A city is a large and permanent human settlement.〔Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.〕〔Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. London: Routledge.〕 Although there is no agreement on how a city is distinguished from a town in general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.
Cities generally have complex systems for sanitation, utilities, land usage, housing, and transportation. The concentration of development greatly facilitates interaction between people and businesses, benefiting both parties in the process, but it also presents challenges to managing urban growth. A big city or metropolis usually has associated suburbs and exurbs. Such cities are usually associated with metropolitan areas and urban areas, creating numerous business commuters traveling to urban centers for employment. Once a city expands far enough to reach another city, this region can be deemed a conurbation or megalopolis. In terms of population, the largest city proper is Shanghai, while the fastest-growing is Dubai.〔The Travel & Tourism Competitiveness Report 2007, Jennifer Blanke, World Economic Forum〕
There is not enough evidence to assert what conditions gave rise to the first cities. Some theorists have speculated on what they consider suitable pre-conditions, and basic mechanisms that might have been important driving forces.
The conventional view holds that cities first formed after the Neolithic revolution. The Neolithic revolution brought agriculture, which made denser human populations possible, thereby supporting city development. The advent of farming encouraged hunter-gatherers to abandon nomadic lifestyles and to settle near others who lived by agricultural production. The increased population-density encouraged by farming and the increased output of food per unit of land created conditions that seem more suitable for city-like activities. In his book, ''Cities and Economic Development'', Paul Bairoch takes up this position in his argument that agricultural activity appears necessary before true cities can form.
According to Vere Gordon Childe, for a settlement to qualify as a city, it must have enough surplus of raw materials to support trade and a relatively large population. Bairoch points out that, due to sparse population densities that would have persisted in pre-Neolithic, hunter-gatherer societies, the amount of land that would be required to produce enough food for subsistence and trade for a large population would make it impossible to control the flow of trade. To illustrate this point, Bairoch offers an example: "Western Europe during the pre-Neolithic, () the density must have been less than 0.1 person per square kilometre". Using this population density as a base for calculation, and allotting 10% of food towards surplus for trade and assuming that city dwellers do no farming, he calculates that "...to maintain a city with a population of 1,000, and without taking the cost of transport into account, an area of 100,000 square kilometres would have been required. When the cost of transport is taken into account, the figure rises to 200,000 square kilometres ...".〔 Bairoch noted that this is roughly the size of Great Britain.
The urban theorist Jane Jacobs suggests that city-formation preceded the birth of agriculture, but this view is not widely accepted.
In his book ''City Economics'', Brendan O'Flaherty asserts "Cities could persist—as they have for thousands of years—only if their advantages offset the disadvantages" . O'Flaherty illustrates two similar attracting advantages known as increasing returns to scale and economies of scale, which are concepts usually associated with firms. Their applications are seen in more basic economic systems as well. Increasing returns to scale occurs when "doubling all inputs more than doubles the output () an activity has economies of scale if doubling output less than doubles cost" . To offer an example of these concepts, O'Flaherty makes use of "one of the oldest reasons why cities were built: military protection" . In this example, the inputs are anything that would be used for protection (e.g., a wall) and the output is the area protected and everything of value contained in it. O'Flaherty then asks that we suppose the protected area is square, and each hectare inside it has the same value of protection. The advantage is expressed as:
:(1) , where O is the output (area protected) and s stands for the length of a side. This equation shows that output is proportional to the square of the length of a side.
The inputs depend on the length of the perimeter:
:(2) , where I stands for the quantity of inputs.
So there are increasing returns to scale:
:(3) . This equation (solving for in (1) and substituting in (2)) shows that with twice the inputs, you produce quadruple the output.
Also, economies of scale:
:(4) . This equation (solving for in equation (3)) shows that the same output requires less input.
"Cities, then, economize on protection, and so protection against marauding barbarian armies is one reason why people have come together to live in cities ..." .
Similarly, "Are Cities Dying?", a paper by Harvard economist Edward L. Glaeser, delves into similar reasons for city formation: reduced transport costs for goods, people and ideas. Discussing the benefits of proximity, Glaeser claims that if a city is doubled in size, workers get a ten percent increase in earnings. Glaeser furthers his argument by stating that bigger cities do not pay more for equal productivity than in a smaller city, so it is reasonable to assume that workers become more productive if they move to a city twice the size as they initially worked in. The workers do not benefit much from the ten percent wage increase, because it is recycled back into the higher cost of living in a larger city. They do gain other benefits from living in cities, though.
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