Has any organism ever extinguished itself as you describe ? This whole human extinction thing… isn’t that catastrophising? We are a 7-8 billion individuals away from extinction.

All plausible theories I've heard for the cause of humanity's unprecedented, historically low birth rates are things that could not occur in less intelligent species. (Birth control, women's rights, hedonism enabled by modern technology, etc.)

> isn’t that catastrophising

Yes, I'm only putting that forward as the worst case scenario to make the point that this can't just be ignored. As I said in other comments, this almost certainly won't actually result in extinction because there are other corrective factors which would occur long before that, but none of those scenarios are particularly desirable either. (E.g. Civilizational collapse returning humanity to pre-industrical birth rates, global takeover by theocratic governments that ban birth control, etc.)

The only non-catastrophic corrective factors that sound plausible to me involve some kind of intentional collective action on our part that reverses the trend, which won't happen if everyone's attitude is the same as the root commenter's. (Granted, maybe there are other possible corrective factors I haven't thought of, but if so I'd like to discuss what those are rather than just have the problem dismissed with a hand wave.)

Yes. Read through https://www.the-scientist.com/universe-25-experiment-69941 to see that, given a good enough environment, mice can wipe themselves out. Here is a particularly telling passage:

Eventually Universe 25 took another disturbing turn. Mice born into the chaos couldn’t form normal social bonds or engage in complex social behaviors such as courtship, mating, and pup-rearing. Instead of interacting with their peers, males compulsively groomed themselves; females stopped getting pregnant. Effectively, says Ramsden, they became “trapped in an infantile state of early development,” even when removed from Universe 25 and introduced to “normal” mice. Ultimately, the colony died out. “There’s no recovery, and that’s what was so shocking to [Calhoun],” says Ramsden.

Like the mice, our population is going into reverse. And that description of behavior, looks awfully prescient when I compare to humans on social media today...