Zwick and Zidar argue that a substantial share of the decline in labor share can be accounted for by changing forms of pay, including pass-throughs and equtiy compensation. In particular, if an employee is paid in stock and that stock increases in value then the tax rules tend to count some of that as capital income (depending on when the capital gains occur) rather than as labor income. Zwick and Zidar point us to Human Capitalists for the details:

Human capitalists are corporate employees who receive significant equity-based compensation such as equity grants and stock options. These employees are partial owners of US firms, and in return for their human capital input, human capitalists accrue a share of firm profits through firm dividends and capital gains in addition to earning wages. We document the stylized facts describing the evolution of human capitalists’ income over time and across industries within the US manufacturing sector.¹ Human capitalists have become an increasingly important class of corporate income earners. Due to measurement challenges, prior work has underestimated the importance of equity pay below the C-suite. Correctly measuring the total income of human capitalists substantially alters conclusions about changes in factor shares and technological complementarity.

Equity-based compensation represents 36% of compensation to human capitalists from 2010 to 2019 and constitutes a 7% share of value added in the manufacturing sector in 2019. Correctly accounting for the total income earned by high-skilled workers has a substantial effect on measured changes in labor shares over the modern era. The addition of equity pay to cash wages reduces the decline implied by the wage-only income share of value added in manufacturing since the 1980s by 32%. Without including equity pay, high-skilled labor’s share decreased from 17% in the 1980s to 11% in the most recent decade. The inclusion of equity-based compensation almost eliminates this decline. The high-skilled share of total labor income increases from one-third at the beginning of the 1960s to two-thirds in the 2010s when equity-based compensation is included.

See also my previous post The Labor Share Fell. So What?