We already have some limited evidence that conventionally religious people are happier
But see Will Wilkinson on this too (arguing that this only really holds in the US, and speculating that it's really about "a good individual fit with prevailing cultural values" rather than religion per se).
That's a good counter-argument, but the linked post doesn't actually measure religion-happiness correlation within those other countries (which is the relevant factor), and it's very plausible that European monopolistic religions are far less effective than American freely competing religions for creating happiness.
I declare this Open Thread open for discussion of Less Wrong topics that have not appeared in recent posts.