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    Real Strategy Requires Cunning

Ilion

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I haven't done a play-thru yet (I have a "proof of concept city" running unattended on my other computer at the moment), so I don't know how fun/interesting this idea/challenge will be. That said, the idea is to grow a city using only "natural growth", that is, births to the Cims already resident in the city at the time the outside connections are cut.

Here is what I've seen so far: if there is any way at all for incoming Cims to get to the residential areas -- even if it is by walking there -- then they will, and growth will be by influx + births. Thus, you can stay connected to the highway *exit* but not to the entrance. Similarly, you probably won't want to allow intercity passenger train or ship or air traffic, and certainly not if there is *any* connection to the residential areas.

The growth of the city will be affected by its initial population when the outside connections are cut and by the lifespans of your Cims. For example, since I long ago tweaked the settings for the "Citizen Lifecycle Rebalance" mod to cause the Cims in my cities to live a number of game-years more closely resembling real-world lifespans, this means that in takes approaching 20 game years for Cim children to become adults and have their own children.

In the "proof of concept city", there were @1205 households and @4000 citizens when I cut the outside connections. After five game-years, there are 1229 households and 4527 citizens. Meanwhile, as I had placed RICO residential assets totaling 2000 potential households, and there are 771 currently empty households, it looks like the concept can work.
 
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Ilion

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I hadn't thought of this earlier, but one can use the 'Vehicle Restrictions' feature of TMPE (along with 'Strict' enforcement) to prevent passenger cars using the highway entrance to the city. Thus, neither tourists nor 'influx' potential-residents can enter the city, while outside cargo still can enter.
 

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I had mis-recalled how I tweaked the settings for the "Citizen Lifecycle Rebalance" mod -- this "natural growth" city I'm attempting would be growing even more slowly had I tweaked the mod as I thought I had; later, I'll post some progress/status reports. At the moment, I'm running a city with *one* family, so that I can chart the live stages of the individual members (unfortunately, all three children left the city after they became adults).

I've read a comment online stating that the average vanilla lifespan of Cities Skylines citizens is about six in-game years. About five years is what I had myself estimated after I started playing the game.

The "Citizen Lifecycle Rebalance" mod triples the average lifespan, so that's maybe 15-18 years. I had doubled the "Citizen Lifecycle Rebalance" mod's lifespan setting, so my Cims live maybe 30-36 years.
 
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Ilion

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Another thing I hadn't mentioned in the OP which can affect a city's population growth is that sometimes the Cims spawn directly at a residence, rather than travelling into the city on one of the transportation networks (road, rail, ship, air).

As I tend to be a "plopper" (of both RICO and growable residences), I was concerned that plopping a lot of extra houses to ensure that newly maturing citizens would have someplace to establish their own homes would add masses of new citizens who hadn't been born in the city. But that doesn't seem to be happening. So, either so many the born-in-the-city citizens are leaving the city upon reaching adulthood that the exodus is masking this influx (if there is an influx, the game's "City Statistics" panel isn't showing it), or perhaps (as appears to be the case in the "single family" city), due to there being no passenger car road connection into the city, they have a difficult time "establishing" themselves in a residence before they despawn as being homeless.
 

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I did a bit of Google-foo, and saw a claim that the Cims have an age-counter, which can go up to a max value of 255 (i.e. it's a one-byte unsigned integer). This sounds reasonable.

In the "Citizen Lifecycle Rebalance" mod 's config file is a comment about this age-counter; for instance, that the lower age value of young adult is 45. I didn't understand this comment until reading about the (one byte) age-counter.

So, my guess is that in vanilla, the age-counter is incremented by one tick for each in-game week, and thus the maximum life-span of a Cim would be 256 weeks (i.e. a few weeks short of 5 years). Looking at the life-cycle stages I had recorded for the Cims in the "single family" test I was running, that bears out.

