That's the thing. I know that it's usually said that women's education is the best predictor of declining birth rates, but I can imagine women's education rates being highly correlated with levels of endocrine disruptive chemical exposure. When countries adopt modern cultural norms, they also tend to start using a lot more plastic and other artificial materials.

Well maybe not epigenetic, but look up the so called 'Mouse Utopia Experiment'.

Basically, when you entirely remove the ability of social mammals to feel like they are actively doing something to succeed in that society... social dynamics invert and social instincts basically now work in an ultimately destructive way.

The mice become territorial despite there being enough resources for all of them.

Extreme mating selection pressure basically causes incels, weaker mice that aren't allowed to mate, that stronger male mice just basically attack for sport.

Then you get looksmaxxers, 'the beautiful ones', some of the incel male mice switch to just preening themselves all the time, never breed, but never try to, so they don't get attacked.

Female mice stop caring for their children, become more polyamorous.

More male become gay or bi. Sometimes consensually, sometimes not, as part of an assault.

As more generations come into being, they learn the behaviors of their parents, of the mice in their environment, and thus basically stop acting like 'normal' mice.

... this all results in an extremely hostile social dynamic that ultimately leads to basically what would be called a slew of mental/behavioral disorders compared to your Gen 0 starter mice, and this leads to population collapse. Tons of unneccessary aggression, way way lower birth rates.

So ... in a situation of abundance, in a species that is evolutionarily hardwired to compete for dominance over scarce resources, thus creating a kind of hierarchy... well the instincts for that do not go away, despite them not actually being needed, strictly speaking.

And those instincts are what causes the population to... correct its overshoot, sort of, but in a catastrophic way.

I would posit that this is not perfectly analagous to humans... but broadly similar.

Theoretically, we have the sentience and sapience to truly realize that we have outgrown some of our instincts, and intentionally replace them with better ones.

In practice?

... TBD, I guess.

Like, imagine trying to explain the concept of a parasocial relationship a human nowadays may have with a streamer, or a massive super fan or brand loyalist to whatever company... try to explain that to a person from 1810.

You could explain it to them. People from 200 years ago are not 'stupider' than people now.

But they would still likely find the entire thing completely insane.