مهاجرت های داخلی رو به کاهش آمریکا: مدارک و شواهد از تعادل فضایی و یا تغییرات ساختاری در بازارهای کار محلی؟

This paper examines whether the significant downward shift in U.S. gross migration rates after 2000 is indicative of the economy nearing a stationary spatial equilibrium characterized by relatively small population growth differentials. Nearing spatial equilibrium would imply that site-specific factors such as amenities and location within the urban hierarchy substantially subside in their influence on net-migration and relative population growth because their values have been capitalized into prices, causing interregional utility levels to become approximately equal. Yet, in an examination of U.S. counties, we find empirical evidence of only slight ebbing of natural amenity-based migration after 2000 and little slowing of population redistribution from peripheral towards core urban areas. Instead, the primary finding is a downward shift in the responsiveness of relative population growth to spatially asymmetric demand shocks post-2000, and associated increased responsiveness of local area labor supply, more consistent with European regional labor markets. Additional sensitivity analysis, including instrumental variable estimation, confirms the result. Quantile regression analysis suggests that our findings are not due to a difference in regional labor market tightness between the 1990s and post-2000.

Amongst the highest in the world, U.S. interregional labor migration flows have long been viewed as a critical component of U.S. labor market flexibility (Obstfeld and Peri, 1998) and economic performance. Internal migration has been shown to smooth out spatially-asymmetric macroeconomic shocks (Blanchard and Katz, 1992 and Partridge and Rickman, 2006b) and the effects of industry restructuring such as those arising from the decline of manufacturing and agriculture (Dennis and Iscan, 2007). Further, internal migration drives regional differences in regional employment and population growth and possibly underlies U.S. advantages in innovation and growth (Crescenzi et al., 2007). The recent decline in gross internal migration, however, raises the possibility that the U.S. economy is nearing a spatial equilibrium. A change in the traditional role of migration in local labor markets would be an alternative explanation. Persistent migration during the latter half of the twentieth century suggests that the U.S. economy was far from a stationary spatial equilibrium. For example, amenity migration has been a primary driver in the redistribution of population from the Frostbelt to the Sunbelt as U.S. income and wealth increased (Graves, 1979, Graves, 1980, Blanchard and Katz, 1992 and Plane, 1993). Interregional migration also has been fueled by urban-hierarchy-based shocks, such as those related to changes in communications and transportation technologies and the ascendancy of higher-ordered services (Plane et al., 2005 and Partridge et al., 2008b). High-skilled workers seeking to earn greater returns on their human capital form a basis for regional innovation and growth (Becker, 1962, Faggian and McCann, 2006, Faggian and McCann, 2009 and Glaeser and Resseger, 2010). An economy approaching a stationary spatial equilibrium would be characterized by greatly diminished net-migration flows as the values of site specific characteristics become capitalized into housing prices and wages (Greenwood et al., 1991). Perhaps consistent with approaching of a spatial equilibrium, the United States recently experienced a secular decline in the rate of interregional migration. As shown in Fig. 1, beginning with the 1970s, the percentage of the population moving across counties or across states generally started to decline, but the decline became more dramatic at the end of the 1990s. The dramatic decline has led to reports that the U.S. has entered an era of “new localism” (Kotkin, 2009) and increased rootedness (Cooke, 2011), raising concerns that jobs would need to be created where people live (Fletcher, 2010). The recent Great Recession that began December 2007 appears to have magnified the post-2000 decline in the rate of migration (Saks and Wozniak, 2007 and Frey, 2008).Regional scientists though focus more on differences in regional growth across U.S. regions than on gross migration flows, as much of gross migration can simply be churning between regions relating to personal considerations (e.g., divorce, marriage) or idiosyncratic matching effects. The more relevant issue then is how the decline in gross migration affected the net migration patterns (reflecting utility differentials) that drive regional growth differentials. Fig. 2 reports the standard deviation of annual county net migration rates (net migration divided by beginning year population) over the 1990–2009 period. The standard deviation of net migration rates experienced a secular decline over the period until 2002, then an upward spike until the peak of the housing bubble in 2006, followed by a sharp decline back to the pre-bubble trend.5 While the peak of the housing bubble period was associated with an increase in net-migration differentials, possibly because the bubble was associated with an increased regional dispersion of housing prices (Sasser, 2010), the collapse of housing prices post-2006 and the ensuing recession that began in December 2007 rapidly reduced these differentials.A question arises then whether net-migration and regional growth differentials will significantly rebound after the recession or whether the long-term trend of declining net migration will continue. U.S. population growth differentials may be more “permanently” at a lower level if the economy is nearing a spatial equilibrium in which location-specific attributes have largely been capitalized into local prices and interregional utility levels are nearly equal. Equalized utility levels would be manifested by an absence of net migration aside from the influences of “short-term” labor demand shocks (which would occur even in spatial equilibrium), or personal idiosyncratic migration. Alternatively, if there has been a change in the role of migration in smoothing out asymmetric demand shocks, migration would have a smaller role in regional economic growth differentials, representing a structural shift in U.S. labor markets. Therefore, this study compares U.S. county population growth and interregional migration during the 1990s with that over the period 2000 to 2007. Because the latter period predates the recession, the comparison establishes whether there has been a longer term, rather than cyclical, shift in interregional migration dynamics. Factors examined include population growth and migration movements related to: natural amenities; proximity in the urban hierarchy; and asymmetric labor demand shocks. The next section contains the theoretical framework, which demonstrates how the various factors can affect interregional migration and how their influence might change over time. Section 3 presents the empirical approach. General regression analysis, including instrumental variable estimation, and quantile regression analysis are described in Section 4. Among the primary results, there is some evidence of the diminishment of natural amenities as a force in the redistribution of population post-2000. We do not find any evidence that net population movements related to proximity in the urban hierarchy are ebbing—i.e., households continue to locate to areas more proximate to larger urban centers. Thus, consistent with the survey findings on well-being across U.S. states by Oswald and Wu (forthcoming) for 2005–2008, differences in utility arising from “innate state differences” (p. 15) do not appear to have been arbitraged away. The most important shift appears to be that migration was the primary labor supply response to spatially-asymmetric labor demand shocks before 2000 while post-2000, the primary labor supply response is a change in the local employment rate. Possible explanations include a slack national labor market, which provide ample labor supply sources in most local labor markets, reducing the impetus for interregional migration. Likewise, increased variability of labor demand shocks may have caused risk-averse households to be less willing to migrate for jobs. Another possible explanation is increased labor mobility across industries, reducing the need for households to geographically migrate with job changes, possibly arising from reduced government regulation, reduced unionization and increased globalization (Kambourov and Manovskii, 2008). Further potential explanations include a decline in military transfers (Pingle, 2007) and the aging of the U.S. population. We explore the plausibility of these potential explanations in sensitivity analysis. A structural shift away from the traditional large labor demand induced migration flows would suggest that U.S. regional labor markets have taken on a European flavor, in which asymmetric labor demand shocks would have the largest effects on local unemployment and labor force participation rates rather than on interregional migration ( Decressin and Fatás, 1995 and Jimeno and Bentolila, 1998). Such a shift would fundamentally alter U.S. economic performance, in which adjustments to shocks may increase structural unemployment rates if labor is not moving to high growth regions, and it would provide additional justification for place-based economic development policies ( Partridge and Rickman, 2006a and Partridge and Rickman, 2006b). A brief summary and discussion of the results and suggestions for future research follow in the final section.

