What is meant by population momentum and what is its cause?

Population momentum is defined as the tendency for population growth to continue even after fertility rates have declined, due to the size of the existing population. It is a result of a population having a larger proportion of individuals in reproductive ages than what would be found in a stable, zero-population-growth (ZPG) situation.

The concept of population momentum was first introduced by demographers in 1967 and has since been used to explain why countries with declining fertility rates may still experience continued growth or stagnation rather than an immediate decrease in overall numbers. This phenomenon occurs because when fertility rates drop, there are fewer children being born which results in fewer people entering adulthood compared to those who are leaving it due to death or other factors. However, this does not immediately reduce the overall size of the population because there are still many people who were born before the fertility rate decline that remain alive and contributing to population growth.

For example, if a given country experienced rapid economic development leading to a sharp reduction in infant mortality and increased access to family planning services over two decades ago such that their current total fertility rate (TFR) is now 2 children per woman instead of 6, then during those two decades most parents will have had four more children than they otherwise would have if TFR had already been at 2. In addition due to improved healthcare standards many more individuals from these families will survive into adulthood creating an ‘overhang’ effect whereby there are far more adults surviving beyond their natural life expectancy than what would occur naturally under ZPG conditions–this creates ‘population momentum’ where even though TFR has dropped drastically, the number of people within each family cohort continues growing rapidly for some time after its introduction due purely numeric reasons based on past high births relative too low deaths at present time periods.

In countries where ZPG status has yet been achieved through policy measures or cultural changes concerning reproduction behavior this can lead rapid levels of human capital accumulation leading further fuelling economic development which can further fuel demographic transition ie from high birth/deaths levels towards lower ones however this also increases disparities between rich & poor as well as urban & rural areas; furthermore although momentum tends generally slow down eventually without policy intervention it could lead potentially negative long term consequences perpetuating intergenerational inequalities & poverty traps etc posing serious challenges for sustainable human development efforts even once desired targets have been reached initially

Thus understanding Population Momentum can aid decision makers insofar as formulating policies allowing them better target resources towards specific needs thereby helping promote equitable & sustainable outcomes while also assisting scientists measure impact public health interventions over longer period timescales enabling them craft better tailored solutions addressing problems related eg immunization programmes child nutrition maternal health care etc .