Can anybody point me to a source of statistics, by country, of average income versus median income?

Question

I suspect that the greater the disparity between these 2 figures, the more social discontent.

Answer 1

The Gini index which is a measure of income inequality is available at
http://www.nationmaster.com/graph/eco_dis_of_fam_inc_gin_ind-distribution-family-income-gini-index
A gini of 1 means one person has all the income and 0 means the all incomes are the same. Also see
http://en.wikipedia.org/wiki/Gini_coefficient
The US Gini has been rising over the last 30 years and is now similar the South American countries. If you theory is right we are in for trouble.
The geographical distribution of the Gini index within the US is at http://www.visualizingeconomics.com/2007/07/17/united-states-income-inequality-map/

Answer 2

cia world factbook:

https://www.cia.gov/library/publications/the-world-factbook/rankorder/2004rank.html