>>212849960echos of WWI and II
every 20-30 you have dip and spike of fertility rate caused by people missing in the demographics and waiting for war to end to start having kids.
population decline is also massively improving economy because the human capital not put into kids can be used for GDP growth
instead of government investing into schools and families buying more food and clothes for kids, the money could be instead invested into industry subsidies and buying stocks
lowering fertility rate is by far the easiest way to rapidly boost your economy
this rapidly rising economy combined with inflation monetary policies raises cost of living, so you cannot afford more kids because reality market and cost of living is rising faster than wages for most people and the whole system gets into feedback loop
less kids => booming economy => less kids