Which factor is most important in determining a country's population growth rate?

The fertility rate is the most important factor in determining a country's population growth rate because it directly influences the number of births in a population. A higher fertility rate indicates that women are having more children, which leads to a larger population over time. This rate reflects not just the number of children born per woman, but it also ties into cultural, economic, and social aspects that can affect family planning and childbearing practices.

While life expectancy, migration patterns, and death rates do play roles in shaping a country's demographic trends, they do not have as direct an impact on the growth rate as the fertility rate. For instance, an increase in life expectancy may result in an older population but does not directly cause population increase without a corresponding fertility rate that encourages births. Similarly, while migration can affect population sizes in specific areas, its overall impact on the growth rate can vary greatly and may not be sustainable over time, as it depends on people leaving and arriving in different regions. The death rate has a similar effect; while a lower death rate can contribute to growth, it is the fertility rate that ultimately serves as the foundation for population increase.