Dear Marianna,
Welcome to the list and good luck. I, too, am preparing to take the exam. 
There is information out there about extended breastfeeding, but mostly it 
is aimed at mother support and establishing that nursing past one or two 
is a normal thing to do. For science, you can look for studies done by K. 
Dettweiler. They do not speak about US children and mothers specifically, 
but it is a good place to start to establish the biological norm/necessity 
of breastfeeding past two years old. I nursed my daughter until she was 6 
1/2  and my son until 4 1/2 (over three years tandem). You are welcome to 
email me and pick my brain. I know of one person currently researching 
this area, but I think it is for a book, not a scientific study. If you 
like, I will put you in touch with her. You can also read some of the 
parenting books by Dr. Sears and check out attachment parenting websites. 
They are proponents of child-led weaning and have documented positive 
behavioral outcomes in children who are allowed to wean when they are 
ready, rather than forced into it too early in their development. I hope 
this information helps you.

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