Final answer:
For a human population to be stable, women need to have on average 2 kids. This represents replacement level fertility, where each generation maintains its size. Real-world scenarios might require a slightly higher rate to account for mortality prior to reproduction.
Step-by-step explanation:
For a human population to be stable, without considering immigration or emigration, women need to have on average 2 kids. This number represents a replacement level fertility rate, which means each generation is being replaced without increasing or decreasing in size. This theory assumes that each woman needs to replace herself and her partner, hence two children per woman is the baseline. However, in real-world scenarios, a slightly higher number may be required to account for the mortality rates before reaching the age of reproduction. For instance, historical hunter-gatherer populations often had three or four children to ensure at least two would survive into adulthood.
Today, fertility rates vary around the world, with the average number of children a Japanese woman has being 1.37, and a Spanish woman 1.47, both below replacement level. Conversely, during times when infant mortality rates were high, families needed to have many children to ensure that some survived to adulthood. In China, the fertility rate has dropped significantly from 5.91 in 1966 to 1.6, due in part to policies like the one-child policy. While fertility rate and family size can differ due to survival rates, they are used to illustrate economic and population dynamics.