The population of the world in Imperator Rome is represented by individual "population units", or “pop” for short. Each population unit has a unique ID in the save game file.
Pops are the basic generator of 
Money (commerce + Taxes), 
Manpower and 
Research Points.
Social classes
A pop belongs to one of the five available social classes. The social class determines what a pop produces.
- Nobles - generate research points and trade routes
- Citizens – generate research points, trade routes and manpower
- Freemen – generate manpower and tax income
- Tribesmen – generate manpower and tax income
- Slaves – generate tax income and additional trade goods
The population of a territory will slowly approach the "desired ratio", which varies by the Nation, Buildings and Civilisation, among other factors. This ratio and the rate of change can be affected by player choices.
There are a few effects that can directly change Social Class:
- When a City is occupied, there is a chance that Citizens will be demoted ⇒ Freemen and Freemen ⇒ Slaves.
- When any territory (including a city) is occupied, some of these generated slaves will be abducted (teleported) to a provincial or state capital of the sacking nation for no cost.
Nobles
Each Noble Pop at 100% Happiness produces per month:
- 0.5 Research Point
- 0.15 Trade Routes (Happiness does not affect this)
- Food Consumption: 0.5
- Base Happiness: -50%
Nobles only have a relevant desired Ratio in cities and metropolises. Their happiness is heavily affected by Civilisation level, and is otherwise rather hard to manage. Both their Ratio and happiness can be buffed via the Library. The Academy also boosts their Ratio by half as much, along with their (significant) Research output.
Citizens
Each Citizen Pop at 100% Happiness produces per month:
- 0.2 Research Point
- 0.03 Trade Routes (Happiness does not affect this)
- 2 Manpower
- Food Consumption: 0.3
- Base Happiness: -34%
Citizens only have a relevant desired Ratio in cities and metropolises. Both their Ratio and happiness can be buffed via the Court of Law.
Freemen
Each Freeman Pop at 100% Happiness produces per month:
- 0.0075 Base Tax
- 3 Manpower
- Food Consumption: 0.2
- Base Happiness: -20%
Freemen can be found in cities. Unless there are barracks, they are usually not found in Settlements. They are slightly easier to maintain than Citizens, but about equally affected by Civilisation Level. They are usually the main source of Manpower, so their usefulness is highly situational depending on your Manpower needs. Both their Ratio and happiness can be buffed via the Forum.
Tribesmen
Each Tribesmen Pop at 100% Happiness produces per month:
- 0.01 Base Tax
- 3 Manpower
- Food Consumption: 0.2
- Base Happiness: 16%
Tribesmen are easy to keep happy, but receive a penalty for Civilisation level. Their values are relatively low - being beaten in income by the Citizens' Commerce and Manpower by the Freemen. However, actually getting those outputs is easier due to high base happiness.
For Tribal nations, they make up 50% of the Settlement Population, limiting the space available for slaves and thus tradegoods and tax production. In cities, they are less common, but still have a noticeable ratio.
For Non-Tribal nations, they are generally of little use, besides as population that can be promoted or demoted to something more useful. For these nations, they have a desired ratio of 0% at any location.
Slaves
Each Slave Pop produces per month:
- 0.02 Base Tax
- Base Happiness: -30%
- Food Consumption: 0.1
- (semi-hidden) Extra Tradegoods
Slaves are the backbone of every economy during the time frame of the game. There is no nation or territory that does not have, want or need slaves. They are mostly found in settlements, where they make up 100% (Civilised) or 50% (Tribal) population, but are also found in cities. Their happiness, output and desired ratio can all be buffed by the Mill. Their happiness can be difficult to keep high, as those captured in war tend to be of the wrong Culture and Religion. The output of slaves, however, does not depend on their happiness.
While they produce more tax than any other Population type, they uniquely produce additional Trade goods. Every 15/18 slaves in settlements/cities (situational modifiers can lower this) in the territory will produce an additional Trade Good.
A territory with 10 or more slaves with less than 50% happiness may spawn a slave uprising.
Productivity
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Depending on their type, pops produce gold, manpower, and research points, and are the main (or only) source of these resources in the game. In addition to the output modifiers for each specific resource, the 
population output modifier will also increase the production of all resources produced by pops, and is in general one of the most powerful economic modifiers. Pop output can also be affected on a class-specific basis with the local/national noble output, 
local/national citizen output, 
local/national freeman output, 
local/national tribesman output, and 
local/national slave output modifiers.
Output modifiers are given from a large variety of sources, such as buildings, deities and omens, laws and government forms, inventions, and more. Notably, population output is also significantly decreased by low 
pop happiness (slaves being the only exception) and high 
local unrest, which means that keeping pops happy and content is important for maintaining a strong economy.
