Final answer:
The replacement fertility rate necessary for population stability is 2.1 children per woman. This accounts for child mortality and other factors that could prevent some children from reproducing. Various factors, such as modernization, have led to fertility rates below this threshold in many developed countries.
Step-by-step explanation:
The replacement fertility rate is the average number of children each woman would need to have to ensure the population remains stable, without taking immigration into account. This rate is generally understood to be 2.1 children per woman. It is slightly above '2' because not all children will survive to adulthood or have children of their own. Therefore, it accounts for the 'extra' children required to compensate for mortality rates before reaching reproductive age and other factors that could prevent some children from having offspring of their own.
In many modernized countries, the fertility rate has declined well below the replacement level, leading to negative population growth. For example, countries like Spain, Italy, and Japan have fertility rates around 1.2, which indicates that these populations are declining. On the other hand, the fertility rate in India has contributed to its population surpassing China's due to higher birth rates.
To maintain population stability without the influence of immigration, it is crucial for the fertility rate to approximate the replacement level. Family size is often used to illustrate trends in population growth or decline, but it is distinct from fertility rate as family size refers to the number of living children raised by parents in the same household and does not directly relate to population stability.