Is global inequality (inequality among world citizens) stable, decreasing or increasing? How high it is? Is it mostly due to inequalities within nations or between nations? Is there a global middle class? How did historically global inequality evolve?

For all of this, see the working papers:

• True world income distribution 1988 and 1993: first calculations based on household surveys alone, World Bank Policy Research Working Paper #2244,
• Decomposing global income distribution: Does the world have a middle class?, World Bank Policy Research Working Paper #2562.
• Global inequality recalculated : the effect of new 2005 PPP estimates on global inequality, World Bank Policy Research Working Paper #5061
• Global inequality : from class to location, from proletarians to migrants, World Bank Policy Research Working Paper #5820.

Household survey data (1988-2005) used in these papers, and in books Worlds Apart: Measuring International and Global Inequality (Princeton University Press, 2005) and The haves and the Have-nots: a Brief and Idiosyncratic History of global inequality (Basic books, 2010) can be downloaded here.  The data are in Stata10 and for five benchmark years: 1988, 1993, 1998, 2002 and 2005.

• Global Income Inequality data, 1988-2005 at approximate 5 year intervals (STATA v10, 538kb)
  (updated March 2012, correction made July 2012)
• World Income Distribution (WYD) variable description (PDF file, 138kb)