We already have one. It's called education. The problem is that education costs money and requires social stability. So the solution to population control is absurdly similar to the solution to all of the world's other problems, stop fucking up society. Really, I see absolutely no reason to believe humanity needs to have its population controlled, we tend to form society in such a manner that the favored number of children leads to a stable population level. Some people just don't understand that there is absolutely no good reason to have more than two or three children these days. If they were educated, they'd figure that out.