Doubt this is the explanation. Fertility has declined because easy access to contraception has completely altered the trade-off of having kids. Before you could

Doubt this is the explanation. Fertility has declined because easy access to contraception has completely altered the trade-off of having kids. Before you could essentially be celibate and not have kids or you could be sexually active and eventually end up with kids. People want to fuck more than they don't want kids. Contraception (especially the pill), has eliminated this trade-off. Now people can have their cake and eat it too.