Once and again, I have experienced heated debates whether "religion" qualifies as an adaptation, an exaptation or "only" as an epiphenomenon. The problem is: Although religion may be described as behaviour towards supernatural agents, there is no consensus available about the exact meaning of the other, biological terms. In fact, most of these cloudy discussions turn out to be rather ideological and emotional debates without much empirical foundation. For anyone really interested in evolutionary science, it is high time to proceed beyond!
Religious Affiliation is Adaptive
Let's go back to basics: If a trait helps phenotypes to reproduce successfully (on average) throughout generations, it is adaptive and will tend to evolve and spread (cp. the Post Homo religiosus - The Natural History of Religion). This is the case with religious behaviour and religious affiliation, as shown by international and Swiss census data (article "The Reproductive Benefits of Religious Affiliation" here, pdf). Did you know that the young Charles Darwin invoked religious language himself as he pondered to marry and to have children? And that his wife-to-be confronted him with the prominent "Gretchen's question"? Do you remember God's very first words according to the Bible? ("Be fruitfull and multiply", Gen 1,28)
For more empirical studies from demographers around, you might also want to take a look at the Web-Resources on Religion and Reproduction.
And for an overview over the most recent works in Evolutionary Religious Studies, see the upcoming (and online already available) "The Biological Evolution of Religious Mind and Behavior" in the Springer Frontiers Collection.
At the end of the article about the reproductive advantages of religious behaviour, I allowed myself to share a surprising observation about the resurgence of Creationism and Intelligent Design: "Evolutionary Theorists brought up far more scientific arguments - but committed believers in supernatural agents brought up far more children. There is a certain irony in here: creationist parents unconsciously defend the reproductive success of their children and communities against evolutionist teachings, whereas some naturalists are trying to get rid of our evolved abilities of religiosity by quoting biology. But from an evolutionary as well as philosophic perspective, it may seem rather odd to try to defeat nature with naturalistic arguments."
Somehow, this song and clip from the biologists of Tübingen University squarely captures the topic!
Download: Blume, M. 2009: "The Reproductive Benefits of Religious Affiliation", Heidelberg 2009