Perhaps the biggest question in American political economy right now is why middle-class wages have been falling. There are roughly three main hypotheses: robots, unions and China.

• The robots theory gets by far the most play in the news media, since it's by far the scariest — if automation is replacing big chunks of the human workforce, things are only going to get worse as robots become more capable and efficient. This interpretation has tentatively been embraced by many on the political right, since it doesn't imply a need for substantial government intervention in the economy (though it might imply a need for redistribution).

• The unions theory is favored by the political left, since it implies that giving more institutional power to this traditional liberal power bloc would shift the distribution of national income toward workers.