From Outscape Wiki
Corruption is a Mechanic introduced in Alpha phase 3 to moderate uncontrolled colonisation.
Currently it is the subject of hot debate among the Alpha testing community with the apparent majority wholeheartedly recommending its removal or heavy modification. It is likely to change as the game develops so whatch this space for updates.
Essentially after a certain tolerance based on the players race, an unhappiness penalty is assessed against ALL (not just new) worlds owned by that player. This necessitates the building of Entertainment Centers to compensate.
The exact metric how it is calculated has not been formally released by the Developers. However a player was vouchsafed the following message which I quot here.
Currently corruption is 0.5 per each colony after max tolerance number specific for civilization. So, if you have 12 colonies, and your tolerance maximum is 5 (Mankind, Ripchee) you will have corruption from 12-5=7 colonies which will equate 7*0.5 = 3.5. To compensate this corruption you will need to construct 3.5/0.1=35 entertainment centers for each planet, which will cost you 105k inhabitants from each planet + some extra personnel for power stations. From such perspective player who accurately selects planets for colonization can reach empire size of 15-20 (twice higher). Using T3 entertainment centers triples this number