# Population growth rate

#### MichaelJanuary

##### Lt. General
Moving this discussion here so it can be more specific.

For me, growth rate would make sense as ....

R = pops/x * e ^ ((availability of housing, hab, jobs, amenities) + (policies, decisions)).

This would necessarily be calculated on a per planet and species basis, with each species growing independently.

Migration/resettlement has to be an entirely separate system. Whether its movement of pops (my preference) or an additive factor.

The model I propose is to calculate growth rate on a per species per planet basis. I haven't modelled the math fully but that's playing with numbers.

THE CURRENT MODEL
R = Pop growth rate. Base in current Stellaris is 3.
P = Growth units required to make up a pop. Base in current Stellaris is 100.
M = time to generate a pop (months). Base in current Stellaris P/R, or 100/3 which is 33 months for a new pop.

Then add in modifiers for species, environment, policies, decisions.

The problem is it does not matter whether you start with 1 pop or 100, it takes 33 months to generate a pop on average. This makes it hard to simulate the population pressures of a large well developed planet or the problems of developing a new colony on a low hab planet dependent only on birth rates. And you have funnies like 10 planets with 10 pops each, generating 1 pop each (10 total), in the same time it takes 1 planet with 100 pops to generate a single pop. Making "wide" super-powerful.

SIMPLISTIC PROPOSAL

 Variable Pops Hab - 80 Housing Open Jobs Amenities Weight 0.1 0.2 0.2 1 0.2 Example Planet 100 70 -10 -2 15 Contribution 100 * 0.1 = 10 (70-80) * 0.2 = -2 (-10 * 0.2) = -2 -2 (15 * 0.2) = 3 Final Growth +7 / 100

The result would be three phases of growth (giving you an S-Surve, that starts out slow, gains momentum, then flattens out as the planet fills up.):

Phase 1: Slow growth. Planets with less than 30(?) pops would generally have very low growth (less than 3). If the hb is less than 80, the growth will be further limited. At 50 or 60 hab or worse, you would have negative growth. However, open jobs, excess housing, will ensure growth stays in low positive values (range 1-5 / 100). Poorly managed planets, or planets with low hab, may even have flat or declining populations.

Phase 2: Rapid growth. As you pass 30 pops, the base growth rate would be about 3 (30 pops / 10), and easily boosted to 5-10 or more with available jobs, housing, amenities.

Phase 3: Declining growth. As you pass 100 pops, the base growth rate from pops might be over 10, but the negative pressures of unemployment, lack of housing, limited amenities, would mean you would rapidly hit a ceiling where the population stabilises. Hab maxes out at +4 growth at 100 hab. Housing and Amenities are self-limiting (the more you focus on them, the fewer total jobs your planet has). Unemployment will be the major limit to growth. The moment your planet passes the max number of jobs you can provide, the growth rate will start to flatten.

@DrFranknfurter small colonies on low hab planets would be very inclined to have negative growth. You would have to be careful about managing jobs, housing, amenities to keep the colony growing. I know this is bound to encourage resettlement for small colonies (its meant to). Covered in a seperate thread.

Some Examples.

Well Developed Planet: Planet with 120 pops and 100 hab would have a base growth rate of 12, plus 4 from hab. To achieve +20 growth you would still need open jobs while maintaining housing and amenities. On the other hand, having 5 or more unemployed, plus say -30 housing and balanced amenities would bring your growth rate down very quickly. The biggest factors here would be population and employment. It would take a large concentrated population to grow fast, but unemployment would stop growth in its tracks.

New colonies: A 60 hab planet with 10 pops would have -1 growth to start with. Having 3 open jobs and 5 open housing would yield a net growth of 3/100 on this planet. Back where we started. Thus Hab, Jobs, Housing are the maindrivers to keep small colonies growing, even a little bit.

Brand New colony: Say 60 hab, 2 pops. Base growth rate would be about -1.8 (decline). But new colonies also generally have open housing and open jobs. And some amenities. Two open jobs would be enough to give it +1 growth. A 50 hab planet. No chance.

The relative weighting of the factors could easily be adjusted if you are wary of the degree of fluctuation involved. Its possible to build 'breeder' planets (focus entirely on housing and amenities, but you ill run out of jobs). if you focus on jobs, you will run out of housing and amenities.

We can discuss migration models separately.

I am also not a fan of how the game manages HAB on planets. Might start a thread on that.

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Why?

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#### DrFranknfurter

##### Major
I almost think I can see what you're trying to accomplish... but I'm not sure I feel comfortable or competent to be wading into the numbers to get your suggestion to a place where it feels good when I don't fully understand the web of interactions and the ramifications of the changes (Mentally looking at it it feels like I'm doing math homework in a different language without a calculator, so bear with me). I hope you don't mind if I stick to a single post on the subject, I wouldn't have commented at all but you mentioned me twice so I felt compelled to do so.

