From Mitchell Hoffman, based on Holocaust data:
I find that (1) Richer countries had many more rescuers than poorer ones, and (2) Within countries, richer people were more likely to be rescuers than poorer people. The individual-level effect of income on being a rescuer remains significant after controlling for ease of rescue variables, such as the number of rooms in one's home, suggesting that the correlation of income and rescue is not solely driven by richer people having more resources for rescue. Given that richer people might be thought to have more to lose by rescuing, the evidence is consistent with the view that altruism increases in income.
Hat tip goes to BPS Digest.