Excuse me while I indulge in unbridled futurism (bordering on science-fiction)…

Population growth is already levelling off in societies where children are more of a liability than an asset. (They can’t help you till the fields anymore, after all.) Once the overpopulation problem peaks, I could imagine it becoming the norm for teens to seek permanent contraception (reversible or not).

So then the question becomes: what sort of person would make the choice to become a “breeder”? And how will that affect the gene pool? There are already theories flying about that there may be genes that cause a predisposition toward conservatism; what if there’s a gene that causes an especially strong desire to procreate? If contraception is the norm, those without such urges would quickly (willingly?) die out.