سرمایه انسانی و رشد شهرهای آمریکایی، 1900-1990

We propose that cities that start out with proportionately more knowledgeable people grow faster in the long run because (a) knowledge spillovers are geographically limited to the city and (b) much knowledge is most productive in the city within which it is acquired. We found that city-aggregates and metropolitan areas with higher average levels of human capital grew faster over the 20th century. The estimated effects of human capital were large: a standard deviation increase in human capital in 1900 was associated with a 38% increase in average annual employment growth of city-aggregates over the period 1900–86. The estimated effects for metropolitan areas were smaller but still economically significant: a standard deviation increase in 1940 human capital was associated with an increase in average annual employment growth over the period 1940–90 of about 15%. Although the rise of the automobile appears to have overwhelmed the importance of human capital in cities dominated by manufacturing early on, human capital seems to have been economically more important in manufacturing cities than in non-manufacturing cities later on. Moreover, the estimated effects of human capital persisted for very long periods of time, suggesting either that adjusting to the steady state is very lengthy, or that shocks to growth are correlated with the presence of human capital.

Between 1860 and 1920 the number of American cities with 10 000 or more people increased from 93 in 1860 to 752 in 1920; the number with 100 000 or more people increased from 9 to 68. As Schlesinger (1933, p. 435) put it, the era witnessed the ‘momentous shift of the center of national equilibrium from the countryside to the city.’1 Not all cities, however, participated equally in the transformation. Some cities experienced unprecedented population growth; other cities were nearly stagnant; and still others shrank. What determined which cities rose and which fell? In this paper we examine the potential for human capital to offer an explanation. This is not the first paper to study the link between city growth and human capital. Empirical papers by Glaeser et al., 1992 and Glaeser et al., 1995); Nardinelli and Simon (1996) and Simon (1998) have identified human capital as a determinant of city growth in the post-World War II period, and Beeson et al. (1999) identified human capital infrastructure as a determinant of county growth over the period 1840–1990. The theoretical starting point of these papers was Lucas (1988), who suggested that the very existence of cities was evidence of the external effects of human capital: ‘What can people be paying Manhattan or downtown Chicago rents for, if not for being near other people?’ (p. 39). 2 In contrast to most previous research, which has examined the relationship between human capital and city growth in the post-World War II period, this paper focuses on the early 20th century, when America had only just embarked on her transition from agriculture to manufacturing. The data also permit examination of longer time periods than most previous studies. This is of interest because the models of Eaton and Eckstein (1997) and Black and Henderson (1999) predict parallel growth of human capital, and therefore, of employment across cities in the long run. Their models suggest that the initial effects of human capital should die out. Our analysis allows us to determine whether this was empirically the case for US cities. Our study has a number of limitations. The fortunes of cities are linked in large part to the industries that inhabit them. The rise of the automobile, in particular, had important ramifications for city growth early in the 20th century. The rise and fall of other sectors must have had similar, albeit smaller effects. Although we make some attempt in the paper to test the importance of human capital against the role of industry composition, this remains an important avenue for future research. Nor did we examine the specific mechanisms by which human capital might affect growth. Human capital could lead to higher growth by enhancing the ability of cities to absorb existing ideas, create new ones, or to adapt to changing economic conditions. Finally, it is necessary in the future to refine our tests to distinguish empirically between theories of local human capital externalities and those based on comparative advantage. Our analysis should therefore be viewed as a first step: establishing whether a correlation between growth and human capital exists in the first place.

We found that American cities with proportionately more individuals with high levels of human capital in 1900 grew more rapidly over the next 86 years. Our evidence is consistent with stories of economic growth that rely on local externalities from human capital. We also found considerable persistence of the effects of human capital, indicating that the distribution of human capital established in the first decade of the twentieth century played a role in current status of American cities. We found some evidence that the presence of human capital is less important today than in the past, perhaps reflecting the decline in the costs of transportation and communication. We also found that the presence of human capital was, at least from 1940 onward, at least as important in manufacturing-dominated cities as in non-manufacturing cities. As noted at the outset, however, much work remains to be done. First, we found that the effects of human capital were overshadowed by the rise of the automobile in manufacturing dominated cities in 1900 and 1920. Although we made some effort to control for the effects of industry mix, it would be worthwhile to develop better measures of industry composition to control for the rise and fall of specific industries. Second, it is important to know whether cities with higher levels of human capital grew faster because they were more successful in absorbing existing ideas, creating new ones, or in adapting to changing economic conditions. Finally, it is important to refine tests to distinguish empirically between theories of local human capital externalities and those based on comparative advantage.