[chad_oliver]

I'm not a biologist, but surely the fewer-than-replacement birth rate could be considered as just a new way for the population to drop to carrying capacity? It may be a response unique to humans, but wouldn't the effect be the same?

[afterburner]

It's not a response to the carrying capacity of the planet/environment, though. It's a response to the pressures and culture of human societies in developed countries.