Religion and Psychology?

From a psychological standpoint, why do you think some people justify existence "God" and why do some have the capability to see existence in terms of nature, science and logic and disregard a belief in a higher power? What in the human mind drives these beliefs or non-beliefs? What differences in behavior, personality or biology can one observe in a religious man and in an atheist? I have my own ideas, but I would love to hear some other opinions.