Check out Michael Shermer’s article “Who Believes in God–and Why?” An excerpt:

In sum, being female and raised by religious parents in a large family appears to make one more religious, whereas being male, educated, in conflict with one’s parents, and older appears to make one less religious. As people become older and more educated, they encounter other belief systems that lead them to see the connection between various personal and social influences and religious beliefs. This helps explain the differences we observed in reasons people give for their own beliefs versus the reasons they attribute to other people’s beliefs.

Here’s what he says are the top five reasons people say they believe in God (sorry, you have to work with his awkward syntax here):

1. The good design / natural beauty / perfection / complexity of the world or universe (28.6%)

2. The experience of God in everyday life (20.6%)

3. Belief in God is comforting, relieving, consoling, and gives meaning and purpose to life (10.3%)

4. The Bible says so (9.8%)

5. Just because / faith / the need to believe in something (8.2%)