Above a certain threshold, rises in per capita income, I hypothesize, bring diminishing returns, and if inequality is as big as in the USA -- the US GINI index in 2004 = 45 (at <https://www.cia.gov/cia/publications/factbook/geos/us.html>) -- a high per capita income leaves large swathes of poverty and illiberalism*.
* Note that liberalism and democracy are not the same thing. IMHO, liberalism requires a certain level of living standard, whereas democracy doesn't.
[WS:] That briefly crossed my mind too, but then, that would imply that the more wealth one has, the more liberal and pro-democracy one is. This kind of reasoning is directly contradicted by a comparative historical analysis by Rueschemeyer at al (_Capitalist Development and Democracy_) showing that it's the "low income and wealth" working class that is driving force for democracy, while the upper class tends to be an impediment to it.
And while we are at that, did not Marx say that having nothing else to loose but their rags and fetters creates the revolutionary potential of the working class? And then there is long line of political activists from Trotsky to Mao and to Pol Pot who saw poverty and manual labor as a pre-requisite to being a vanguard force in society.
I think that the relationship is far more complex than variations in income and wealth. It above all depends on institutional arrangements that either facilitate or impede the mobilization of pro-democracy and liberal social forces, or alternatively counter-mobilization of the reaction. High income and growing income can be conducive to democratic ideals, while high and shrinking income - to reactionary ones (cf. the US today). Likewise, working class organization may align themselves with other progressive social forces, or with the elite and reaction (cf. Italy and Germany between WW1 And WW2 - see Berman "Civil society and the Collapse of the Weimer Republic" (1997) and Kwon, "Associations, Civic Norms and democracy: revisiting the Italian case (2004)).
In short, liberalism or reactionism depend on historical particularities and accidents, or different combinations of social, economic and political forces, which may or may not include income differentials.
I am pretty much convinced that the Islamic reaction has very little to do with income, and everything to do with the traditionalism and fundamentalism of Arab societies - or rather gradual weakening of traditional forms of social organization and religious institutions in favor of secular authoritarianism in those societies in modern times (initiated btw by Ata Turk around WW1 rather than the "Great Satan" and its support for Israel today). Income may play only an opportunistic role in mobilizing supporters for that reaction.
Wojtek