Societies change as they develop. Incomes increase, and health care often improves. Changes in roles and expectations are probably even more important. For whatever reasons, women come to have fewer children on average, and both men and women tend to live longer. In sociology this is called the Demographic Transition-populations change from a high death rate and a high birth rate, to lower birth and death rates. The demographic transition comes with massive social change.
This transition occurs in four stages. First death rates and birth rates are high and populations grow slowly. Then death rates decline while birth rates remain high and populations increase quickly. Next birth rates also begin to decline and growth slows. Finally birth and death rates find a new balance, in which population growth again slows or may even stop.
High birth and death rates
In some societies birth and death rates are still very high, for reasons that are not clear. Women may have no access to birth control and may view childbearing as the only accessible lifestyle in societies that denigrate female economic contributions. Both parents may view their children as labor and as insurance against impoverished old age. Both may be unacquainted with lives in which parents do not have, and lose, many children.
Children die young in impoverished societies. In countries with low average life spans, it is children's deaths that drive the numbers. Many die before they reach age 5, often from disorders that would be trivial in developed countries.
Death rates decline
Death rates decline before birth rates do. Medical care and public health measures, such as providing safe water, save lives. Vaccinations stop epidemics by producing herd immunity. Shots make enough people immune that a disease cannot easily spread from person to person and protect children who will never be vaccinated.
Insecticides and mosquito nets save more lives. Public health education saves lives as well, teaching parents sanitation, nutrition and food safety.
With improved standards of living, parents get jobs or can sell their products at higher prices and can afford to take better care of their family. All these factors lower the death rate but do not at first lower the birth rate. Parents, who naturally love their children, may even feel they can afford to raise more.
Birth rates decline
Yet birth rates do eventually decline, and often rapidly, as change spreads through a society. Nowadays, all governments actively encourage or tacitly allow access to birth control. Some women have the opportunity to discover new roles in opening societies, and many men find life easier with fewer children to support.
At last, the population stabilizes. Some countries in Europe have zero population growth or something very close to it. The population of Italy is actually shrinking. Japan has experienced population shrinkage, with falling birth rates and virtually no immigrants. The United States might have no population growth as well if it were not for immigration.
Zero population growth is a mixed blessing. Japan already has an aging population that it has trouble supporting. The United States sees Social Security problems coming, which are related to falling numbers of workers and rising numbers of the retired.
Thomas Malthus said that populations grow until they are stopped by famine, disease or war. Events have contradicted his theories in detail, but a kernel of truth remains. A crowded world cannot be comfortable for the humans who share it, and excess people harm the earth itself with pollution and environmental degradation. It is fortunate that development itself slows population growth.