Using county level data, this paper examined whether the post-2000 downward shift in U.S. gross migration rates signaled that the economy was approaching a stationary spatial equilibrium where established site-specific forces driving population shifts were diminishing in influence. These include the long-standing movements of population to areas possessing a pleasant climate and other desired natural amenity attributes, as well as the traditional movements of population from peripheral to urban core areas. Our findings suggest that these population movements continued post-2000, with amenity-based population shifts becoming only somewhat muted. Thus, current and future differentials in area attractiveness related to natural amenities and urbanization have not been fully capitalized into factor prices to produce a spatial equilibrium in which population growth would be more evenly spread across space. Forces underlying these processes appear to continue producing divergence in well-being across the United States, consistent with the findings of Oswald and Wu (forthcoming). Nevertheless, we do find a downward shift in population (net migration) responsiveness to spatially asymmetric demand shocks. Pre-2000, net migration was the primary labor supply response to regionally-variant demand shocks. Post-2000, however, demand shocks almost exclusively have been satisfied by changes in local labor supply. This raises the possibility that the tepid job growth associated with strong productivity growth post-2000 provided ample sources of local labor supply, precluding the need for in-migrants to fill new jobs. Yet, using quantile regression analysis, we generally did not find that the marginal population response to a unit labor demand shock in areas with strong growth exceeded the corresponding response in areas experiencing weak growth. This contrasts with an expectation that population would be more responsive in faster growing areas with tighter labor markets than in those with more excess labor supply if the difference in national employment growth across the two period was responsible for the shift in migration responsiveness. The lack of migration response to differential economic shocks suggests a structural shift, in which U.S. regional labor markets took on more of a European flavor, potentially affecting macroeconomic performance and adjustments to shocks. For example, only post-2000, did we find greater variability in demand shocks to be associated with lower net migration, suggesting increased risk aversion as reducing migration. If so, government policies would need to be oriented more towards creating jobs where the people are, rather than assuming that people will move to the jobs (Partridge and Rickman, 2006a). Yet, amenity migration, albeit somewhat muted, and core–periphery migration, indicated some continued willingness to move. Thus, other potential explanations for the changing role of migration in satisfying spatially asymmetric demand shocks should be explored. One possibility is a rise in mobility across industries, as suggested by the findings of Kambourov and Manovskii (2008) for 1968–1997. Among the possible explanations they offered were changes in government regulation, unionization and increased globalization. Greater use of contingent workers also could facilitate such industry mobility. In our case, if workers could more readily shift across industries within their occupation, workers would be more affected by occupation (skill) shocks and less affected by shocks to industry. Indeed, we find some preliminary evidence that geographical mobility is increasingly tied to occupationally-based demand shocks and less so by industry-based shocks, which would reflect a major shift in the functioning of regional labor markets. It is unclear how such a trend would affect regional adjustments to asymmetric economic shocks or the overall flexibility of the U.S. labor market. Future research could use micro-data to further explore the potential nexus between occupational mobility and dwindling U.S. internal migration, and the implications for U.S. regional labor market dynamics and economic performance.