Population In Stellaris is made up of units called POPs.
POPs live on planets, where every tile can be inhabited by one POP. Each POP represents a seemingly variable number of individuals. The most important characteristic of all POPs is [species]], but they also have happiness, ethics, traits and faction membership.
If there is a food surplus on a planet new POPs will grow on it until the planet's population capacity is reached or the food surplus ends. The rate of growth depends on the amount of surplus food.
Happiness is a percentage that determines how statisfied a POP is with their living condition. Unhappy POPs are less efficient than happy ones and will also join factions.
Population size affects research speed.
Migration is voluntary movement of POPs to a more attractive planet, for example a minority POP of aliens might try to move to its species homeworld. Once migration is activated for a POP, a modifier icon appears on it's portrait in the planet surface view and after a ceratin amount of time it relocates to the destination. Fanatic Individualist empires have to allow free migration for their primary species. A migration access trade deal with another empire allows migration to foreign territory, in which case the empire of origin loses control over the mobing POPs.
Resettlement is forced movement of POPs by the governemnt. In order to resettle a POP an empire needs to activate the 'Allowed' Resettlement policy. After that, the resettlement menu can be opened through any POP. In this menu the planets and POPs involved in the resettlement can be chosen and the resettlement may be implemented at a cost of 100 influence. There is also a Resettlement is only possible on planets that have a Planetary Administration, so new colonies are excluded. Resettlement is not available to Individualist empires.
Non-Individualist empires can enslave POPs, which increses their resources production but lowers their science output. Xenophobes can restrict slavery to alien species. Collectivist POPs are more accepting of being enslaved. Slave POPs will join slave factions, which can revolt and will establish a new empire on success.
With the right policies, undesirable POPs can be killed through purging. However, this angers most empires as well as other POPs. Depending on POP ethos, this reaction can be reduced or removed.
Known happiness modifiers
|Capital World||+10%||Is located on the empire's capital|
|Communal||+5%||Has the Communal trait|
|Fanatic Spiritualist||+10%||Is Fanatic Spiritualist|
|Governor||+n%||[?] modifier from Governor|
|Moral Democracy||+10%||Empire is a Moral Democracy|
|Neighbors with Wrong Ethos||-10%||Neighboring POP has opposite[?] ethos|
|[Policy Group] ([Policy])||+5%/-5%||Every empire policy matching/against POP ethos|
|Recently Conquered||-20%||POP was recently conquered by its current empire|
|Solitary||-5%||Has the Solitary trait|
|Xenophilia||+5%||Is Xenophile and has [?] aliens|
|Xenophobia||-5%||Is Xenophobe and has [?] aliens|
Everything marked with [?] is speculative.