Three factors determine how a population grows or shrinks: births, deaths, and migration. These are the only ways a population changes size. More births and incoming migrants push the number up, while more deaths and outgoing migrants pull it down. Understanding how these three components interact explains why some countries are booming while others are steadily losing people.
Births: The Engine of Growth
The birth rate is the number of live births in a population over a given period, usually expressed per 1,000 people per year. When birth rates are high relative to death rates, a population expands. The key benchmark here is the replacement fertility rate: an average of 2.1 children per woman in developed countries. That number accounts for the small percentage of children who won’t survive to adulthood. A population consistently below 2.1 will eventually shrink unless migration fills the gap.
Birth rates aren’t static. They respond to economics, culture, and policy. During Europe’s early industrialization, fertility actually rose for a time because men’s wages increased and families could afford more children. Eventually, as women’s wages climbed enough to draw them into the workforce, the cost of raising children grew faster than household income, and birth rates fell. This pattern, called the demographic transition, has played out across most of the world’s developed economies and is now unfolding in many developing ones.
Japan illustrates what happens when fertility stays low for decades. With a rate of roughly 1.4 births per woman, Japan’s population has been falling since 2011. Italy is in a similar position, with births at an all-time low since the country’s unification. Bosnia and Herzegovina sits at just 1.26 births per woman, well below replacement level.
Deaths: The Counterbalance
The death rate works in the opposite direction. When a society reduces infant mortality, improves sanitation, or expands access to healthcare, fewer people die each year and the population grows, even if birth rates stay the same. In Western Europe, the decline in mortality started nearly a century before fertility began to drop, which is why populations surged during that window.
The combination of births minus deaths is called natural change (or natural increase when the number is positive). A country can have a positive natural change, meaning more people are born than die, or a negative one. Ukraine, for example, faces the compounding problem of high death rates paired with low birth rates, producing a steep natural decline that emigration only worsens.
Age structure matters here, too. A population with a large share of young people has built-in momentum: even if each couple starts having fewer children, the sheer number of people entering their childbearing years keeps total births high for a generation or more. Researchers have found that in both statistical models and real populations, a one-year increase in the average age of a population translates to about 4.5 percent more total growth before the population stabilizes. That momentum works in reverse as well. An aging population with fewer young people will continue shrinking for years even if fertility ticks back up.
Migration: The Wildcard
Migration is the movement of people from one area to another, and its effect on population size is measured as net migration: the number of people arriving minus the number leaving. A positive net migration means more people are moving in than out. A negative one means the opposite.
For some countries, migration is a bigger driver of population change than births or deaths. Eastern Europe is the clearest example. Bulgaria and Lithuania are among the fastest-shrinking countries in the world, and in both cases, mass emigration is the single largest contributor. Latvia’s decline comes from a combination of economic migration and low birth rates. An estimated 38 percent of Albania’s population lives abroad. Poland, Georgia, Cuba, and Italy are all losing young workers to countries with stronger job markets.
On the receiving end, migration can offset low fertility. Countries like Canada, Australia, and the United States have historically relied on immigration to maintain population growth even as their domestic birth rates fell below replacement level.
How the Three Factors Work Together
No single factor operates in isolation. A country might have a healthy birth rate but lose population because emigration drains working-age adults faster than babies are born. Another might have very low fertility but still grow because immigration more than compensates. Croatia’s population has been declining due to low birth rates and an aging population, while Moldova’s struggles combine poverty-driven emigration with high mortality among those who stay behind.
The formula is straightforward:
Population change = (births − deaths) + (immigrants − emigrants)
That’s it. Every projection, every census estimate, every headline about population boom or bust traces back to those two terms: natural change and net migration. The U.S. Census Bureau uses exactly this framework to track population shifts year to year.
Why Age Distribution Amplifies These Factors
While births, deaths, and migration are the three direct inputs, the age structure of a population shapes how powerfully each one plays out. A country where 40 percent of the population is under 15 will see rising births for decades, regardless of what fertility rates do in the short term. A country where a large share is over 65 will see rising deaths simply because more people are reaching the end of their lives.
This is why demographers pay close attention to the proportion of a population under 30 versus over 65. When a fast-growing population suddenly drops to replacement-level fertility, the population below age 30 essentially stops growing, but the total population keeps expanding for years because of the large young cohort already alive. Greece’s population began declining in 2011, the same year it recorded its first negative birth rate, because it no longer had enough young people to generate momentum.
Countries navigating the demographic transition, moving from high birth and death rates to low ones, often experience a sweet spot where death rates have fallen but birth rates haven’t caught up yet. During that window, the working-age population balloons relative to dependents, and economies can convert more of their productivity gains into rising income per person. Once birth rates eventually fall and the population ages, that advantage fades.