This means that people who get their income primarily from labor have lost a great deal of power vis a vis those whose income comes disproportionately from ownership of capital.
Questions and some guesses: 1) Why is this? If labor can extract less value, that would suggest that it has less bargaining power. That is, labor is more fungible. This fits in with what we experience as developers: one of the great emphases that (successful) companies place is on making developers replaceable and their labor roughly equivalent, through an increased emphasis on frameworks and general languages. It also makes sense for less professionalized labor. Heavy industry requires skills to operate effectively and runs the risk of shutdowns at centralized production facilities, while services and light industry are much easier workplaces to dominate workers.
2)How do the unemployed and underemployed fit in? This is complicated, as women have entered to workforce to make two-income households, while zero-marginal product men have left it as they cannot be employed profitably.
3) Was the threat of communism an enabling force for the middle and working classes in the "good times" leading up to the 80s when it finally started to crack and fall? Before the ruling class in the United States had to optimize the economy to be as effective as possible against the threat of Communism. This meant sacrificing some of its own well-being to buy off the rest of our consent for the ability to play a major role in geopolitics ( this buy off taking the form of concessions to unions, more forceful labor market interventions, healthcare, investments in public education).
4) Could the increasing inequality somehow be a statistical artifact? Smaller countries tend to be more egalitarian than richer ones. Most individual countries in the EU and states in the USA have a lower Gini coefficient than the EU or USA as a whole. Could a similar mechanism cause a larger and more sophisticated economy to feel pressures to become more unequal?
Its tricky to simply blame it on USA policies, as most Western countries have also become significantly less egalitarian than they were in 1970. Which isn't to say that US policies didn't cause it in the USA; it's just that those policies didn't appear in a vacuum, and you've got to identify the forces that caused it around the world.