I had always vaguely understood that world wide weaning age ranged 2-4
years and this was attributed to Mead and Newton. I tried to pin it down
and the closest I got was a chapter in "Childbearing - Its Social and
Psychological Aspects, edited by Stephen A. Richardson and Alan F.
Guttmacher, Williams and Wilkins, 1967.  The chapter is "Cultural
Patterning of Perinatal Behavior" by Margaret Mead and Niles Newton.  They
discuss the "transition period" as the period between delivery of the
infant and the establishment of total physiological separateness.  "In
modern American culture this period often lasts less than one minute -
until the cord is cut before the delivery of the placenta.  In many
primitive cultures the transition period may last three years or even
longer."

Dettwyler's  chapter "A Time to Wean: The Hominid Blueprint for the Natural
Age of Weaning in Modern Human Populations  in  Breastfeeding: Biocultural
Perspectives, Patricia Stuart-Macadam & Katherine A. Dettwyler, editors,
Aldine DeGruyter, 1995 mentions that before the widespread use of formulas,
the cross-cultural literature was consistent in reporting weaning ages of
2-5 years of age.

I agree that I would treat 4.2 years very lightly as a "fact."

I was taught in my master's program that a text like Lawrence's was OK for
a general overview, but always, always go back and read the original
sources for yourself.  In other words, don't believe "facts" just because
they are in a nice looking, authoritative  text.

Sincerely, Pat Young in SNJ, USA