[crdoconnor]

I'm pretty sure that it's determined by optimism about the future. Germans think that the next generation will have it about the same or even slightly worse than them.

The Greatest Generation were absolutely positive that the future was going to be very bright once they returned home from the war, hence the baby boom and baby boomers.

Once you account for access to contraceptives, etc. (i.e. once childbearing becomes a choice), I think this is largely what determines the birthrate, which is why most efforts to 'engineer' a higher birthrate have been so futile.