The religious shall inherit the earth: I wanted to quantify the pattern of religious people having more children than the irreligious. Starting with the U.S, the General Social Survey (2004) indicates that Americans who never go to church average 1.67 kids: it's 2.25 for those who go weekly and a slightly lower 2.14 for those who go more often. By the way, the fertile "attends weekly" group is not fringe: it is the modal category of religious behavior in America.
As for the world, I looked at the World Values Survey and calculated that those who never attend religious services have 1.72 children, compared to 2.47 for those going more than once a week. (I suspect that this sample is a bit skewed toward more developed countries).
I'm reminded of William James' statement that (went something like) believers will always have more vitality than non-believers.