William Easterly relies on his own research as support for parts of  
chapter 13 from "The Elusive Quest for Growth", although it isn't only  
income inequality that hinders economic growth. Rather it is mixed in  
with other elements that tend to divide societies. In his paper "The  
Middle Class Consensus and Economic Development", he finds greater  
homogeneity of the population and a greater share of income going to the  
middle class is associated with higher incomes and higher economic  
growth. Here is the abstract for the paper:  
  
"Modern political economy stresses "society's polarization" as a  
determinant of development outcomes. Among the most common sources of  
social polarization are class divisions and ethnic divisions. A middle  
class consensus is defined as a high share of income for the middle  
class and a low degree of ethnic divisons. A middle class consensus  
distinguishes development successes from failures. The paper links the  
existence of a middle class consensus to exogenous country  
characteristics like resource endowments, along the lines of the  
provocative thesis of Engerman and Sokoloff 1997 that tropical commodity  
exporters are more unequal than other societies. This hypothesis is  
confirmed with cross-country data. This paper exploits this association  
using resource endowments as instruments for inequality. A higher share  
of income for the middle class and lower ethnic divisions are  
empirically associated with higher income and higher growth. These  
associations are robust to a number of alternative controls. A middle  
class consensus is also associated with more education, better health,  
better infrastructure, better economic policies, less political  
instability, less civil war and ethnic minorities at risk, more social  
'modernization' and more democracy."  
  
He refined this thesis in a later paper titled "Social Cohesion,  
Institutions and Growth", which will be published next month. Now he  
argues that social cohesion, as measured by income inequality and ethnic  
fractionalization, endogenously determine institutional quality, which  
in turn determines growth. The papers are available on the research page  
of his website:  
http://www.nyu.edu/fas/institute/dri/Easterly/Research.html  
  
Brad Andrew