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As a civilization grows, it becomes harder and harder to control leaders of new colonies. Bribery, theft, fraud and blackmail all become commonplace and difficult to control. In-Game Description
Corruption is an in-game mechanic first introduced in Alpha 3 to moderate uncontrolled colonisation.
In Alpha 2, the Corruption mechanic was not present, meaning players could in theory expand and create massive individual empires. With the introduction of the Corruption mechanic, this is no longer possible. Once a player has colonised a certain number of planets, a Happiness penalty is assessed against all worlds owned by that player. This necessitates the building of Entertainment Centers to compensate.
Initially, these effects are minor and can be easily handled, but will increase exponentially as a player's empire grows, to a maximum of -10/h at 36 planets - the largest empire possible under this new system. Hence, players should think carefully about which planets to colonise, and they may reach a point where taking a new world would cause insurmountable problems in terms of happiness upkeep. Players should also make sure to upgrade their entertainment buildings on each of their worlds as their empire grows.
Mankind and Ripchee empires will have Corruption kick in at six planets taken, while The People's Realm will have to deal with Corruption at seven planets. As the Syntis are unaffected by Happiness, they are also immune to the effects of Corruption. Instead, they have a hard planet cap (visible in the Main UI), which can be increased by researching the relevant Technologies available to the Syntis player.