How about simple spontaneous population stability... I live in a country with negative birth rate but the population is increasing due to immigration nevertheless. This state of affairs hasn't been legislated into existence, it just happened, and may be a natural behaviour of large human populations. Perhaps once the whole world reaches western standards of living the whole world will stop growing exponentially, with pockets of negative growth being compensated by low but positive growth in others... in the long term the trend could even be for decreasing population...