I didnt see anyone mention it but it seems a garrison reduces or affects corruption as well.
Last night I bribed an out of the way rebel town and took it over. I happened to have the city detail window up and was buying peasants, after about 5 units I noticed my corruption dropped a bit, it went from 239 to about 200.
I always use my detail screen as for some reason even tho you dont actually have a unit made or structure built the detail screen will still reflect future puchases.
As far as how much garrison affects it, I dont know, I am not smart enough to figure that out..:)
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