This fascinating images came my way via Paul Kedrosky:
I read Jared Diamond's "Guns, Germs, and Steel" a few years back, and am also familiar with Edward O. Wilson's population biology. The two have led me to believe that climate and geographic circumstance have a huge impact on social outcomes. And so when I look at that chart, I see that in the tropics, religion is common; and in the temperate zones and in the northern regions, it is far less common.
This does not have to be a direct causal relationship. There is a lot more poverty, disease, and tragically shortened lifespan in the tropics. There is more civil war in the tropics. But it can certainly be an indirect causal relationship, as people wracked by suffering brought about in part by the effects of their climate turn to the solace of the hereafter.
What a fascinating image.