Derren Brown informed us this week (http://derrenbrown.co.uk/blog/2011/01/atheists-dying-breed-nature-favours-faithful/) that those who worship at least once a week have 2.5 children, compared to the 1.7 of those who never worship. He puts this down to religious people being encouraged to reproduce as a religious duty.
I think that there are other factors too. Catholicism, for example, has a set of beliefs which seem to be designed with the purpose of breeding more Catholics. Hang on a sec while I get a condom. What was that? We're not allowed contraception? Well that's ok, we'll just have to have more abortions. Er...what? Why not? Oh, you have got to be kidding. And the stereotypically huge Catholic family is born... [see Monty Python's 'Meaning of Life']
I used Catholicism as an example because it's the largest religion in the world, but most of the others have a similar attitude to these things. And once the kids are born, the have to be baptised ASAP, before they have any say in the matter. No wonder the religious out-procreate the non religious. Or perhaps religious people just have better sex lives.
Also, the condemning by the majority of the world's religious folk of homosexuality is telling, because, let's be honest, it's not a pursuit well-designed for creating offspring. So that leaves us with only allowing naughty business between a man and a woman, without contraception, and no abortions, and when the kid is born it has to join the church IMMEDIATELY or else! How can the non-religious possibly keep up? I guess we just have to rely on people changing their minds once they're old enough to know better.