At first glance I can see issues with:
1. Massively overvaluing free jobs. If jobs directly impact growth instead of merely migration/the allocation of growth then strange things happen, like Commerce Megaplexes would produce more growth than gene clinics, servant jobs could be massively overpowered as the planet would never have unemployment etc.
2. One uncapped variable could swamp the rest. I'd want variables to have minimum and maximum values or a predicted range to balance them against each other to prevent one from dominating the rest (otherwise yuht cleansing could kill the planet from lowered housing from devastation or similar unexpected outcomes).
3. Losing colonies is annoying. Colonies could potentially decline before you/the AI could build districts to save them. I'd favour growth reduction over colonies collapsing (losing colonies is annoying for players, crippling for the AI)
4. Unstable equilibrium. The numbers look to be balanced precariously on a knifes edge, with the possibility of being wiped out by the addition of some uncharismatic solitary pops, or the construction of a few robots who use all the housing, amenities and jobs and thereby killing organic growth and eventually kill the rulers off - reducing amenity production and stability to make it even worse.
5. Growth being split on a "per species" basis means the pop numbers you work with will be much, much smaller in certain situations (xenophiles) than the table indicates and all the other variables would need to be percentage modifiers to base growth from pops rather than independent, flat modifiers or it would result in the situation where having 10 species growing would result in 10x the growth added from housing/jobs/amenities/habitability. Or just a long delay and then 10 different species growing a pop at the same time and the sudden gain in pops triggering all species to deline, bouncing back and forth if there are enough species on a world. There's also the issue of growing the first pop on a planet (when population for that species is 0).
5. Robots? Pop growth suggestions need to cover robotic growth or it's only half-done.

It's the sort of thing that needs lots of graphs and competing models looking for edge-cases, then modding and playtesting to see if it's actually fun in practice. Lots of work to perfect and it's probably easier to wait a week and see what the devs are planning and help them improve that.

For me to fully endorse a new growth system I'd want:
1. Fun new interactions (e.g. clearing Dangerous Wildlife to increase local growth, or excess food converted to growth)
2. Simple to understand (e.g. 5 global growth, if you have 2 equal planets with room to grow then you have 2.5 growth on each. Or if you have +5 growth from excess food and 5 planets growing then you have +1 growth per planet. No growth is lost or wasted while a growth target exists)
3. Optimal gameplay is fun (either significantly easier to manage many planets, or fewer planets need to be managed thanks to the changes)
4. Robotic growth seemlessly integrated (robots use the same system rather than breaking organic growth when added on top)

I do like trying to tie in lots of variables to make unconnected systems interact with one another, I'm just not keen on using those particular variables as you've suggested for the reasons above.

Sorry that I can't really be of help

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#### MichaelJanuary

##### Lt. General
I deliberately kept the post thin and avoided discussing buildings, multi species planets, and different ethics. I would rather develop a ground up solution than a bandaid to the games current problems. I set out to develop a population driven growth model that is influenced by the environment. The rest are details (weighting, costs, limits, ethics, species, inter-species breeding, etc.).

1. Gene clinic's could be replaced by a previous building we used to have. I seem to recall a medical type building in an earlier version of the game (field clinic?) which gave the planet +5%/+10% hab. This would directly impact growth especially on new colonies. It was a valuable building to have before habitation was devalued.

2. Assimilator jobs, servant jobs, purge jobs etc are not calculated correctly even now. So yes, what their role is would have to be looked at and fixed. But thats true in the current model. Technically, servants/purged/assimilate/cattle should be a "class", not a job. Their growth (or decline) will follow the same rules, except they wont be influenced by jobs. Jobs are irrelevant to them.

3. Multi species planets. Yes each species would grow at it's own pace (hab, traits will vary, though other factors are shared). This is more realistic than one species growing at a time, we do have to look at mechanisms to stagger the arrival of pops. I considered partial pops (growing pops). On the old tile system, you would have a partial pop as it was being grown. We could have something similar here, with each species growth being represented by a partial pop. The problem is that the game has a burgeoning species problem , even before you allow for inter-species breeding /smh. These can all be included, but the modelling would be tricky. I would probably NOT distinguish between sub-species, and to create a half-breed of two species their growth would have to be combined.

4. Building out empty jobs to boost growth. You would have to spend the resources and swallow the upkeep, but sure a mid game empire can afford that. Weighting jobs at 0.5 growth would flatten the curve slightly, but you would still have an S curve. However, there are not infinite jobs on a planet, and you could waste a lot of resources building out commerce megaplexes early on and then replacing them with forges later. There is also no reason why job weight would be the same for all classes of pops. Slaves can only occupy specific jobs, so only those should be counted, and slaves don't desire jobs (the jobs desire them) so the weighting can change for slaves versus residents versus citizens.

5. Robots are manufactured in a factory, they dont reproduce. The factorys rate of production doesn't vary because you have more robots in the world. Robots would need energy and spare parts, (why would they need housing?) Regardless. Yes, they occupy jobs, but their manufacturing rate is low and steady and predictable. I don't see robots as a problem.

6. New colonies start with open jobs and spare housing. So they are more resilient than you suggest. However, hab will be much more important. Colonizing a 30 hab planet will probably be a death sentence for those pops. And it should be. it should also be easy enough to flag planets in the outliner according to their growth rate. However, PDS has been especially bad at making proper use of the outliner.

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