The population density, still low as the general average of the confederation, with 26 residents / km 2, is significantly increasing in the southern and western areas, while it is stable in the increasingly populous states. In the Mid Atlantic (states of New York, New Jersey and Pennsylvania) it was 140 residents / km 2 in 1970 and is still the same in 1990, while in Texas and California it increased by 35 ÷ 40% over the same period. The demographic center of gravity has continued to move westward, following a constant trend for centuries: at the beginning of the twentieth century it was in Indiana, in 1970 in Illinois, in 1990 in Missouri, near Steelville. Furthermore, in the last decade the center of gravity has dropped from the 39th to the 38th parallel.
The urban population has grown in global terms, but is now stable in percentage terms (73.5% in 1970; 73.7% in 1980, approximately 75.0% in 1990). The influx to metropolitan areas and to smaller cities is partly offset by the opposite movement towards villages and areas considered to be rural. After the 1980 census, a new classification of metropolitan areas was made, now called MSA (Metropolitan Statistical Areas), with criteria that adhere more than in the past to the concept of ” functional area ”; the areas thus identified have at least 100,000 residents and a central locality of at least 50,000 residents, while the largest may have subordinate central localities. 284 have been defined as such, hosting 193 million residents. (1990), on an area of 1,. Eleven of these large urban units exceed 3 million residents, while the largest (New York-New Jersey-Long Island) exceeds 18 million. These metropolitan areas do not have a single administration, on the contrary they often cover many counties and extend into several neighboring states, but their identification helps to highlight the enormous extent of uninterrupted urban systems. Moreover, the various neighboring administrations are oriented towards forms of cooperation to solve the complex problems of land management. The increase in the large centers of the South is evident, where medium-small centers are also growing, at even higher rates. On the other hand, the large northern cities have almost all shown stability in recent years, after a period (1970-80) of even decline.
The 1980 census had already classified 11,400 villages, officially the centers with less than 2,500 residents, not included in metropolitan areas. Their number is even higher than that of twenty years earlier, when the agricultural population was much more consistent. The phenomenon, still in progress, is explained by the transformation of the functions of small settlements: no longer commercial centers serving the countryside, but places of various advanced and industrial tertiary activities. Decentralization, which has seen particular success in the West, South and Great Plains, is facilitated by the fact that the village can offer a better quality of life, more accessible services and cheaply available space. In addition, the current new residents feel little isolation, easily connected as they are to large centers,
Another aspect of the same phenomenon is urban decentralization. Within the same city, the peripheral areas have grown in the face of a depopulation of the central districts, which more often than not have suffered social and physical degradation. The most recent trend is that of an urban revival of the central districts, where imposing buildings with spectacular architectural qualities are built. In Manhattan skyscrapers are being built again, in Detroit the Renaissance Center has been built, in Baltimore the degraded Inner Harbor has seen luxurious buildings rise on its banks; the same happens in Chicago, St. Louis, Dallas. But the reconstruction of the central neighborhoods further deprives them of population, as this type of building is dedicated to offices, hotels, shopping centers and, only to a modest extent, to housing.
