From what I've read, browsing thru thousands of articles as we build the CBI Research Library (yes, reading slows down my efficiency, but some of this stuff is plain fascinating!)
season of the year when the baby is born in the developing world, impacts mother's milk production and probably leads to earlier supplementation.  The opposite also being true, good season and mom BF longer before bothering with foods.   Other things that impact are cultural practices - these can have a big influence as seen in what happened in the 50s in the US - cereal at 2 weeks!  Another impact is the baby, who grabs and tries foods before we think they "should" and the baby who could care less until 7-8 mo.
 
Natalie or anyone,  you're welcome to come dig thru the paper  :-) We haven't secured copyright privileges yet :-( but on site searches are welcome.  Pat in SNJ

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