The “universe 25” theory does make sense in densely populated urban areas where young people do not see a way to have space/land/single family home for their kids.
That is certainly the case in Japan and probably Taiwan.
In the US urban dwellers can move to lower cost areas with land and much more affordable single family homes if they want to do so.
In agricultural societies children are put to work at a young age—my wife grew up on a farm and she was taking care of the animals are bringing in firewood when she was ten years old.
Western societies have made child labor illegal so kids cannot contribute to family finances at all—once they are adults they can leave the nest if they wish.
The economics of having children do not work in that environment.
Humans have a tendency to act first and justify after the fact—so I am not persuaded that cultural attitudes caused people to act differently.
Just read the laws and settlements regarding divorce.
I look at it as a lack of purpose. There is no challenge to stay alive. I do not know what to do about it, but as the author pointed out, we are in an unprecedented era of abundance and safety, to which we may well be psychologically maladapted.
I think for humans, the “Universe 25” analogy more closely fits with regard to the resource factor. Also, even in Universe 25, the rats had more than enough space to spread out if they had wanted to. They didn’t. They never even got close to the theoretical capacity of the environment they were in.