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

The fertility rate is the most critical factor in determining a country's population growth rate because it directly impacts the number of children born in a given population over time. A higher fertility rate indicates that women are having more children, which contributes to an increase in the overall population size. When a country has a fertility rate above the replacement level (typically around 2.1 children per woman), it usually means the population will grow, assuming other factors remain constant.

While life expectancy also plays a role in population dynamics by influencing how long people live and can contribute to the population, it does not directly dictate how many new individuals are born. Migration patterns can affect population size, but they are often more variable and contingent on external factors, such as economic opportunities or political situations. The death rate, while also important, is a reflection of population health and does not actively contribute to growth; rather, it can slow it down if it surpasses the birth rate. In the context of population growth, the fertility rate stands out as the primary driver.