The fertility rate refers to the average number of children born to women over their lifetime. This metric offers insights into reproductive patterns and is important for predicting future population changes. For a population to maintain its size, a specific fertility rate is necessary to ensure each generation replaces the one before it.
The Concept of Population Stability
Population stability signifies a state where births approximately balance deaths, leading to little or no change in total population size. This balance is achieved at “replacement level fertility,” the average number of children a woman must have to replace herself and her partner, ensuring the population neither grows nor shrinks in the long run, assuming no net migration. Globally, the accepted average for replacement level fertility is approximately 2.1 children per woman. This figure ensures enough offspring survive to reproductive age to replace both parents, leading to a stable population size.
Why the Rate is Not Simply Two
Replacement level fertility is above 2.0 children per woman due to two factors: infant and child mortality, and the sex ratio at birth. Even with advanced healthcare, some children do not survive to reproductive age; the slight excess above two compensates for these early deaths. The natural sex ratio at birth also contributes, with approximately 1.05 males born for every 1.00 female globally. Since population replacement is based on women replacing themselves, a woman needs to give birth to slightly more than one daughter on average to ensure her replacement, accounting for the higher proportion of male births. This slight imbalance in the sex ratio at birth, combined with the reality of child mortality, necessitates a fertility rate marginally above 2.0 to achieve generational replacement.
Factors Affecting the Required Rate
While 2.1 children per woman is a common global average, the exact figure varies between populations and over time. A country’s child mortality rates determine its unique replacement level fertility rate; higher mortality means women need more children to ensure enough survive. For instance, a nation with very low child mortality might have a replacement rate close to 2.1, while higher mortality requires a higher rate. Life expectancy also plays a role, as longer life expectancies generally require a slightly lower fertility rate for stability. These variations highlight that replacement level fertility adapts to a population’s demographic conditions.
Consequences of Varying Fertility Rates
When a population’s fertility rate consistently deviates from the replacement level, it leads to significant demographic and societal consequences. A fertility rate consistently above the replacement level results in population growth. This growth can place increased demand on natural resources like food, water, and energy, potentially leading to environmental degradation and resource scarcity. Such populations also tend to have a younger age structure, with a larger proportion of children and young adults.
Conversely, a fertility rate consistently below the replacement level leads to population decline over time. This trend often results in an aging population, where the proportion of older individuals increases relative to younger ones. An aging population can present economic challenges, such as a shrinking workforce, increased strain on social security and healthcare systems, and a higher dependency ratio where fewer working-age individuals support a larger elderly population.