Zero population growth (ZPG) is the point at which a population stops growing or shrinking, holding steady at roughly the same size over time. It happens when births plus immigration equal deaths plus emigration. For most of human history, populations were stable because high birth rates and high death rates canceled each other out. Today, dozens of countries are approaching or have passed this balance point for the opposite reason: both birth rates and death rates are low.
How the Balance Works
A population’s growth rate equals its birth rate minus its death rate, plus net migration. When those forces balance to zero, you get ZPG. In practice, reaching that balance depends heavily on fertility. Demographers use “replacement-level fertility” as a benchmark: the average number of children per woman needed to keep the population stable without migration. In developed countries, that number is about 2.1 children per woman. The extra 0.1 accounts for children who don’t survive to adulthood. In countries with higher child mortality, replacement level can be significantly higher.
Migration complicates the picture. A country can have far fewer births than deaths and still maintain a stable population if enough people move in. Conversely, a country with above-replacement fertility can lose population through emigration. ZPG describes the net outcome of all these forces, not just what’s happening in maternity wards.
Why Populations Naturally Stabilize
The shift toward ZPG follows a well-documented pattern called the demographic transition. Every country that has industrialized has moved through roughly the same stages. First, death rates drop as sanitation, nutrition, and medicine improve. Birth rates stay high for a generation or two, causing a population boom. Then birth rates fall as families gain access to contraception, women enter the workforce, and children shift from economic assets to economic costs. Eventually birth and death rates settle at similarly low levels, and growth slows to a crawl or stops entirely.
The key insight is that rapid population growth is a temporary phenomenon driven by timing. Deaths decline first, births decline later. The gap between those two shifts is what produces the explosive growth the world experienced in the 20th century. Once both rates converge at low levels, a country enters what demographers call stage four of the transition. If fertility drops below replacement and stays there, countries enter stage five: population decline.
Where ZPG Is Already Happening
Several dozen countries have already reached or passed zero growth. Japan is the most prominent example, with approximately two deaths for every birth. Its population has been shrinking since 2010. South Korea, Italy, Greece, and much of Eastern Europe face similar trajectories, with fertility rates well below replacement and limited immigration to offset the gap.
The United Nations projects that the global population will peak in the mid-2080s at around 10.3 billion, then slowly decline to about 10.2 billion by 2100. That projection means the entire world is on track to reach ZPG within this century, though the timing varies enormously by region. Sub-Saharan Africa still has high fertility rates, while East Asia and Europe are already contracting.
The Environmental Case for Stable Populations
Fewer people consuming resources means less pressure on ecosystems, which is why ZPG has been a goal of environmental advocates since the 1960s. The logic is straightforward: total environmental impact is a function of how many people exist, how much each person consumes, and how efficiently that consumption is produced. Stabilizing any one of those variables helps.
The carbon footprint alone illustrates the scale of the challenge. Humanity’s carbon footprint grew more than 700% between 1961 and 2002. Under business-as-usual projections, it was expected to rise from 7.1 billion global hectares in 2002 to 11.3 billion by 2050. More aggressive scenarios that combine emission cuts with slower population growth could bring that figure down to 3.5 billion global hectares by mid-century, and 2.1 billion by 2100. Population stabilization doesn’t solve the problem alone, but modeling consistently shows it’s one of the necessary ingredients. Even under moderate population projections, per-person consumption of cropland and grazing land would need to drop by about 25% to leave enough room for biodiversity conservation.
The Economic Tradeoffs
A stable or shrinking population creates a distinct economic challenge: fewer working-age people supporting a larger share of retirees. This is often measured by the old-age support ratio, which compares people over 65 to the working-age population. In the United Kingdom, that ratio is projected to rise from 27% in the 2005-2010 period to 41% by 2045-2050. Japan, further along the curve, is already grappling with the consequences as a growing share of its population exits the workforce.
Countries that fund healthcare and pensions primarily through payroll taxes face a particular squeeze. Fewer workers means less revenue per person, even if the economy is otherwise healthy. Policy responses generally fall into three categories: encouraging people to work longer, raising tax rates, or diversifying government revenue away from labor-market contributions. None of these is painless, but the framing of an aging population as a “catastrophe” overstates the problem. Many older adults remain healthy and productive well past traditional retirement ages, and the assumption that everyone over 65 is economically dependent is increasingly outdated.
How Governments Try to Influence Growth
Countries rarely aim for ZPG as an explicit policy target. Instead, they push fertility in whichever direction feels urgent. Nations with below-replacement fertility, like South Korea and much of Europe, offer financial incentives for larger families, expand parental leave, and promote workplace gender equality to make it easier for women to have children without sacrificing careers. The results have been modest at best. No country has successfully reversed a sustained fertility decline through policy alone.
On the other side, countries that have tried to slow growth have used a wider toolkit. Iran’s experience is one of the most dramatic examples. After the 1979 revolution, the government suspended family planning programs and subsidized larger families. Fertility soared. By the late 1980s, the government reversed course, launching a comprehensive family planning program that provided widespread contraception education, affordable birth control, and legal reforms raising the marriage age. Fertility plummeted. More recently, Iran has shifted back toward pro-natalist policies, limiting contraceptive access and offering financial incentives for more children.
These swings illustrate a broader pattern: governments can accelerate fertility changes that are already underway, but they struggle to reverse deep structural trends. Access to contraception and education for women consistently lower birth rates. Taking those away can slow the decline, but rarely stops it permanently.
Origins of the ZPG Movement
Zero population growth entered the public vocabulary in the late 1960s, riding a wave of environmental concern. Paul Ehrlich’s 1968 book “The Population Bomb” warned of mass starvation and ecological collapse driven by overpopulation, and it became a bestseller. Around the same time, Lincoln and Alice Day’s “Too Many Americans” made a similar case focused on the United States. These ideas fed directly into the first Earth Day in 1970, where population was a central topic alongside pollution and conservation.
An organization called Zero Population Growth (later renamed Population Connection) advocated for limiting families to two children, making birth control universally available, and restructuring tax laws to stop rewarding large families. The movement’s predictions of imminent famine proved too dire, but its core argument, that a finite planet can’t support infinite growth, became a foundational principle of modern environmentalism. The irony is that many of the countries where ZPG advocates were most active have now overshot the target, with fertility so far below replacement that population decline, not growth, is the more pressing demographic concern.