a population of low average age and at each successive census the population of any age group exceeded the corresponding figure at the preceding census. When, therefore, death rates in all age groups fell by an average of about 33 per cent, as they did between 1880 and 1910, the result was first a very low crude death rate which helped to maintain the population increase in spite of a fall in the birth rate, and, secondly, a gradual increase in the average age of the population.
After the first world war the birth rate fell to less than half the nineteenth-century rate. Even so, the population continued to increase slowly, though its average age rose more rapidly.
Owing to the changing age composition, crude death rates remained nearly stationary at around 1-2 per cent though the death rates continued to fall heavily in all age groups, particularly among pre-school children, school children, and adults in their thirties and forties. From 1932 onwards the birth rate steadied itself and thereafter rose slowly up to the outbreak of war, after which all orderly movement was interrupted by the disturbed conditions of the war years. The 1947 birth rate (20-7 per thousand of the population) was the highest since 1921, but the 1948, 1949, 1950, 1951 and 1952 birth rates were progressively lower, though higher than pre-war. The 1952 birth rate was 15-7 per thousand of the population.
Mortality Causes. The causes of the decline in mortality include better nutrition, rising standards of living, the advance of medical science, the growth of medical facilities, improved health measures, better working conditions, education in personal hygiene, public and private schemes to make the health services generally available, and the smaller size of the family, which has reduced the strain on mothers and enabled them to take greater care of their children.
Mortality from acute infective diseases and infant and maternal mortality have declined very sharply. Mortality from the main acute infective diseases of childhood is less than one-thirtieth, and mortality from tuberculosis is less than one-tenth of the rates prevailing in the mid-nineteenth century. Infant mortality has fallen by about 80 per cent since 1900, and between 1934 and 1942 maternal mortality was halved and has since continued to fall. The reported mortality from many of the chronic diseases of middle and old age has risen during the twentieth century, and this rise, though exaggerated by the improvement in diagnosis, is at least partly real. Medicine has not yet discovered effective measures to combat some of these diseases, and improvement in positive health does not always bring increased immunity or resistance to them.
Fertility Trends. The fall in births during the twentieth century has taken place in spite of an increase in the marriage rate and a drop in the usual age of marriage for women. It is due mainly to a decline in the number of children born per married couple (the average size of the family). Couples married in the mid-Victorian era produced on the average five to six liveborn children. Among the couples married in the years 1925-29 the figure may be estimated at 2.2.
At first the decline in family size was most marked among the professional and salaried classes. Among couples married between 1900 and 1930 the families of manual workers were about 40 per cent larger than those of non-manual workers, but this class difference appears to have been diminishing. The decline in family size has been considerably slower among Roman Catholics than in the rest of the population, and slower in Scotland and Northern Ireland than in England and Wales.
Changing social habits and the disturbing effects of war preclude any reliable estimate of long-term trends in family size since 1939.
Migration. During the inter-censal period 1931-51 the net balance of migration to and from the United Kingdom was inward for the first time in the past century. The net gain to the United Kingdom from civilian migration was about half a million, a net gain of
