This new paper for "The Social Science Journal" sounds interesting. Here's the abstract:
This paper reviews recent studies that claim to provide support, through statistical analysis of survey data, for the traditional proposition that being religious makes people more generous. The studies have serious shortcomings. First, the data consist exclusively of self-reports. Second, the dependent and independent variables are conceptually problematic and ill-defined. Third, even if there is a positive correlation between religious involvement and personal generosity, it may be due to selection bias. Thus, these studies do not provide serious evidence for the traditional hypothesis. Moreover, it has been directly controverted by experimental studies of economic and other behaviors. LINK.