Life expectancy is the average number of years a person is expected to live, based on current mortality rates. This measure reflects overall mortality patterns across all age groups. Population growth refers to the increase in individuals within a population over time. Life expectancy significantly influences how populations change in size and structure.
Direct Contribution to Population Size
When individuals live longer, there is a direct impact on the total number of people. Reduced mortality rates across all age groups mean fewer deaths, allowing more individuals to survive into adulthood and old age. This has been a primary cause of global population growth for decades. Advances in healthcare, improved nutrition, and better sanitation have significantly contributed to this decline in mortality, resulting in a larger population by keeping more people alive longer.
Reshaping Population Age Structures
Increased life expectancy fundamentally alters a society’s age distribution. As people live longer, the proportion of older adults increases, leading to an “aging population.” This can result in a smaller proportion of younger individuals. Globally, the number of people over 64 surpassed children under 5 for the first time in 2018.
This demographic shift changes a population’s age structure from a pyramid shape to a more rectangular or top-heavy form. More individuals are living into their seventies, eighties, and beyond. This reshaping of the age structure has broad societal implications, affecting workforce dynamics and healthcare systems.
Influence on Birth Rates
Increased life expectancy also has an indirect influence on birth rates. A primary factor is reduced infant and child mortality rates. When parents are confident their children will survive to adulthood, they often choose to have fewer children. This “child survival hypothesis” suggests that fewer early-life deaths lead to reduced fertility.
For example, historical data from Bangladesh showed that families with no child deaths had an average of 2.6 children, while those who experienced three or more child deaths had 8.3 children. This adjustment in family size contributes to declining birth rates. While more people live longer, the rate at which new individuals are born can slow down.
The Overall Picture of Population Growth
Increased life expectancy, birth rates, and population structure create a complex picture of population growth. Longer lifespans directly contribute to a larger population by reducing deaths and increasing older individuals. However, their indirect effect on fertility can lead to slower overall growth. A population can thus experience both increased numbers and a decreased growth rate simultaneously.
In many developed nations, high life expectancy and lower birth rates have resulted in aging populations, and sometimes, long-term population decline. Initial population growth acceleration from reduced mortality is often followed by deceleration as fertility rates adjust downwards. Understanding these interconnected demographic forces helps anticipate future population trajectories.