Population as a System
Human populations behave like systems: they have inputs that increase size and outputs that reduce it. At any scale, from a single town to the entire planet, the balance between these flows determines whether a population grows, shrinks, or stays stable.
The four flows are:
- Inputs: births and immigration
- Outputs: deaths and emigration
All four can be measured as rates, allowing populations at very different scales to be compared on the same basis.
Births and Immigration as Population Inputs
The number of live births per 1,000 people in a population per year.
The number of immigrants per 1,000 people in a population per year.
A CBR of 20 means that for every 1,000 people living in an area, 20 babies are born in a given year. CBR tends to be higher where access to contraception is limited, where cultural norms favour large families, and where economic development is low. Niger, for example, has a CBR of roughly 44 per 1,000, driven by limited contraceptive access and strong cultural preference for large families. Japan, by contrast, sits at around 7 per 1,000, reflecting an ageing population, high living costs, and career-focused lifestyles.
The immigration rate captures a different kind of input: people arriving from elsewhere. Immigration is driven by economic opportunity, conflict, political instability, and, increasingly, environmental change. Germany's immigration rate spiked to around 12 per 1,000 in 2015 as it received large numbers of refugees from Syria, Afghanistan, and Iraq.
Both rates apply at any scale: a city planning its school capacity needs local birth and immigration data just as a national government does when setting housing policy.