I'm only thinking out loud here, but it seems to me that part of the entire principle of nationhood is based on the idea of cumulative self-interest. This makes me wonder if worldwide government could ever feasibly be a solution prior to either the colonization of other planets or the discovery of alien civilizations.

I want to walk through this, so let's start small. Imagine that the entire world is one family. Traditionally, families are small enough that in the past they were organized like a dictatorship, with the father in charge. But let's pretend that this family is democratic. Each person in the family, adult or child, gets one vote on any important issue. Therefore, each person will vote with self-interest in mind, or at best will organize alliances to get what they want.

But suddenly this democratic family discovers that there's a whole village out there, with lots of different families. Suddenly, the self-interest perspective shifts, and each individual family member realizes that they have more in common with each other, and more shared interests with each other, than they have with the other families. When it comes to governing the village, each family gets one vote. Now the family starts operating in the self-interest of the family, not of the individuals that comprise the family. Each family wants to do better than the other families, so in the interest of doing that, the members of the family put aside their individual differences adn vote for what's best for the family as a whole.

Next, the village discovers that there are actually lots of different villages out there. Suddenly the issue of survival and success is based on the concept of what's best for the village. So the perspective shifts again. Suddenly the families in the village have more in common with each other than with the people in those other villages. So the individual families in the village start to put aside their differences and work together to do what's best so that the entire village will survive and prosper. When voting at the conclave of villages, each village represents its own interests.

But lo, this group of villages discover that there are other groups of villages. Each group calls itself a nation. The members of one nation decide they have more in common with each other than with the people in a different nation, and want to do what's best to survive and prosper. Voting is now based on what's best for the nation as a whole.

The next step, of course, is when all the different nations discover that there are other groups of nations out there on other planets. At that point the people of one world (Earth) would be more likely to decide that they have more in common with each other than they do with the people of the other worlds, and that they need to vote together to represent what's best for the planet, instead of what's best for the individual nations.

If this progression is reasonably correct, then it may be that we need the knowledge of other worlds, and the necessity of survival and prosperity in competition with them, before we could really set aside our differences and have a true world government. Which is a tremendously cliche sci-fi notion, but maybe it's cliche for a reason.

After worlds comes galaxies binding together to compete with other galaxies. Then parallel universes. Then things get complicated.