Motivation aside, though, it's still interesting, even if it's not surprising. As people get less religious, they get wealthier. Or perhaps the other way around. Or perhaps there's something else behind both trends.
But whichever it is (probably a bit of all three), the United States, as usual, is an outlier. The United States is always an outlier on these kinds of things. Which reminds me: I'd love to see this same scatterplot for states or counties within the U.S. Has anyone ever done that?