That is the title of a new NBER paper from Gene M. Grossman, Elhanan Helpman, Ezra Oberfield, and Thomas Sampson. It is a very simple hypothesis, but they do show it can explain much of the observed decline in labor’s share:
We explore the possibility that a global productivity slowdown is responsible for the widespread decline in the labor share of national income. In a neoclassical growth model with endogenous human capital accumulation a la Ben Porath (1967) and capital-skill complementarity a la Grossman et al. (2017), the steady-state labor share is positively correlated with the rates of capital-augmenting and labor-augmenting technological progress. We calibrate the key parameters describing the balanced growth path to U.S. data for the early postwar period and find that a one percentage point slowdown in the growth rate of per capita income can account for between one half and all of the observed decline in the U.S. labor share.
In other words, the decreased bargaining power of labor, or for that matter globalization, are not necessarily playing major roles. Here is yet another (ungated) version of the paper.