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.
I am convinced that agricultural societies create great incentives to have lots of kids while industrial and post-industrial societies have negative incentives for having kids.
Humans react to incentives—no matter what they claim or say...