In the context of a population continuing to increase even after reaching replacement rate fertility, one key concept that helps explain this phenomenon is demographic momentum. Demographic momentum occurs when a population keeps growing due to a large proportion of women being in their childbearing years, even if fertility rates have dropped to replacement levels.
1. **Crude birth rate:** The crude birth rate is the number of live births per 1,000 people in a population. While it is a factor that influences population growth, it alone may not fully explain why a population continues to increase for several decades after reaching replacement rate fertility.
2. **Total fertility rate:** The total fertility rate represents the average number of children born to a woman in her lifetime. Even if the total fertility rate drops to replacement level (around 2.1 children per woman), the population may still increase due to demographic momentum.
3. **Rate of natural increase:** The rate of natural increase is the difference between the crude birth rate and the crude death rate in a population. It contributes to population growth but may not fully elucidate why a population keeps growing despite achieving replacement rate fertility.
4. **Demographic momentum:** This concept explains why even after reaching replacement fertility levels, a population may continue to grow. It is primarily influenced by the large number of women of reproductive age, leading to a high number of births that outpace the declining fertility rates.
Understanding demographic momentum is crucial in comprehending why a population can experience growth for decades despite achieving replacement rate fertility. It highlights the complex interplay between fertility rates, age distributions, and population growth patterns.
