Human populations have undergone rapid growth, yet in some developed countries, populations have stopped growing. The demographic transition model shows how these population changes happen. The theory behind the model is that industrial development causes economic and social progress that then affects population growth rates. Figure 19-13 compares general trends in birth rates, death rates, and population sizes during four stages.
In the first stage of the model, the birth rate and the death rate are both at high levels, and the population size is stable. In the second stage, a population explosion occurs. Death rates decline as hygiene, nutrition, and education improve. But birth rates remain high, so the population grows very fast. In the third stage, population growth slows because the birth rate decreases. As the birth rate becomes close to the death rate, the population size stabilizes. In the fourth stage, the birth rate drops below replacement level, so the size of the population begins to decrease. It has taken from one to three generations for the demographic transition to occur in most developed countries.
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