World Bank data shows that per capita Gross Domestic Product (at constant 2000 prices) for the entire Arab world actually declined from an average of $2,671 for the decade of the 1980s to $2,556 this decade (going even lower to $2035 for the decade of the 1990s in between). In other words, in the last 30 years, the average income or personal wealth of Arabs on average has been simultaneously low, dropping and erratic. For every BMW and Mercedes car you see in Arab capitals there are 50 families you do not see that cannot provide their children with sufficient nutrition, school supplies or heat in winter.
Rami Khouri, presenting five points about the Arab predicament. This one requires further verification but is quite striking. I'd like to see a chart not of average income but of the rate of income growth over the 1950-2010 period, to see whether the acceleration (or deceleration) of the rate of growth of average household income. Of course it might be more meaningful to do this per country than for the whole region.
Update: See this chart and the long comment by Non-Arab Arab below.