A substantial part of U.S. employment is related to manufacturing, either directly or indirectly. Most cities were founded and experienced their major periods of growth when manufacturing was the primary factor in urban growth. Other important sources of variation include differences in labor availability or labor skills, in the quality of transportation facilities, and in local political attitudes. Regions tend to specialize in the production of whatever it is that they can best produce. Goods and people move freely within and between regions of the country.
Relative isolation is uncommon, but it does exist.
Nearly 20 percent of all Americans change their residence in any one year. Although much of this residential migration is local in nature, it does result in substantial interregional population movement. Until the last decade of the 19th century, there was a strong westward population shift toward frontier agricultural lands. The focus of opportunity then changed and migration shifted to urban areas. Such employment is much more flexible in its location, and there has been a more rapid growth in such employment in areas that appear to contain greater amenities.