population density

Individual humans can't be relied on to participate in a population control program. Corporations (McDonalds, CocaCola corp, PhillipMorris, etc) provide a certain amount of population control by giving people what they want: instant pleasure followed by slow painful death, but this has not been completely effective due to efforts by well-meaning groups to increase awareness of the dangers of smoking and poor diet. Increased operational costs in addition to better informed consumers has impacted the profitability and effectiveness of corporations who provide products aimed at the self-destructive. Currently we are living in a society partially controlled by the profitability of such corporations, to the benefit of the corporations and their investors, at the cost of the consumers, with the side effects of human suffering and population reduction. It can be argued that these companies are not doing a very good job of population reduction, and it is probably in their interest not to. When called to court in some class action lawsuit, these corporations readily admit to neglecting their primary benefit to the world. After all, what good is a dead smoker? The dead don't buy cigarettes. Greed and consumer awareness prevents them from being effective at population control.

People are generally stupid or careless enough that they don't mind paying huge companies to kill them slowly and painfully. For example, I smoke either Camel or Marlborough cigarettes. Neither are very healthy. Both cost me about $7 per pack.
Humans are always screaming about how terrible these companies are for giving people what they want. They are afraid of becoming a race of weak, bloated, gouty, cancerous diabetics. Everywhere you look you see the sweaty-pale-sagging faces of the future: the ultra-consumer. But take their freedom to consume away, and they will scream and riot about that. We allow the corporations to provide people with what they want because that's what freedom is all about. Our great Capitalist Republics exist to provide a place where humans can be free to pursue happiness. Consumerism is meant for everyone; no race or class is exempt. However, the strongest population control measures are aimed at minorities. Exploitation is a major profitability driver; many corporations keep from crossing the line between profit and loss by some form of exploitation.
The above should be obvious. The idea that humans can rule their own destiny as a species goes against the natural order of the Universe. Simple minded optimists believe that eventually, something like democracy will provide a government that lasts forever. After all, the problem right now is that we just don't all have a say, right? Our votes don't really count! If only all our votes counted, We, as a whole, would make the right decisions.
Wrong. This is extremely naive. We may have some incredibly intelligent individuals, but combined we are less intelligent than some of the lowest lifeforms. As a whole, we function similarly to algae. As long as there is the right mix of radiation and nutrients, algae will continue to reproduce until it chokes itself and everything else near it to death.
Computers. Compared to our own brains, computers are primitive. However, they are evolving much faster than our own brains did. Since they are engineered by an intelligent creature, they can skip much of the extremely slow cycles of random mutation and natural selection.
So far we have not engineered anything that actually 'thinks' like higher functioning animals do.
Some people believe that computers could never think for themselves, never have a 'soul', never have desires or love or any kind of what we call 'feelings'. Almost nothing is completely improbable. Regardless, a computer can be considered to be much 'smarter' than the smartest human, even though they may need to be purpose built for specific tasks. No computer (yet) does everything better or faster than a human can.
Anyway, to get to the point: government can't be provided by human intelligence. No human is entirely impartial. No human can decide what is best for the human race. Currently the Earth and the Universe itself are the ultimate government. We exist at the mercy of the planet, which exists at the mercy of the universe. However improbable, some quantum phase shift could completely obliterate time and space as we know it in the next five minutes. Don't panic.
All probabilities aside, what is known is that we are responsible for destroying the surface of the planet on which we depend for our survival. Decisions regarding sustainability of the planet (including humans, birds, bees, fish, kangaroos, trees, flowers and all other things necessary for our survival) as influenced by the actions of humans should be calculated by computers. This includes pretty much everything we currently just kind of DO willy-nilly.
Reproduction:
If you've ever read Orwell or Huxley, the following will be very familiar:
Computers should be responsible for reproduction. Yes, I know, it sounds awful. Only a man would say something like that. I'm not saying All reproduction, just most of human reproduction. There would always be contingency groups of naturally reproducing humans as a fail-safe. All but a few humans would have their reproductive cells extracted at birth, incubated, analyzed and separated and stored. Reproduction would be performed as needed, based on unbiased criteria which computers create and adapt continuously.
This sounds like a nightmare to a lot of people. I realize that most people wouldn't want that kind of thing to happen. Freedom, etc.