  The evolution of Gini coefficient for personal incomes in the USA between
1947 and 2005 is analyzed and modeled. There are several versions of personal
income distribution (PID) provided by the US Census Bureau (US CB) for this
period with various levels of resolution. Effectively, these PIDs result in
different Gini coefficients due to the differences between discrete and
continuous representations. When all persons of 15 years of age and over are
included in the PIDs, Gini coefficient drops from 0.64 in 1947 to 0.54 in 1990.
This effect is observed due to a significant decrease in the portion of people
without income. For the PIDs not including persons without income, Gini
coefficient is varying around 0.51 between 1960 and 2005 with standard
deviation of 0.004, i.e. is in fact constant. This Gini coefficient is
practically independent on the portion of population included in the PIDs
according to any revision of income definitions. The driving force of the model
describing the evolution of individual incomes (microeconomic level) and their
aggregate value (macroeconomic level) is the change in nominal GDP per capita.
The model accurately predicts the evolution of Gini coefficient for the PIDs
for people with income. The model gives practically unchanged (normalized) PIDs
and Gini coefficient between 1947 and 2005. The empirical Gini curves converge
to the predicted one when the number of people without income decreases.
Asymptotically, the empirical curves should collapse to the theoretical one
when all the working age population obtains an appropriate definition of
income. Therefore the model Gini coefficient potentially better describes true
behavior of inequality in the USA because the definitions of income used by the
US Census Bureau apparently fail to describe true income distribution.
