Detecting Differences in Interstate Migration Streams Using An Enhanced Gravity Model

Southeastern Geographer Vol. XXXVIII, No. 1, M ay 1998, pp. 22-40 DETECTING DIFFERENCES IN INTERSTATE MIGRATION STREAMS USING AN ENHANCED GRAVITY MODEL i Alexander C. Vias Over the last 30 years, most statistical studies modeling population redistribution in the United States have focused on patterns and processes o f migration at the national and regional scale. However, the very nature o f these generalized studies results in a loss o f inform ation on indi­ vidual migration streams. This paper represents a prelim inary attem pt to rem edy this problem by focusing on two o f the m ost im portant m igration streams in the United States— the m ove­ m ent o f people to Arizona and Florida. In particular, an enhanced version o f the gravity model (utilizing ordinary least squares regression) is used to detect differences in the m igration stream s to each state during the five-year period o f 1985-1990. Results show that the structure o f movem ent into each o f these states is different, highlighting that m igration to the Sunbelt is far from homogenous, and that studies using m igration flows to and from every state may hide these im portant differences. INTRODUCTION. Migration processes have long fascinated social scientists from a number of disciplines including geography, economics, and sociology. One of the most common goals of researchers has been to detect, explore, and model patterns of migration. In the United States, a great deal of interest has been stimu­ lated by the massive Snowbelt-to-Sunbelt migration stream (Berry and Dahmann, 1977; Greenwood, 1985). Since World War II, millions of Americans have left the colder and more economically distressed regions of the United States and moved to areas in the southern and western portions of the nation. As a result of intense scrutiny by researchers, much more is known today about what motivates the decision to move and what factors determine the migrant’s final destination. In addition, researchers now have a clearer picture of the streams of migration that have dominated movement around the country. While these macro-level analyses have offered valuable insights on migration processes at the national scale, the very nature of these generalized studies results in a loss of information on individual migration streams. For example, migration models that utilize data on every migration stream in the U.S. may not offer much information on the specific characteristics of migration to a particular state. This paper represents a preliminary attempt to remedy this problem by focusing on two of the most important migration streams in the U.S.—the movement of people to Arizona and Florida. In particular, the well-known gravity model is used in an or­ dinary least squares (OLS) regression to define differences in the migration streams to each state during the five-year period of 1985-1990. Results show that Mr. Vias is a graduate student in the Department o f Geography and Regional Development, University o f Arizona, Tucson, AZ 85721. V o l . X X X V I I I , N o . 1 23 the movement of people into each of these states is in fact quite different and that generalized studies that use a cross-section of every state’s migration flows may hide these important differences. The first section of this paper provides a brief review ofthe literature on recent migration patterns in the U.S. and on the determinants of migration. This section is followed by a discussion of recent migration patterns to the two states that are the focus ofthis study, Arizona and Florida. In the next section a model is developed that can differentiate characteristics of the migration that took place to Arizona and Florida during the time in question. Besides the basic model, a number of other variables are also introduced, along with discussion of the data. Finally, the regression results for each of the model specifications are presented, along with a discussion of these results. The last section summarizes the goals and findings of this paper and outlines areas for further research. RECENT RESEARCH ON INTERSTATE MIGRATION. Over the past 50 years, thousands of papers have appeared...