Zero population growth (ZPG) represents a state of demographic balance where a population neither grows nor declines over time. The concept emerged prominently in the 1960s and 1970s, fueled by concerns over the environmental impact of rapid population expansion and the finite nature of Earth’s resources. ZPG is widely seen as necessary for long-term environmental sustainability and a stable resource base. Although the global population continues to increase, the rate of growth has been decelerating since the 1960s, making the goal of stability relevant in environmental and economic planning today.
Defining Zero Population Growth
Zero population growth is a condition of demographic equilibrium where the number of people in a specified population remains constant. Globally, this occurs when total births equal total deaths. For a country or region, the calculation requires births plus in-migrants to equal deaths plus out-migrants.
The concept is closely linked to the replacement level fertility rate, the average number of children a woman must have to replace herself and her partner. In developed nations, this rate is typically estimated at 2.1 children per woman, accounting for children who do not survive to reproductive age. Reaching this fertility rate is a prerequisite for ZPG, but it is not immediately sufficient due to population momentum.
Population momentum is the tendency for a population to continue growing even after the fertility rate has reached the replacement level. This happens because past high fertility rates result in a large proportion of women still being in their childbearing years. The number of births remains high because the number of potential parents is large. True ZPG is reached only when the replacement fertility rate is maintained, mortality rates are constant, and the population’s age structure adjusts, which can take several decades.
The Demographic Transition to Stability
The shift toward ZPG is a natural outcome of the final stages of the Demographic Transition Model, which describes the historical shift from high birth and death rates to low rates. This transition is driven by a convergence of socioeconomic changes that alter family decision-making. The primary change is the initial decline in infant and child mortality rates, reducing the need for parents to have many children to ensure some survive to support them in old age.
As societies modernize, urbanization and economic development make children less economically valuable as farm labor and more costly to raise and educate. This encourages smaller family sizes, reinforced by the rising status and education of women. Educated women tend to delay marriage and childbearing, gain access to the labor market, and recognize a wider range of life choices beyond motherhood.
The widespread availability of effective family planning services and contraception allows individuals to control their reproductive choices. This combination of greater female autonomy, reduced child mortality, and the rising cost of raising children drives a sharp decline in the total fertility rate, moving the population toward the replacement level and eventual stability.
Global Case Studies of Near-Zero Growth
Many developed economies have completed the demographic transition and are at or below the replacement level fertility rate. Countries like Japan and South Korea illustrate near-zero or negative population growth, driven by very low fertility rates. Japan’s population has barely changed since 2000, fluctuating around a stationary mark, while South Korea has one of the world’s lowest fertility rates (0.68 as of 2024).
Across Europe, numerous countries exhibit population stagnation or decline, particularly in Eastern and Southern regions. Nations such as Germany, Italy, Latvia, and Lithuania have experienced population shrinkage due to persistent low fertility and, in some cases, outmigration. These examples demonstrate that maintaining ZPG is difficult, often resulting in a slight decline rather than perfect equilibrium.
Even China reached a population plateau in 2022, largely due to economic growth and past family planning policies. The experience of these nations highlights that once the fertility rate falls significantly below replacement level, the momentum can reverse, leading to a sustained decline in population size.
Socioeconomic and Environmental Consequences
Environmental Benefits
The primary benefit of ZPG is the reduction of strain on the planet’s natural systems. A stable population size lowers the overall demand for resources such as land, water, and energy, which helps mitigate environmental degradation. Stabilizing population size improves the opportunity for sustainable development practices, including conservation and the adoption of environmentally friendly technologies. A stationary population also leads to lower aggregate carbon emissions, although the environmental impact is heavily mediated by per capita consumption levels. Halting population growth removes the pressure to continuously expand infrastructure, allowing resources to be redirected toward improving the quality of life for existing citizens.
Socioeconomic Challenges
Achieving ZPG often brings significant socioeconomic drawbacks, primarily related to an aging population structure. When fertility rates remain low, the proportion of older, non-working individuals increases relative to the younger, working-age population. This demographic shift strains public finance systems, particularly social security and healthcare, as fewer workers are available to fund the retirement and medical needs of a growing number of retirees.
Economic stagnation is also a concern. A smaller working-age population may lead to labor shortages and a slower pace of technological innovation and consumption. Countries with very low fertility rates must maintain economic dynamism while supporting an increasingly older society. This requires policy adjustments, such as increasing worker productivity, encouraging higher labor force participation among older adults, or managing immigration to balance the age structure.