On average, religious people have more children than non-religious people. Now, that's a sweeping generalisation, of course. However, statistically it seems to hold good, to different degrees, for all the societies that I've seen examined.
But why? It's an important question. A common answer is that this is evidence that religion is evolutionarily advantageous. The idea here is that religious belief in some way facilitates having lots of children (perhaps by making you a nicer, trustworthy person), which gives you a head start in the race to pass on your genes to the next generation.
It's a view that I think is plain wrong.
I think the link is not with religious belief and fertility, but rather with conservative family values and fertility. And, crucially, I think that link is a recent innovation.
Here's some new research to back that up.
Markus Jokela, at the University of Helsinki, has analysed the changing relationship between personality traits and fertility in people living in the USA who were born in the decades 1920 to 1960 - a period of huge cultural innovation, especially with regard to women's rights.
He looked at the conventional "5-factor" model of personality, which rates individuals on their extraversion, neuroticism, agreeableness, conscientiousness and openess.
He found that three of these traits (extraversion, neuroticism, and agreeableness) were consistently related (either positively or negatively) to fertility over time.