Most Americans don't have the mathematical rigor to understand a rent vs. buy calculator, but I love the idea that every American without kids understands how to calculate the present value of a deferred annuity against the cost of raising a kid and have made the multi-decade commitment to forgo family life simply as a result of that calculation and knowing that they will come out ahead in the end.
And yet in times century ago every simple farmer understood it. The principle is simple, watching your childless neighbors relatively suffer is not hard to understand.
That calculus totally changes if the childless neighbor can just send armed men to collect the neighbor child's income and redistribute it to themselves.
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.
