Different regions have different rates of population growth , but in the 20th century , the world saw the biggest increase in its population in human history due to medical advances and massive increase in agricultural productivity made by the Green Revolution .
However , in some countries there is a negative population growth , especially in the Central and Eastern Europe , mainly due to low fertility rate ,and Southern Africa due to the high number of HIV – related deaths .Within the next decades , Japan and some countries in Western Europe are also expected to encounter a negative population growth due to sub-replacement fertility rates .
Population growth which exceeds the carrying capacity of an area will result in overpopulation . On the contrary , such areas may be considered “ under-populated ” if the population is not large enough to maintain an economic system .
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