Output efficiency factors summed up and then applied multiplicative to the base output of each social class. Output efficiency is the sum of:
- Happiness of class
- Code of Rights (freemen) +2.5%
- Right to be Heard (citizen) +5%
- Tyranny: (slaves)+0.5*Tyranny Score
- some trade goods surplus in province (hemp: +2% for slaves)
- Province policies (Harsh Treatment -32% (scales ??), Local Autonomy -60%)
- (Province Investments: reduction for 2 years, needs verification)
- (Province Procurator investment +4%, needs verification)
- Capital Region +10% all classes
- Capital Province +10% all classes
- Capital Territory+10% all classes
- Dominant culture is not primary culture -20% (all classes)
- Settlement -25%
- Religion: Mercury +10%
- Governor Traits (Harsh: slave output +10%)
- Governor Bonus 2.5% per finesse point, (needs verification)
- Foundry +1%
- Harsh Taxation: national slave output +10%
Happiness
Happiness has two main effects on a pop:
- Output: a pop's output (except for slaves') is scaled to its current happiness. For instance, a freeman pop at 50% happiness generates only 5 Manpower instead of 10.
- Unrest: any pop with happiness less than 50% generates unrest to the city it currently resides in. A city with high enough unrest triggers a rebellion.
Each of the five pop types comes with base happiness values of -50%, -34%, -20%, 16% and -30%. Then, the game defines a great deal of modifiers, such as:
- City's civilization value
- Ideas
- Inventions
- Laws
- Trade goods
- Religions and cultures different to the state's; such pops are also affected by assimilation and/or conversion (where applicable)
Aggressive expansion points additionally affect pops with divergent cultures and religions
In general, there are 3 different sets of modifiers that will each affect a different set of pops:
- Pop type modifiers local/national noble happiness, (
local/national citizen happiness, 
local/national freeman happiness, 
local/national tribesman happiness, and 
local/national slave happiness modifiers) - Culture modifiers (
primary culture happiness, 
wrong culture happiness, and 
wrong culture-group happiness modifiers) - Religion modifiers (
state religion happiness and 
state religion pantheon deity modifiers)
Population culture and religion
As each state has its own State Culture and Religion, so does each pop unit have its own as well. When a pop's religion and/or culture differs from the state in which they resides, that pop's happiness is affected (usually negatively). Ensuring that a majority of the state's pops are of the state's Culture and Religion is a crucial step in securing the stability and prosperity of the state.
Population Change
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Population of each social class can change through several means: Growth/Death and Conquest.
Growth/Death
The base rate of population growth in each city in the game is 0.04%/month. When this number reaches 100% a new pop of that type is born in that city. There are many modifiers to this value. The population growth in a city is altered by factors such as terrain type, Stability, and availability of trade goods. Note that all bonuses are additive, therefore population growth omen power may multiple the growth rate in every city you have.
- Religion: Hellenic Diety of Fertility Demeter gives a passive effect of National Population Growth: +0.1%
- Being in peace gives 0.04%/month.
- Above 50 stability gives progressively more growth rate. Having 100 stability gives 0.5%/month, while having 50 stability or lower gives 0%/month.
- Several laws give various, but very low rate of growth, typically between 0.01%/month and 0.05%/month.
- Every 12 months of stored food applies a bonus of 0.05% population growth up to a maximum of 0.5%. Provincial food storage capacity is increased by granaries and salt surplus.
- Monarchy Taxation Law: Infrastructure Policy gives National Population Growth: +0.1% and increases governor loyalty by +10 while decreasing general loyalty by -10.
If aiming to boost your population size, aim for stability (while it gives the highest potential growth rate, it is the most costly to keep high), and stored food. Having multiple of the trade goods or other modifiers have an extremely low benefit.
When a new pop starts to grow, its social class is determined by chance, from the available pops already living in the city. Its culture and religion is also determined upon it starting to grow. They are also randomly selected from the religion and culture already present in the city. For example if a city has no Tribesman then it will never start to grow. It is possible to promote all of your (for example) tribesman into freeman, and still end up with one, if it was already growing when you have promoted the already grown ones.
Population growth in a city can also become negative (by modifiers). If this happens, the growing pop will shrink instead. If there is no growing pop, an existing pop will be chosen (at random) and a progress bar toward 0 will start. If it reaches 0%, that pop will die.
Conquest
Population is also gained through warfare. As cities are sacked, POWs are taken to the state capital or to the provincial capitals as slaves. It should be noted, that pops within the newly conquered city may be demoted:
- Citizen ⇒ Freemen (50% chance)
- Freemen ⇒ Slaves (50% chance)
Tribesmen and Slaves never change their social class upon conquest.
Pop Movement
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Pops move between cities under 4 circumstances:
- Migration
- Enslavement (From either war occupation or slave raiding. Slaves are heavily weighted to go to national or province capitals.)
- Scripted Events
- Manual movements of slaves (but not freemen or higher) by spending gold
Tribesman Pops
- Tribal nations can move tribesman pops as if they are slaves.
- Tribal nations can also trigger a "tribal migration" via the Government Interface.
A slave pop can be moved from a territory to any territory in the same province or an adjacent territory.
Moving slaves costs a base of 5 gold.
- Capital surplus of vegetables lowers cost by 1 gold.
- Exporting vegetables lowers cost by 0.5 gold.
Colonizing
- 主条目：Colonization
Pops are used to colonize unowned territories. At least 10 pops of your country's primary culture are needed in any neighboring province or up to 2 sea tiles away. Colonizing has a two year cool-down for the colonizing territory.
