Exodus from the California Core: Using Demographic Effectiveness and Migration Impact Measures to Examine Population Redistribution Within the Western United States Christopher J. Henrie David A. Plane Published online: 11 October 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Increasingly since the 1960s and 1970s, population migration trends within the United States have been driven by the development of a second western population core. The burgeoning concentration of population along the Pacific Coast has fueled the emergence of a significant interconnected system of western metropolitan areas that increasingly rivals the primacy of the long-established northeastern core. During the 1990s the dispersal of population downward within the western urban hierarchy supplanted a much diminished Frostbelt-to-Sunbelt trend to become the most salient aspect of national population redistribution. The Southern California and Bay Area conurbations are serving as the primary pivots fueling the extension of a western urban subsystem. In this study we use county- level IRS matched tax return data and the newly defined Core Based Statistical Area (CBSA) units to explore the recent (1995–2000) flows of U.S. internal migrants within the functional urban system of the western United States. We present maps based on demographic effectiveness and on a new migration impact measure to examine and illustrate the evolving spatial patterns characteristic of current popu- lation redistribution across the West. This report is released to inform interested parties of research and to encourage discussion. The views expressed are those of the authors and not necessarily those of the U.S. Census Bureau. Dr. Henrie was on the faculty of the Department of Social Sciences, Pittsburg State University when this paper was submitted for publication. C. J. Henrie (&) Geography Division, U.S. Census Bureau, 4600 Silver Hill Road, Mail stop: 7400, Washington, DC 20233-7400, USA e-mail: christopher.j.henrie@census.gov D. A. Plane Department of Geography & Regional Development, University of Arizona, Tucson, AZ 85721, USA 123 Popul Res Policy Rev (2008) 27:43–64 DOI 10.1007/s11113-007-9053-6