People in rural areas, after having long outnumbered those in cities and towns, became the minority in 1969. More than four-fifths of the population was rural at the time of independence in 1878, and more than three-quarters was still rural in 1947. The movement to the towns accelerated with the post-World War II industrialization. Towns that attracted industries have grown by factors of five or more since 1920, and by far the most dramatic growth has occurred since 1947. With 8.7 million people occupying 42,800 square miles in 1972, the average population density for the country was 203 persons per square mile. Regions where the densities were highest include the Sofia Basin and the southwestern portion of the Thracian Plain. The population was more dense than average in the western and central portion of the Danubian plateau, in the lower eastern Rodopi, and in the vicinities of Varna and Burgas on the Black Sea coast. It was least dense in the higher mountains, particularly in the high western Rodopi, the Pirin and the Rila, and along the narrow high ridge of the Stara Planina. Dynamics Warfare that was endemic to the Balkan Peninsula throughout much of its early history, exploitation by the Ottomans, and living conditions that contributed to a short life expectancy served to hold down the population of the area before independence. Since 1878, although the country has participated in four wars and most migratory movements have been at Bulgaria's expense, the population has tripled. Growth has been comparatively steady during the century of independence. Its rate has fluctuated but not widely. Until 1910 it was high. It dropped during the 1910-20 decade, which included the Balkan wars and World War I. The period of greatest growth occurred between the great wars, and the three decades since 1941 have been the periods of least growth. Vital statistics supplied by the Bulgarian government to the United Nations in 1972 indicated an annual growth rate of 0.7 percent. This was based on 16.3 births per each 1,000 of the population, as against 9.1 deaths. Infant mortality, included in the overall death rate, was 27.3 deaths during the first year for each 1,000 live births. In early 1973 the government was alarmed at an apparent change in the statistical trend. Complete information for 1971 showed that, instead of 16.3 births per 1,000, the actual figure was 15.9. Indications were that in 1972 it was dropping to 15.4. Internal migrations since 1878 have consisted largely of the initial movement of the rural population from the hills to the plains and the later movement of people from the rural areas to the towns. External migrations have been more complex. The earliest occurred in the aftermath of the liberation; later ones have resulted from the animosities and territorial changes associated with the various wars in which the country has been involved.