Population Capacity
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Each territory has a certain population capacity, which determines how large of a 
population a territory's terrain and infrastructure can support. Territories have a base population capacity of 10, which is modified by a number of modifiers from terrain, technology, buildings, events, and more. Knowing which territories are good for population growth is essential in city placement and economic planning; it is almost always better to construct new 
cities in territories with a higher base population capacity, as larger cities are almost always richer and more productive than smaller ones, and population capacity is usually the main factor that restricts the growth of cities. Note that population capacity is a different value and modifier, though related, from 
, which also services as a limitation to the population of a territory.
Some of the most important modifiers to population capacity include:
- +5 for
settlements - +20 for cities
- +30 and +10% for
metropolises - +0.25% per point of
civilization value - +10% for
farmland terrain - -10% for
forest terrain - -10% for
marsh terrain - -15% for
jungle terrain - -20% for
mountains terrain - -30% for
desert terrain - +5% if there is a
nearby river - +10% if adjacent to a major river
- +5% for
coastal territories, if there is no 
port - +10% if the territory has a port
- +5% for territories with a Warm Climate
- -15% for territories with a Arid Climate
- -20% for territories with a Frigid Climate
- -25% for territories with a Alpine Climate
- -75% if the province has run out of and has a Critical Food Supply
- +10 in the
capital territory - +6 in
province capitals - +20% from
barracks, 
slave estates, 
mines, 
farming settlements, and 
tribal settlements - +4 from
aqueducts - +2.5% from each
State Infrastructure province investment - +25% from the
City Planning national idea - +2% from each level of
Civic Advances - +10% from the
Pedagoguery invention ( Civic Advances 8) - +5% from the
塞琉古帝国 unique Imperial Calendar invention (
Oratory Advances 1) - -20% for
tribal countries - +10% as a passive modifier of many deities (+12.5% if the
holy site is held) - +5% from
Heritage of Chandragupta (Maurya) - +10% from River Plains group heritage
It is theoretically possible to raise the population capacity of any territory as high as desired by stacking bonuses from the State Infrastructure province investment and Civic Advances, but the small percentage increases means that this is prohibitively expensive. However, as in cities and metropolises every 10 pops unlocks a new 
building slot, it is possible to stack city population capacity arbitrarily high (given enough time) by building aqueducts if the total of the percentage modifiers is at least 150% - possible with a reasonable number of State Infrastructure province investments by the midgame - which will make each aqueduct give +10 population capacity, enough to create another building slot to build another aqueduct once the population capacity has been filled. It is usually strongest to do this in the capital, which has a number of bonuses that makes each individual pop even more productive.
If a territory exceeds its population capacity, it will get one instance of the Overpopulation modifier for every pop over the population capacity, giving:
-3 Migration Attraction- -4% Local Population Happiness
- +1 Migration Speed
- -0.04% Local Population Growth
The population of territory with any significant amount of overpopulation will therefore usually quickly fall back down through migration and population death, while population growth from natural growth, immigration, and enslavement will be quickly stalled, providing an effective soft cap on the population that a territory can sustainably have. It is generally rarely worth having an overpopulated territory, unless the territory only needs to go slightly over the population capacity to meet the threshold to produce another trade good.
Population promotion/demotion
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In every territory (city or settlement), one pop at time can be promoted and one can be demoted to reach the desired pop type ratio of the population in that territory.
- Settlements: in monarchies and republics the base ratio is 100% slaves, in tribal nations is 50% tribesmen and 50% slaves. This can be modified building Barracks that increase the desired ratio of freemen in settlements by +75%.
- Cities: the base ratio in monarchies is 35% citizens, 35% freemen, 0% tribesmen, 30% slaves; in republics is 34% citizens, 39% freemen, 0% tribesmen, 27% slaves; in tribal nations is 33% citizens, 33% freemen, 4% tribesmen, 30% slaves.
These ratios can be modified by the 
local/national citizen ratio, 
local/national freeman ratio, 
local/national tribesman ratio, and 
local/national slave ratio modifiers, most prominently available from various laws and buildings (e.g. Libraries increase desired ratio of citizen, Mills increase ratio of slaves).
The player can increase migration of non slaves pops from settlements to cities in order to promote more pops to citizen status. This is generally recommended because citizens are the only source of research and are needed to keep up in technologies.
Every social class has a promotion speed value:
- Slave promoting: +4
- Tribesman promoting: +0.4
- Freeman promoting: +4
this can be directly modified building Academies (+0.5 each), using the Social mobility edict in the province (+% based on governor finesse), enacting the second Land reform law in republics (+6).
To the values listed above are applied different modifiers. Some common modifiers include:
- Settlement: -25%
- Capital territory: 25%
- Province capital: 10%
- Coastal port: 10%
- Unrest: -1% every 0.1 unrest
- Number of Trade route: 5% every active trade route
- Road Network in territory: 2.5% for each road entering the territory
- Total population: 2.5% every pop living in the territory
- Positive centralization: 2.5% at 100 centralization
- Civic invention Granted Manumission at tech 10: 10%
- Civic invention Gradated Citizenship at tech 17: 20%
参考资料