In addition to the other noted reasons, I'd like to point out social security changes the dynamic significantly.
In modern day, society says it's selfish and bad to have kids for your own retirement. Your kids are supposed to be the tax-stock for everybody else to use up for their retirement. So you take on the lions shares of the losses to raise the kid and then society sucks up the tax money and distributes it to anyone on social security including those who didn't have kids.
This creates a massive "free-rider effect" where the incentive is not to have kids yourself, while enjoying the benefits of their labor and distribution payments in your own retirement. Why have kids when others can do the lions share of the investment and you still get your full share of social security money.
I don’t really think this is true. Both Korea and Japan have poor elder social security (and elder poverty rates to match) yet are some of the lowest fertility countries. China is in a similar position.
Can you explain how I can tease out this multivariate issue to rule out these economic incentives are at play?
I think looking at a two dimension "elder social security" and "fertility rate" you are going to see a correlation, but it is confounded by lots of other things including "cost vs personal benefit of raising child", access to contraceptives, education level, etc. I think it would be absolutely wild if you could just look at the numbers for Korea and Japan and just nope out on this one.
If you want to do deep data analysis, it helps that the OECD collects a lot of data from Japan, South Korea, the US, and the member countries which are all generally developed.
I just don't think, that if it is a factor, that it has much impact, given that the worst countries do not have this factor; the child(ren) is still pretty much the extent of the social safety net. Japan is also notable in that they actually give significantly lower social security payments to women, especially unmarried or divorcees.
In modern day, society says it's selfish and bad to have kids for your own retirement. Your kids are supposed to be the tax-stock for everybody else to use up for their retirement. So you take on the lions shares of the losses to raise the kid and then society sucks up the tax money and distributes it to anyone on social security including those who didn't have kids.
This creates a massive "free-rider effect" where the incentive is not to have kids yourself, while enjoying the benefits of their labor and distribution payments in your own retirement. Why have kids when others can do the lions share of the investment and you still get your full share of social security money.