Population Reference Bureau Population Bulletin Vol. The third component of population change is migration, the movement of people into or out of a specific geographic area. Migration adds to or subtracts from an area's population depending on whether more people move in or out. Migration usually has the greatest impact on population change in small geographic areas and where there is little or no natural increase from the excess of births over deaths. Migration is the most complex and volatile demographic variable.
Migration is selective.
More educated and more adventuresome people are more likely than other people to move, for example. Migration is closely tied to the life cycle. People are most likely to move at certain stages of their lives, especially when they marry, divorce, or retire. Migration is also more difficult to measure than fertility and mortality. Most countries do not have an easy and accurate way to track population movements.