Nobody is certain the exact sustainable population on earth. Some experts say only 1 billion, others say 5 billion. We are at 7.6 billion, so the only agreement is that we are over the sustainable amount. It isn't obvious to all because overpopulation "works" in the short term. More consumers. More taxpayers. Higher property values. And then . . .
China's one child policy can be seen as a success or a failure. It was actually a "one birth" policy. If a woman had triplets, they didn't put two back. It only reduced the Chinese population increase by 400 million. Sort of a drop in the bucket and could be seen as a failure based on the world population. On the other hand, it has had a long term effect on Chinese women (and to a lesser extent, men). Women found that they could control their own fertility. It turns out that pooping out a baby a year for 14 years wasn't how many women wanted to live their lives. Who would have guessed? (Hint: not the men.) The effect of the policy continues. The same has been the case with educating women in other societies. Once they learn a little, the birth rate drops.
So it turns out the solution isn't limited to war, plague, euthanasia, or some other disaster by those who claim that a sustainable population is a commie plot. The solution is education. Learning that there is more to life than family. Ooops, forget I said that. Instead, let's kill sea lions as we whistle past the graveyard.
Now if I could just find more beer in the fridge 
