Who 'owns' a system?

Yes, who ever has the most influence in a sector
owns that sector.

I thought it was population, not influence. Uninhabited sectors are considered "unowned". I thought that "most influence" is what shows up on the minimap and counts for a culture victory, but population was what mattered for UP resolutions and whether or not the AI considers a starbase or troop movement to be infringing its territory.

I have had unihabited sectors show up as mine due to the influence of the surrounding sectors. In fact, I have "owned" sectors that had a star system inhabited by a major race. My surrounding planets were simply more influencial, easing the enculturaion of the neighboring sector and it's star systems. I am sure population must play a role however. I don't know how much...

I'm saying that influence-domination, for purposes of the minimap and a culture win, is different from "ownership" in the sense of a UP resolution.

I remember reading somewhere that, for UP resolution purposes, ownership = most population. I can't find the reference right now, and the source may be wrong.

The only way to be sure is to do an experiment.

Sector ownership goes to the race with the most influence in a given sector.

However more population generates more influence, but this can be boosted a great deal by other means.

~SDC~

Does this affect whether you can build a culture enhancing starbase in a sector? I seem to get the AI demand I give it to him even if I have 2 or 3 planets in the sector to his 1. I guess if he has any planets in the sector you can't build influence bonus starbases?

If you already have the most influence in a sector
the AI probably wont demand the starbase if you build
one

If you dont have the most influence, he will
probably demand it.