I think it comes down to incentives. Traditionally the economic benefit of children was that they would basically work for you, including supporting you into o

I think it comes down to incentives. Traditionally the economic benefit of children was that they would basically work for you, including supporting you into old age. This made the enormous investment a lot more worthwhile. Also the childcare obligations were much less (a 12 year old could be pretty much self sufficient by working). With pensions, no child labour, etc, the trade-off is not like that any more in economic terms. Having children is basically a total economic loss and a rational person would tend to avoid it. It's seen as a luxury nowadays, something most can't afford.