What do you mean by that?
On a farm children are free labor, in an urban enviornment their an expensive investment, and in an environment with social security people might decide that its a good move not to have children because in theory the goverment should fund their retirement.
(Skal most likely has coments about this)
I frequently disagree with soil on a lot of things but this is a common reason. I happen to think there are a multitude of factors at play.