Last year Kopczuk and Saez published estimates of top wealth shares in the U.S. from 1916 to 2000, making inferences from estate tax data.
They found that levels of inequality were highest from 1916 to about 1930, that they fell during the depression and the Second World War, and that they'd remained fairly stable at the lower level since the end of the World War.
The most plausible explanations for the facts are perhaps the development of progressive income and estate taxation which has dramatically impaired the ability of large wealth holders to maintain their fortunes, and the democratization of stock ownership which now spreads stock market gains and losses much more widely than in the past.
(Wojciech Kopczuk and Emmanuel Saez, "Top Wealth Shares in the United States, 1916-2000: Evidence from Estate Tax Returns" NBER Working Paper 10399, March 2004).
Now Rosenbloom and Stutes have pushed back a little further - to 1870 - using data on property ownership from the 1870 Census. They find evidence that overall wealth inequality in 1870 was similar to, and maybe somewhat higher than, the post-World War II distribution.
The figure on the right compares their 1870 point for the share of wealth held by the top 1% to the shares estimated by Kopczuk and Saez. (If you click on the figure it should get bigger.)
Rosenbloom and Stutes conclude:
Compared to estimates for the early twentieth century, the distribution of wealth at the national level... was relatively equal. In 1870 the top 1 percent of wealth holders owned 27.9 percent of all property, about one-third less than was the case in 1916. Thus, wealth inequality increased substantially during the period of rapid American industrialization in the late nineteenth and early twentieth centuries.
The rise in inequality associated with increasing industrialization was prefigured in the pattern of cross-sectional variation in inequality in 1870. Inequality varied considerably across states, and much of this variation reflected differences in urbanization and manufacturing employment across states. For the most part more rural and agricultural states enjoyed a higher level of equality. The exception to this rule was, of course, the South, which remained in 1870 highly rural and agricultural. This exception is explained, however, by the legacy of slavery, which apparently permitted the emergence during the antebellum period of a much more unequal distribution of property than occurred in the North. This inequality managed to survive after the Civil War despite the strong negative effect of emancipation on overall levels of wealth holding in the South.
(Joshua L. Rosenbloom and Gregory W. Stutes, "Reexamining the Distribution of Wealth in 1870"; NBER Working Paper 11482, June 2005)