Final answer:
Sub-replacement birth-rate occurs when the fertility rate in a country is below the replacement level of 2.1 children per woman, leading to a population decline without immigration. It reflects a demographic shift often associated with economic development and social changes such as increased women's education and reduced child mortality.
Step-by-step explanation:
The term sub-replacement birth-rate refers to a situation in a country where the fertility rate is below the level required for the population to replace itself without immigration. This level, known as the replacement fertility rate, is typically around 2.1 children per woman. It is slightly higher than simply replacing each parent due to the fact that not all children will survive to reproductive age or have children of their own.
Factors influencing fertility rates include economic security, infant mortality rates, and education, especially for women. As a country develops economically and socially, a demographic shift often occurs where birth rates decline. This has led to many developed nations experiencing slower population growth or even a decline in their native population sizes.
Moreover, there's a differentiation between the concept of fertility rate and the term family size. The former represents the average number of children a woman will have during her reproductive years while the latter indicates the number of living children being raised by parents in a household. Both are used to understand population dynamics, such as growth or decline.
Understanding the implications of the fertility rate is essential for grasping the challenges and planning required to address population changes, whether it's growth or decline. These population metrics influence environmental, social, and economic aspects of a society. Hence, maintaining a balance in fertility rates is crucial for sustainable development.