I've been slowly reading Tony Judt's quite marvelous Postwar: A History of Europe Since 1945. One of the many, many fascinating bits is his account of what drove the Western European 'age of affluence', between the early 1950s and the end of the 1970s.
The "trigger for the European economic miracle," he says, "and the social and cultural upheaval that followed in its wake, was the rapid and sustained increase in Europe's population." (at 330.) He then goes on to discuss why Europe had such difficulty in having sustained population growth over the centuries - the limitations of traditional agriculture, war and disease, and emigration abroad.
And after reviewing the data of the post WWII baby boom in Western Europe in the fifties and sixties, he adds: "There are many explanations for the recovery of European fertility after World War Two, but most of them reduce to a combination of optimism plus free milk." (at 331.)