The Gabriel Tarde reference article from the English Wikipedia on 24-Jul-2004
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Gabriel Tarde

Time you got around to sponsoring a child
Gabriel Tarde (1843 - 1904) French sociologist and social psychologist who conceived sociology as based on small psychological interactions among individuals (much as if it were chemistry), the fundamental forces being imitation and innovation.

Among the concepts that Tarde initiated were the "group mind" (taken up and developed by Gustav Le Bon, and sometimes advanced to explain so-called herd behaviour), and economic psychology, where he anticipated a number of modern developments.