Osler's Razor

I would add a fifth driver: Education. Notwithstanding the visible changes in our economy, we continue to educate people for the 1970s. Right now, more than 6 million jobs (generally well-paying jobs at that) are unfilled, largely because we do not have appropriately trained workers to fill them. At the same time, American education has largely fallen behind the rest of the developed world in educational outcomes. The reasons for this are myriad and complex, but that a sizable chunk of our workforce is either untrained or wrongly-trained for today's economy goes a long way to explain the trend in income inequality since the mid-80s.

Technology is another factor, and it's broader than both automation of manufacturing specifically and globalization generally. It's true that we don't need as many people to build a car as we used to. Robots do much of that work. But we also don't need as many people to build a house or to drill an oil well, thanks largely to advances in materials and processes. We hardly need people at all in agriculture anymore, except for the small category of produce too delicate to be machine harvested (which can still be machine planted).

And it's not just traditional "labor" that's offset by technology. White-shoe law firms, for example, used to need one staff person for every lawyer for clerical work, research, filing, transcription, etc. Today, firms across the board are moving to a 5-to-1 ratio (lawyers to staff) as everything from communication to document review goes online. Lawyers themselves are becoming less valuable, too. Need a will? LegalZoom. When even an industry with such high barriers to entry sees personnel/value attrition due to technology, it's safe to assume that those who survive will get richer, while those who do not will get poorer.