Introduction [to Demographic aspects of migration]

International migration between countries and internal migration within countries have become the main source of population change within recent decades. Prior to about 1750, mortality fluctuations were the major determinant of population dynamics. In the pre-industrial period in currently developed countries, fertility was relatively high and population movements were either relatively local � between nearby villages or within primarily rural agricultural areas � and limited to small groups of merchants or infrequent actions of military forces. Under these demographic conditions, the regular and often large fluctuations of mortality were the principal factor affecting population change. In the worst years � such as during the mid-1300s when the bubonic plague moved out of Central Asia and swept through Europe � mortality losses claimed more than one-fourth of the existing population. During times of heavy mortality, the number of deaths greatly exceeded the contributions of births and substantial population losses occurred. In other years, mortality was more modest and population would slowly increase. These periods of modest mortality should not be thought of as �normal�, however, because the characteristic demographic situation of pre-industrial societies was fluctuating mortality, evidenced by periods of modest increase and times of heavy population losses