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something in the environment is detected as unsuitable for childrenYep: compelling alternatives and contraceptives have appeared, and the pressures to have children have diminished.
People in the past didn't have many of those, so they "chose" to have kids.
Having kids is hard. If you have other options, you're gonna take them. It's as simple as that. I genuinely think people complaining about the conditions haven't done enough introspection. In the abstract, they may feel that having kids is something they want or should do, despite their revealed preference being to not do so. They're just hoping that, somehow, someway, the prospect of raising families will finally seem ultra-compelling and they'll do it and live happily ever, having their cake and eating it too. But that won't happen. The sad reality is that the premise rested on more suffering than currently exists.
But it's okay:
> If this kind of birthdate collapse happened in other organized animal societies we’d have lots of news pointing to specific environmental hazards, like that insecticide and bird eggs.
Other animals won't be able to defy biology and grow transcended versions of themselves in pods, like we're surely gonna do eventually. That's the future, and it's unironically the solution to all the shitty Ponzi schemes our biological legacy left us with. :p Until then, all you can do is hang tight. (The only other solution is to go back to the Dark Ages, which is even worse.)