How much the wealth inequality increased in the USA in the last decades? Are there any studies trying to measure the increase of the wealth inequality in the USA?

Say for example how much it increased, comparing with 1990. Or compared with any other moment in time after 1970.
This statistic shows the Gini coefficient, an index for measuring
  income distribution, for U.S. households from 1990 to 2016. A Gini
  coefficient of zero expresses perfect equality, where all would have
  the same income,a Gini coefficient of one expresses maximal inequality
  among values. In 2016, the Gini coefficient for household income was
  0,48. This is an increase since 1990 of 0.5 points, indicating an
  increase of income inequality in the United States.


Source: Statista

In case you are interested in how data for such indeces are gathered, a recent study explaining its data process is Wealth Inequality in the United States since 1913.