Various and sundry = technological advances, the rise of the corporate farm, the migration of populations from rural to urban and suburban, the economic boom following WWII accompanied by a population with a larger number of college graduates, the rise of unions in still thriving industrial sectors, the creation of suburban infratstructure, including the proliferation of shopping malls, an increasingly educated female population and entrance of an increasing number of women into the workforce, which took off circa 1973. All of which furthers your central contention about changing family structures not at all. The model has, and is, the same...even though there are lesser and greater adherences to it. While shifts in economics and such are undeniable, the only true deviation in the family structure I can see over the past hundred years, and that being corrosive in nature, is the rise of the two-headed household.