Thomas Piketty and his data on income and wealth disparity:

It speaks of his position that the rate of return on capital (r) will be greater than the rate of economic growth (g), and as such will result in rising wealth disparity, especially at the upper end of the income spectrum.
A more detailed summary:
Piketty’s Thesis:
Piketty’s main argument, as presented in his book “Capital in the Twenty-First Century,” is that the underlying force behind wealth inequality is the interaction between the rate of return on capital (r) and the rate of economic growth (g).
The r > g Inequality:
If r is larger than g, then capital increases at a faster rate than income, causing wealth to concentrate in the hands of the top 10% and 1%.
Impact on Wealth Inequality:
This gap between r and g helps to cause rising wealth inequality because the owners of capital earn money at a higher rate than other individuals who work and earn mostly wages or salaries.
State Intervention:
Piketty believes that state intervention can counteract or reverse wealth inequality through progressive taxation and other policies that transfer wealth.