Katherine Dettwyler is a great source for info on extended bf - here's her site with lots of research from an anthropological standpoint on bf and other topics:
http://www.kathydettwyler.org/dettwyler.html
Quote from:
Beyond Toddlerhood: The Breastfeeding Relationship Continues
by Priscilla Young Colletto
http://www.lalecheleague.org/lllead...FebMar98p3.html
"The scientific evidence on extended breastfeeding is just now beginning to accumulate. A number of the health benefits are now being found to be related to the length of nursing with an increasing amount of benefit correlating with increased duration. This is the case, research has indicated, with breastfeeding's protective effect in maternal breast cancer, osteoporosis, childhood ear infections and malocclusion anomalies (misaligned teeth).
The word "benefit" is perhaps misleading here, for these "benefits" are what nature intended to be the human norm. Breastfeeding is normal. It is artificial feeding substitutes and premature weaning that are, in fact, abnormal from a biological viewpoint. It is these abnormal practices that place the child at increased risk of illness and compromised intelligence. "
American Academy of Family Physicians:
"If the child is younger than two years of age, the child is at increased risk of illness if weaned."
http://www.aafp.org/x6633.xml
According to Sally Kneidel in "Nursing Beyond One Year" (New Beginnings, Vol. 6 No. 4, July-August 1990, pp. 99-103.):
" One study that dealt specifically with babies nursed longer than a year showed a significant link between the duration of nursing and mothers' and teachers' ratings of social adjustment in six- to eight-year-old children (Ferguson et al, 1987). In the words of the researchers, 'There are statistically significant tendencies for conduct disorder scores to decline with increasing duration of breastfeeding.'"
And from milkofhumankindness.org

no longer a valid link, if anyone finds a current web address for this site I'd love to have it.)
"Cross-cultural research indicates a positive correlation between a culture's norm for duration of breastfeeding and a its level of peacefulness. Breastfeeding also promotes development in the parts of the human brain that regulate emotions and help us solve problems non-violently."