Human population has increased dramatically over the last few centuries. In 1830, more than 900 million people inhabited Earth. As the twenty-first century approaches, Earth’s population is nearly six billion. At the same time, extraordinarily large and dense clusters of people are growing: Tokyo has already reached a population in excess of 25 million. The geographically informed person must understand that the growth, distribution, and movements of people on Earth’s surface are the driving forces behind not only human events—social, cultural, political, and economic—but also certain physical events—large-scale flooding, resource depletion, and ecological breakdown.

Students need to develop an understanding of the interaction of the human and environmental factors that help to explain the characteristics of human populations, as well as their distribution and movements. The distribution and density of Earth’s population reflect the planet’s topography, soils, vegetation, and climate types (ecosystems); available resources; and level of economic development. Population growth rates are influenced by such factors as education (especially of women), religion, telecommunications, urbanization, and employment opportunities. Mortality rates are influenced by the availability of medical services, food, shelter, health services, and the overall age and sex distribution of the population.

Another key population characteristic is growth, which may be described in terms of fertility and mortality, crude birth- and death rates, natural increase and doubling time, and population structure (age and sex distribution). These basic demographic concepts help bring focus to the human factors that explain population distributions and densities, growth patterns, and population projections. Population pyramids, for example, indicate the differential effects of past events, such as wars, disease, famine, improved sanitation, and vaccination programs, on birth- and death rates and gender. An analysis of specific age cohorts enables predictions to be made. For example, a large proportion zero to 15 years old suggests rapid population growth whereas a large proportion 45 to 60 years old suggests a mature population, which will soon require significant resources to support the elderly. Both predictions could have significant geographic implications for a community; for example, a young population could create a need for more housing and schools, whereas an older population could create a need for more retirement and medical facilities. Such demographic analyses can be performed at all scales.

Almost every country is experiencing increased urbanization. Across Earth peasant and pastoral life is giving way to the more economically promising lure of life in cities, as people seeking better jobs or more income move to areas where opportunities are better. The majority of the world’s people are moving toward a way of life that only a minority of people experienced less than a century ago. Population geographers predict that Tokyo, São Paulo, Bombay, Shanghai, Lagos, and Mexico City will be the next century’s most massive population centers. However, people in some developed countries are giving up the economic advantages of city life for the ease and attractions of suburbs and small towns, especially those with access to employment in metropolitan areas.

Migration is one of the most distinctive and visible characteristics of human populations, and it leads to significant reshaping of population distribution and character. It is a dynamic process that is constantly changing Earth’s landscapes and modifying its cultures. It takes place at a variety of scales and in different contexts. At international scales geographers track the flows of immigrants and emigrants. At national scales they consider net regional balances of in-and out-migrants or the flows from rural to urban areas, which are a principal cause of urbanization. At a local scale they consider the continuous mobility of college students, retirees, and tourists or the changes of address that occur without necessarily resulting in a job change or change in friendship patterns.

The context of migration varies from voluntary and discretionary (the search for a better place to live), to voluntary but unavoidable (the search for a place to live), to involuntary and unavoidable (the denial of the right to choose a place to live).

In the two voluntary contexts, migration often results from the weighing of factors at the point of origin and at potential destinations against the costs (financial and emotional) of moving. “Pull” factors may make another place seem more attractive and therefore influence the decision to move. Other factors are unpleasant enough to “push” the migrant out of the local setting and toward another area. These factors reflect people’s objective knowledge of places and also their secondhand impressions. As a consequence, many countries have experienced waves of people going from settled areas to new lands in the interior (e.g., the westward movement in the United States in the nineteenth century and the move from the southeast coast to the interior of Brazil starting in the 1960s, when the new capital city of Brasilia was built).

Voluntary and unavoidable migration occurs when much of a region’s or country’s population is impelled into migration streams, such as the millions of Irish who fled to the United States in the 1840s because of the potato famine or the millions of Somalis, Sudanese, and Rwandans who moved in the 1990s because of drought, famine, and civil war. However, some migrations are forced and involuntary. Such was the case with African Americans who were taken to North and South America in the seventeenth, eighteenth, and nineteenth centuries to work as slave laborers on sugar, cotton, and tobacco plantations.

Demographic shifts rearrange patterns of population and create new human landscapes. Natural increase, war, famine, and disease play decisive roles in influencing why many people live where they do. Migration sets people in motion as they leave one place, strike out for a second, and possibly settle in a third. Intervening obstacles influence the pattern of migration. Physical barriers such as deserts, mountains, rivers, and seas or cultural barriers such as political boundaries, languages, economic conditions, and cultural traditions determine how people move and where they settle.

It is essential that students develop an understanding of the dynamics of population characteristics, distribution, and migration, and in particular of how population distribution (in terms of size and characteristics) is linked to the components of fertility, mortality, and mobility.