As best as I can tell, there are two plausible explanations, both of which could be true to some extent. One is that technology has taken a turn that places labor at a disadvantage; the other is that we’re looking at the effects of a sharp increase in monopoly power. Think of these two stories as emphasizing robots on one side, robber barons on the other.Matt Yglesias advances what I think is a better explanation:
To put it nonpolemically, you can see in the chart that not only is there a structural trend in the labor share of output, there's also a strong cyclical trend. The labor share declines during recessions and rises during booms. And the problem of the Federal Reserve is that over the past 30 years, it has a perfect track record of never allowing inflation (which is to say a sustained period in which wages rise faster than productivity), but it doesn't have a perfect track record of never allowing recessions. The inevitable consequence of this asymmetrical success is for the labor share to steadily decline.I like this argument for the simplicity: it is based on a clear policy choice that was made and which continues to this day. There is no need for an appeal to "the world has changed" over and above the decision to be willing to hold price stability in place at the cost of employment. This is directly relevant to today as the Federal Reserve has a dual mandate. On one side of the mandate, they have the requirement to ensure price stability. On the other side of the mandate they need to encourage full employment. It's pretty clear that they are doing much better with one piece of this mandate than the other.