So, in vanilla, the life cycle seems to be:
Child: 0-@14 weeks
Teen: @15-44 weeks
Youth: 45-89 weeks
Adult: 90-179 weeks
Senior: 180-255 weeks

And, in my city, due to my modification to the "Citizen Lifecycle Rebalance" mod 's config file, the life cycle would then be:
Child: 0-@89 weeks (@1.7 years)
Teen: @90-269 weeks (@3.4 years)
Youth: 270-539 weeks (@5.2 years)
Adult: 540-1079 weeks (@10.4 years)
Senior: 1080-1535 weeks (@8.6 years)
With the max lifespan being 1536 weeks / @29.53 years
 
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Ilion

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Observations and hypotheses about "births" and settling of Cims --

- Residences with two Adult or Young Adult residents may spawn children, though the game seems to prefer the "parents" to be Adult:
-- When a residence has 3 Child/Teen residents, it seems not to spawn further children (*), or the odds are so low that I haven't yet noticed any.
-- I have seen immigrant families with one Adult (so far, with only a "father" (**) ) and one or more Children/Teens/Young Adults;
-- I have seen a couple of residences populated by two "brothers" spawn children (including a second set after the first children "left home");
-- In the same time period, I've been watching three residences each populated by two "sisters", and so far, no children have been spawned in any of them;
- During the period while Cims are Young Adults, they may move to a different residence (perhaps more than one time), either with one other YA/A Cim, or as "roommates" with several other YA/A Cims. They seem especially drawn to the "roommate" situation in high density residences. But, even in the "roommate" situation, they seem eventually to pair-up with one other Cim ... or to emigrate from the city.
- When Cims age from Young Adult to Adult, they tend move to a different residence (or to emigrate from the city altogether):
-- If the newly Adult Cims now "leaving home" had been the "parents" of any children spawned by their former residence, this may lead to the odd situation of the residence being populated solely by "orphan" Teens/Children;
-- After Young Adults/(new)Adults "leave home", if their "parents" are still alive, the residence may spawn more children; again, seeming to cap at three; this is probably why one sometimes notices a residence populated by one or two Seniors and one or more Children/Teens.

- I speculate that the key to encouraging Young Adults/(new)Adults to settle in the city, rather than to emigrate, may have to do with the city's "attractiveness score", or perhaps having plenty of unoccupied residences with a favorable land value (or both, of course):
-- In the short "single family" test city I recently ran, all three born-in-the-city children emigrated, despite there being several open residences. The older two "stayed at home" until they became Adults, and then emigrated shortly thereafter; while the youngest "left home" as Young Adult, settling in an empty residence, but then later emigrated after becoming an Adult.
-- On the other hand, in the "Natural Growth" city I'm currently playing (now at 40+ years with "natural growth" only), the Young Adults/(new)Adults seem to be settling in the city, and spawning new children, rather than emigrating.


(*) I may be misremembering, but I think I recall when I first started playing (and before creating the "Custom Names Lists" mod) noticing a family of new arrivals, just as they were moving into their house, with four children. I re-named the Cims after my parents and myself and siblings. I later happened to find the one I'd renamed after myself ... and the surnames of his children were reverted to his original surname.

(**) At the same time, I have seen immigrant families in which the "father" and some children drove into the city in a different vehicle from the "mother" and some children. Though, in the "father only" case just mentioned, they spawned directly out of a just plopped residence and left for work/school.
 
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Ilion

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This graph shows the population changes of 'Isolatia' from its founding (January 2021) until 2042.01.01, that being the first twenty years of the city's life as a "natural growth" only experiment.

In late December 2021, the population of 'Isolatia' has reached the 'Boom Town' milestone (pop: 2200). At that point, I used the 'Vehicle Restrictions' function of TMPE to prohibit passenger cars entering the city via the highway. Thus, no new 'influx' residents nor tourists can enter the city. It took a year and a half to two years for the tourists then in the city to finally all leave.

At the (IRL) time I isolated 'Isolatia', my modifications to the "Citizen Lifecycle Rebalance" mod 's config file were as discussed above; giving a maximum Cim lifespan of @29.5 years.

With new births, the city's population continued to grow for a few years, reaching @2880 in early 2027. Then the Death Wave hit, carrying off the elder citizens for a few years, with the population dropping below 2550 in early 2033. In a city growing due to the influx of new citizens, this Death Wave wouldn't even have been noticed.

After the Death Wave had subsided, the city began to grow again, reaching 3440 by 2042.01.01.

Isolatia(GO)_Pop.2042.01.01.png
 
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Ilion

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This graph shows the population changes of 'Isolatia' through 2062.0101, at which time the city had 4445 residents. It reached the 'Busy Town' milestone (pop: 4600) in mid-2064.

