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The term "demographic transition" denotes the effects on population of the social and economic

Figure 1

changes of the Industrial Revolution, which transformed Europe in the eighteenth and nineteenth centuries and spread to the rest of the world in the twentieth century. In simple terms the complete transition would begin with a stationary population with birth and death rates of about 30 per 1,000 persons, a triangular population pyramid, and life expectancy of about 30 years. The transition would end with a stationary population with birth and death rates of 10 per 1,000 persons, and a life expectancy close to the biological limit of 100 years. This final state was not reached by the beginning of the twenty-first century, but in some developed countries the life expectancy of females is over 80 years. Figure 1 shows an example of this transition, with data for England and Wales, the first countries to experience the Industrial Revolution.

The first effect of the transition was a reduction in the death rate, which continued throughout the transition period. The birth rate increased slightly at first, but later fell to the same lower level as the death rate. During the transition, the excess birth rate over the death rate (the rate of natural increase) produced a large increase in the size of the population. Not all European countries experienced the transition in exactly the same way. In particular, the fall in the birth rate began in France in the early nineteenth century, and later spread to other countries, not reaching Ireland and Russia until the twentieth century.

As an empirical generalization, the above model has proved to be reasonably accurate (see Figure1). By 1990 the transition to equal birth and death rates was almost completed in the developed regions of the world, especially in Europe. The transition is underway in the less-developed regions of the world, with birth rates falling steeply in all regions except sub-Saharan Africa. The world as a whole was, in 1990, at the same stage of the transition as were the developed regions in 1950.

It is generally accepted that the fall in mortality associated with industrialization was due to improved production and distribution of food, which removed the risk of famine and increased resistance to infectious disease. The risk of epidemic disease was also reduced by public health measures such as vaccination against smallpox, the control of waterborne infections by improved sanitation, and of milk-borne infections by pasteurization. Improved medical treatment had little real effect until the middle of the twentieth century.

The cause of the subsequent fall in fertility, which began in the middle of the nineteenth century, is more complex. In preindustrial societies, fertility is primarily controlled through restrictions on the age at which people can marry. Marital fertility in these societies is high, since children are a valuable resource for families involved in agriculture and domestic industries such as spinning and weaving. A fall in mortality, however, will tend to delay succession to land and hence tighten the restrictions on marriage. Improvements in health will also increase the spacing of children, primarily due to the increased survival of infants and a prolongation of the average duration of lactation. Industrialization might tend to increase fertility at first by providing opportunities for earlier marriage. However, especially after the introduction of legislation controlling the employment of children in factories, industrialization will tend to reduce the income obtained from additional children.

GERRY B. HILL

(SEE ALSO: Birthrate; Chronic Illness; Demography; Epidemiologic Transition; Mortality Rates; Noncommunicable Disease Control; Population Pyramid)