  Personal income distribution in the USA has a well-defined two-class
structure. The majority of population (97-99%) belongs to the lower class
characterized by the exponential Boltzmann-Gibbs ("thermal") distribution,
whereas the upper class (1-3% of population) has a Pareto power-law
("superthermal") distribution. By analyzing income data for 1983-2001, we show
that the "thermal" part is stationary in time, save for a gradual increase of
the effective temperature, whereas the "superthermal" tail swells and shrinks
following the stock market. We discuss the concept of equilibrium inequality in
a society, based on the principle of maximal entropy, and quantitatively show
that it applies to the majority of population.
