ. Teachers' academic ratings were significantly higher in the experimental group for both reading and writing. The researchers conclude that their results demonstrate that prolonged and exclusive breastfeeding improves children's cognitive development.

Kramer MS, Aboud F, Mironova E. (2008) Breastfeeding and child cognitive development: New evidence from a large randomized trial. Arch Gen Psychiatry 2008: 65; 578-584.