Went looking for literature, this doesn't quite address in-group charity versus general causes, but FWIW:
> All else held constant, rural respondents were almost 5.2 percentage points less likely to be a
charitable donor, and donated less, on average, than urban donors, after controlling for human
and social capital variables, such as education level, income level, health status, religious
affiliation, family composition, and others. Rural respondents were significantly less likely to be
donors to secular charities, and gave less on average to secular causes overall. These differences
do not exist when comparing the religious giving of rural and urban respondents.
> However, rural donors donated a statistically significantly higher percentage of their income to
charity than did urban donors. Also, rural donors are more likely to donate to religious causes
than secular causes. Frequent religious attendance is associated with a higher probability of
giving for rural residents, as is itemization of deductions on income tax returns.
> All else held constant, rural respondents were almost 5.2 percentage points less likely to be a charitable donor, and donated less, on average, than urban donors, after controlling for human and social capital variables, such as education level, income level, health status, religious affiliation, family composition, and others. Rural respondents were significantly less likely to be donors to secular charities, and gave less on average to secular causes overall. These differences do not exist when comparing the religious giving of rural and urban respondents.
> However, rural donors donated a statistically significantly higher percentage of their income to charity than did urban donors. Also, rural donors are more likely to donate to religious causes than secular causes. Frequent religious attendance is associated with a higher probability of giving for rural residents, as is itemization of deductions on income tax returns.
-- Comparing Donation Patterns of Rural and Urban Donors -- https://scholarworks.iupui.edu/server/api/core/bitstreams/00...