Migration Flow
Every city breathes. Each day a few families and single adults show up at the edge of town wanting in, and some residents decide they can no longer make a life here and leave. The city has two doors, the way in and the way out, and one rule guards both, the same a landlord lives by: the city won’t admit anyone it would only have to evict. At the way in stands the bouncer: our name for the algorithm that decides who gets to move in, the way a club’s doorman waves some people through and turns others away.
The door in
The lazy way to grow a city drops a perfect resident into every empty home: nobody is ever too broke for the rent, nobody is ever turned away, and you only ever meet the people who made it. That is survivorship bias, the trap of studying the winners and missing everyone who never showed: it would have you believe, wrongly, that life here is frictionless. The bouncer breaks it by sampling one honest would-be arrival per open home, flaws and all, and asking a single question:
Can this person make a life here, or will they wash right back out?
What sets the line. How many the bouncer will try to seat comes from a push and a pull: what draws people to your city, thinned by what makes them think twice. Your draw is the open jobs you offer plus the culture you have built (counted in aura points, with City Hall worth one from day one); then the city’s livability scales that draw, crime and bare ground thinning the line, safe streets and good parks keeping it full.
There is no dice roll and no re-rolls: the city tries to fill the whole line each day with one honest applicant per home, and a slot whose roll can’t pay just waits for tomorrow.
The gate. An applicant gets in when they can afford the home: after-tax income of at least 3× the rent, or two years’ rent in the bank (24×), either leg alone.
The door out
People leave the way they are admitted: on whether they can still make a life here. A sitting tenant’s rent is locked at signing and never climbs, so no one is squeezed out by a rising number. What pushes a household out is its income falling below that same affordability rule, almost always a lost job.
Priced out. When a household can no longer clear the gate on its locked rent, the city first tries to move it somewhere cheaper. Only when nothing affordable is open does it leave the city, and only if a working-age professional lives there: someone who can land a job in another town. That is the luxury of leaving. Everyone else, retirees and the unskilled, stays put.
Unhoused. A household that falls two months behind on rent loses the home and joins the shelter-and-rehousing line. Homelessness here bites: worse health, and a harder time getting hired, the real trap. Again only a working-age professional packs up for another city rather than wait for a home; the rest stay, because staying is all they can do. Fire and the bulldozer turn residents out too, straight onto that same line.
Flowing through
This push and pull is the spine of Everett Lee’s A Theory of Migration, a 1966 paper that still shapes how demographers think about who moves and why.
Your city is the destination, the wider world the origin, and your city’s economic, social, political and environmental conditions are the obstacles between.
| Pushes people away | Draws them in | |
|---|---|---|
| Economic | No work in your trade | Open jobs to fill |
| Social | No classroom for the kids | Good education system |
| Political | Crime | Safe streets |
| Environmental | Treeless concrete | Good parks |
Player levers
- Raise the line. Jobs and culture add up to set the count; safe streets and green parks keep it full, crime and concrete thin it.
- Build the workplaces you want filled. The door seats arrivals into real, posted jobs, so an empty office draws no one to fill it.
- Build schools before families arrive, or a family whose children can’t get a seat turns around at the door.
- Keep somewhere cheaper to land. Vacancy is the release valve: with nothing open to downsize into, a priced-out professional leaves instead of just moving across town.
Parameters
Family ratio
0.08
Probability that an immigrant arrives with family (spouse plus 0-2 children).
Bachelors degree ratio
0.48
Share of arrivals who show up with a bachelor’s degree or higher. Whether they actually work as professionals depends on the workplaces you build; otherwise they move in and take lower work.
High school ratio
0.72
Among immigrants without a bachelor’s degree, the ratio who have at least a high school diploma.
Min immigration cap
1
Floor on the intake ceiling. Even when jobs, culture, safety, and parks are all at their worst, a city still seats at least this many newcomers (who then face the usual school, housing, and affordability gates), so growth never stalls dead.
Source: Everett S. Lee, A Theory of Migration (Demography, 1966)