From the May 8, 2002 issue of The Journal of the American Medical Association ( JAMA) reports a study that suggests that the longer an infant breast feeds, the more likely their intelligence level will be higher when they become an adult.  According to the study the results showed that the duration of breastfeeding was associated with significantly higher scores on the Verbal, Performance, and Full Scale IQs testing.  

The study was conducted in a sample of 3253 men and women, all of whom were born in Copenhagen, Denmark, between October 1959 and December 1961. The subjects were divided into 5 categories based on duration of breastfeeding, as assessed by physician interview with mothers at a 1-year examination.  The average results showed that for infants that breast fed for less than 1 month the average adult IQ at age 27 was 99.4.  Conversely infants that breast fed for more than 9 months showed an average IQ at age 27 of 104.0.   Infants in between the breast fed durations of less than one month and more than 9 months showed IQs that were between 99.4 and 104.0.  

This data convinced researchers of a direct correlation between length of time that an infant breast feeds and the probable adult IQ rate later in life. The researchers concluded, "Independent of a wide range of possible confounding factors, a significant positive association between duration of breastfeeding and intelligence was observed in 2 independent samples of young adults, assessed with 2 different intelligence tests."