I assemble two rarely used data sets to measure aggregate concentration in the U.S. in the 1980s and 1990s. Despite the merger waves of those decades, aggregate concentration declined in the 1980s and the early 1990s, but rose modestly in the late 1990s. The levels at the end of the decade were at or below the levels of the late 1980s or early 1990s. The average size of firm and the relative importance of larger size classes of firms increased, however. Gini coefficients for employment and payroll shares of companies showed moderate but steady increases from 1988 through 1999.
"Trends in Aggregate Concentration in the United States ."
Journal of Economic Perspectives,