You may notice that the population growth isn't as steep after 2042.01.01. as before. This is because I changed some of the settings in the "Citizen Lifecycle Rebalance" mod 's config file: I increased the lifespan multiplier from 6 to 20, and I increased the odds that very old Seniors would die of age or ill health.

Changing the lifespan multiplier to 20 does give the Cims a lifespan comparable to real-world lifespans. Since I'm spending the time to build (for some values of 'build') this experimental city, I decided that I might as well see what happens with an expanded lifespan. While the city is growing at a painfully slow rate, I do like the longer lifespans.

Changing the lifespan multiplier to 20 has these effects on Cim life-cycles:
Child: 0-299 weeks / 0-5.7 years old (@5.76 years)
Teen: 300-899 weeks / 5.7-17.3 years old (@11.53 years)
Youth: 900-1799 weeks / 17.3-34.6 years old (@17.3 years)
Adult: 1800-3599 weeks / 34.6-69.2 years old (@34.71 years)
Senior: 3600-5119 weeks / 69.2-98.4 years old (@29.23 years)
With the max lifespan being 5120 weeks / @98.4 years

Because the Senior stage of the Cims' life-cycle is nearly 30% of their maximum potential lifespan, the demographics of Cities Skylines cities may eventually look like Golden Age Communities. In 'Isolatia', the Seniors component of the population had grown to at least 16% before the Death Wave hit. After the city began to grow again, the Seniors component of the population had grown to 12-13% by 2049. But after that date, the changes I'd made to the settings in the "Citizen Lifecycle Rebalance" mod 's config file began to show their effect in the population. Due to the growing numbers of births and earlier Senior deaths, by 2062, the Seniors component of the total population had fallen to 7%; and by 2066, it was at 5%.


Isolatia(GO)_Pop.2062.01.01.png
 
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Ilion

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I did some testing with regard to new "influx" residents arriving by ship.

First, even though Isolatia isn't yet large enough to support ports/harbors, I plopped a passenger harbor across the river from the city proper (I'd say 1-1.5 kilometers). I connected it to the city via a single one-way highway exit lane. I zoned some commercial and plopped a unique building with a relatively high draw.

As expected, tourists, but not new "influx" residents, began to arrive on cruse ships. Also, as expected, the city's residents began to drive to this harbor district, to work or shop or visit the attractions. Of course, since the road connection was one'way, they couldn't drive home, but instead "teleported" (leaving their cars behind).

Then I upgraded the road connection between the harbor district and the city from a single one-way highway exit lane to a two-way national road. As expected, the city's residents could now drive back home from the harbor district. What I *didn't* expect, for I had a couple of hundred unoccupied residences, is that no new "influx" residents began to arrive on the cruse ships, it was still just tourists. I didn't think to check whether the tourists started driving into the city proper, but I don't recall a lot of traffic on the connecting national road.

Next, I zoned some high density residential in the harbor district. As expected, the cruse ships now began to bring not only tourists, but also new residents, who quickly filled up the housing in the harbor district, while still not going for the available housing across the river in the city proper. Since some of the Cims move around quite a bit during their Young Adult stage, it's possible that had I run the test longer, some of the new "influx" residents might have made their way across the river in time.

====
So, I was wrong in the OP when I said that "if there is any way at all for incoming Cims to get to the residential areas ... then they will, and growth will be by influx + births". Clearly, there is some other factor than mere connectivity.

Speculation: Because the connection between the harbor district and the city proper was a national road, that is, a road with no pedestrian pathway, the game's AI didn't see the road as a viable route for new arrivals to find homes in the city proper.
 
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Ilion

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Interesting sidenote: I'm building Isolatia on the vanilla "Grand River" map (*), which doesn't contain the oil resource. Nearly 90 years into the build, and no oil has been imported by any industry.

EDIT: In the 100th year, a recently built factory made the city's first import of oil products.

(*) This is the first time I've used this map ... and, too far into the build, I realized that I don't really like it. But, I'll soldier on: Isolatia is closing in on 10K inhabitants.

