The lower line in the graph shows that the amount of income Messrs. Piketty and Saez attribute to the top 1% accounted for 10.6% of personal income in 2004. That 10.6% figure looks much higher than it was in 1980. Yet most of that increase was, as they explained, "concentrated in two years, 1987 and 1988, just after the Tax Reform Act of 1986." As Mr. Saez added, "It seems clear that the sharp, and unprecedented, increase in incomes from 1986 to 1988 is related to the large decrease in marginal tax rates that happened exactly during those years.".....
The top line in the graph shows how much of the top 1%'s income came from business profits. In 1981, only 7.8% of the income attributed to the top 1% came from business, because, as Mr. Saez explained, "the standard C-corporation form was more advantageous for high-income individual owners because the top individual tax rate was much higher than the corporate tax rate and taxes on capital gains were relatively low." More businesses began to file under the individual tax when individual tax rates came down in 1983. This trend became a stampede in 1987-1988 when the business share of top percentile income suddenly increased by 10 percentage points. The business share increased again in recent years, accounting for 28.4% of the top 1%'s income in 2004.
As was well-documented years ago by economists Roger Gordon and Joel Slemrod, a great deal of the apparent increase in reported high incomes has been due to "tax shifting." That is, lower individual tax rates induced thousands of businesses to shift from filing under the corporate tax system to filing under the individual tax system, often as limited liability companies or Subchapter S corporations.