Are the religious more generous?

It is common to see figures or claims that suggest that religious believers are more generous in terms of donations to charity than the non-religious. (One thing that often concerned me about such figures is that they generally include donations made to their church, which even when limited to money used for actually charitable purposes often have an ulterior motive - proselytization.)

Today I saw a post in reddit's r/atheism that pointed to this information on Red Cross donations by country:

http://www.nationmaster.com/graph/hea_red_cro_don-health-red-cross-donations

Curious about potential drivers of the figures, I decided to try to adjust for GDP, so I got the GDP (PPP) figures from here:

http://www.nationmaster.com/graph/eco_gdp_ppp-economy-gdp-ppp

So I then divided the Red Cross donations by millions of dollars of GDP to give dollars donated per million dollars GDP, a kind of "generosity" measure in a rough sense (a better measure would reflect donations per "spare" dollar after basic needs). I discovered a down-trend of log-generosity against log-GDP (richer countries donate a somewhat smaller proportion on average). Wondering about other drivers, I then took the irreligion figures from here:

http://en.wikipedia.org/wiki/Irreligion_by_country

(specifically, the 2007-8 Gallup figures, which uses the very broad definition implied by the question "Is religion important?" - and I used those figures because they covered the most countries) and plotted log-generosity against the irreligion percentage figures, which give this plot:



[The blue curve is a lowess smooth of the relationship, with f = 0.9]

And what do we find? In spite of the fact that this measure of generosity decreases with GDP, this measure of generosity increases with the percentage of people who see religion is unimportant. Is this the whole story? No - there's all sorts of other things that could be adjusted for.

But it certainly doesn't support the usual story - and if anything, suggests the opposite. There's at least some suggestion here that maybe it goes the other way.