My favorite map is the gorgeous "Dragon Gorges" ( https://steamcommunity.com/sharedfiles/filedetails/?id=469525646 ), and my second favorite is "Otherworld" aka "Jeo Allotaer" ( https://steamcommunity.com/sharedfiles/filedetails/?id=662660854 )
 
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Ilion

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This graph shows the population changes of 'Isolatia' from its founding (January 2021) until 2122.01.01, that being the first one hundred years of the city's life as a "natural growth" only experiment. This is the last population graph I'll post. Over the course of a century, the population of 'Isolatia' has grown from 2200 to 13570.

The graph becomes noticeably steeper from the late 2050s. I *think* this is because I started keeping a *lot* of available/empty housing for Young Adults to move into, and that this encouraged more of them to stay in the city, rather than emigrating.

Isolatia(GO)_PopGraph.2122.01.01.png
 
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Ilion

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This spreadsheet shows the year-to-year demographics of 'Isolatia' for its first 50 years as a "natural growth" only experiment.

In case it's not clear, the "Households" columns are occupied / available; the "Population" percentage is the increase/decrease year-to-year; and the other percentages are each demographic's proportion of the total population.

Early on, most of the unoccupied households were due to a number of Senior Residences I had placed before the Death Wave. After that, It took most of the city's history for them to fill again. I didn't place any further Senior Residences, as I had noticed that frequently the individual Cims I was monitoring would move there once they became Seniors ... sometimes leaving Teens and even Children unattended in their former residences.

Isolatia(GO)_PopAnnual_1-40.png
 
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Ilion

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This spreadsheet shows the decade-to-decade demographics of 'Isolatia' for its first 100 years as a "natural growth" only experiment.

In case it's not clear, the "Households" columns are occupied / available; the "Population" percentage is the increase/decrease decade-to-decade; and the other percentages are each demographic's proportion of the total population.

Isolatia(GO)_PopDecade_1-10.png
 

Ilion

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Observations --

1) I never noticed any household with more than 5 inhabitants; so I must be misremembering seeing a family with 2 adults and 4 children when I first got CS.

2) The Cims don't always change their surnames when they "marry" ... and, sometimes, the male takes the female's surname. When they "divorce", they generally (always?) revert their surname (or perhaps take a new one entirely) or may take that of a new "spouse".

2a) When the "married" Cims each keep their surnames, the children spawned by their residence will have the surname of either one or the other, but never mixed.

3) Apparently, Ghey "marriage" is a thing in CS; usually, but not always, one or the other takes the surname of the other. Actually, I expect that this is a coding error, as I'll explain below.

3a) A household of two (adult) male Cims frequently spawns children. This can begin when they are Young Adults, and can continue even when one is a Senior (I don't know about when both are Seniors). I observed one such household, both Cims being born-in-the-city, which spawned a total of 9 children over the years, as the older children established their owh households.

3b) A household of two (adult) female Cims seems never to spawn children.

3c) A household of one (adult) male Cim and one (adult) female Cim generally, but not always, spawns children. It appears that children are not spawned unless the female is Adult, though the male can be Young Adult.

3d) It *seems* to be the case that a household of two (adult) male Cims spawns children more frequently than a mixed-sex household.

4) The combination of 3B) and 3d) is why I suspect that there is a logic flaw in the coding which spawns children. I suspect that the code checks to see whether the male householder is at least Young Adult and that the female is Adult (but not Senior) ... but doesn't check that the householders are one of each sex.

5) I think the key to inducing the Young Adults to both pair-up and to stay in a "natural growth only" city (rather than emigrating), and thus to spawn children to add to the city's population, is to have a *lot* of available housing for them to choose from. In a normal connected-to-the-world city, empty housing frequently gets nabbed by immigrants, and so it's probably the case that a majority of the born-in-the-city Young Adults eventually emigrate, as they cannot find an empty household in which to pair-up with another Cim.

5a) As is commonly known, Young Adults prefer to move to High Density housing. However, when they do, they generally "take roommates", that is, all five slots in the household are frequently taken by Young Adults and/or Adults, and thus, even if two of them are paired, the household can't spawn children until some of the "roommates" either find new places to live or emigrate. I had long noticed this dynamic; I mean, long before this experiment.

5b) In 'Isolatia', after I started keeping 500 (or more) empty households, it appears that more of the born-in-the-city Young Adults stayed in the city, rather than emigrating. I *think* this is why the population graph "bends up" in the 2050s. I still saw a lot of "roommate" households, and also a lot of "singles" (who often subsequently found another Cim with whom to pair).
 
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