I believe every time you place a new city (or capture one, etc) the corruption is recalculated. I've noticed this in a recent game where I got a hut settler and settled it a good distance away from the capital, and then once I placed my third city closer the corruption increased a noticable amount.
There also seems to be a minor slow down (especially with a lot of cities) when placing cities so thats a clue that some intensive calculations are going on. It can only be cultural borders and/or corruption.
There also seems to be a minor slow down (especially with a lot of cities) when placing cities so thats a clue that some intensive calculations are going on. It can only be cultural borders and/or corruption.