690 International Organization as a contested social boundary that becomes politically significant only when compe- tition for scarce resources arises between the native and the immigrant populations. Susan Olzak,for example,argues that"competition potentially occurs when two or more groups come to exploit the same realized niches."6"Niche overlap"triggers ethnic competition and,with it,anti-immigrant sentiment.From the"competition" perspective,anti-immigrant sentiment and hence political opposition to immigration are not constant but change in light of changing circumstances:the size of the immi- grant community,its rate of growth,and other socioeconomic factors. Each of these alternative hypotheses has been criticized on both logical and empiri- cal grounds.7 However,rather than discard the hypotheses,I suggest that because immigrants are geographically concentrated,the analysis must be disaggregated to the level of the salient local political units.Then,each of these perspectives may provide some insight into the dynamics of immigration policy.Those who emphasize national identity suggest that the analysis must also be sensitive to factors that mediate between public opinion and policy outputs,that is,the political institutions of the nation.Those who emphasize economic variables suggest that important political actors are not limited to those concerned with national identity. Societies have political actors who actively promote immigration as well as those who oppose it.Those who focus on the interaction of cultural and economic variables suggest that the intensity and hence the political significance of national- ethnic identity may vary,and that explanations should incorporate variables that affect the intensity of public opinion.In this sense,the three types of analyses are incomplete but complement each other.The explanation I offer builds on these in- sights while emphasizing a fourth element,the geographic concentration of the immi- grant community. The Political Geography of Immigration Control The Geographic Concentration of Migrants Among demographers and sociologists,the geographic specificity of migratory pat- terns is well known,on both the sending and receiving end."Migrants arriving in a particular country do not spread out randomly throughout all possible destina- tions."18 Examples abound.In the United States,six states of fifty-California,Texas, Florida,New York,New Jersey,and Illinois-host 73 percent of the immigrant popu- lation;33 percent of the foreign-born population reside in California alone.In 1990, 8 percent of the U.S.population were foreign born,but they comprised 22 percent of California's population and 16 percent of New York's.This pattern holds in Australia as well.There,in 1991,22 percent of the population were foreign born;this popula- tion is concentrated in the cities and suburbs of Sydney and Melbourne.Of the 147 16.01zak1992. 17.In conformity with academic fashion,all of the authors cited provide a critique of the altemative theoretical frameworks while making the case for their own. 18.White1993b,52.as a contested social boundary that becomes politicallysigniŽ cant only when compe￾tition for scarce resources arises between the native and the immigrant populations. Susan Olzak, for example, argues that ‘‘competition potentially occurs when two or more groups come to exploit the same realized niches.’’ 16 ‘‘Niche overlap’’ triggers ethnic competition and, with it, anti-immigrant sentiment. From the ‘‘competition’’ perspective, anti-immigrant sentiment and hence political opposition to immigration are not constant but change in light of changing circumstances: the size of the immi￾grant community, its rate of growth, and other socioeconomic factors. Each of these alternativehypotheseshas been criticized on both logical and empiri￾cal grounds.17 However, rather than discard the hypotheses, I suggest that because immigrants are geographically concentrated, the analysis must be disaggregated to the level of the salient local political units. Then, each of these perspectives may provide some insight into the dynamics of immigration policy. Those who emphasize national identity suggest that the analysis must also be sensitive to factors that mediate between public opinion and policy outputs, that is, the political institutions of the nation. Those who emphasize economic variables suggest that important political actors are not limited to those concerned with national identity. Societies have political actors who actively promote immigration as well as those who oppose it. Those who focus on the interaction of cultural and economic variables suggest that the intensity and hence the political signiŽ cance of national– ethnic identity may vary, and that explanations should incorporate variables that affect the intensity of public opinion. In this sense, the three types of analyses are incomplete but complement each other. The explanation I offer builds on these in￾sights while emphasizing a fourth element, the geographic concentrationof the immi￾grant community. The Political Geography of Immigration Control The Geographic Concentration of Migrants Among demographers and sociologists, the geographic speciŽ city of migratory pat￾terns is well known, on both the sending and receiving end. ‘‘Migrants arriving in a particular country do not spread out randomly throughout all possible destina￾tions.’’ 18 Examples abound.In the United States,six states of Ž fty—California,Texas, Florida, New York, New Jersey, and Illinois—host 73 percent of the immigrant popu￾lation; 33 percent of the foreign-born population reside in California alone. In 1990, 8 percent of the U.S. populationwere foreign born, but they comprised 22 percent of California’s population and 16 percent of New York’s. This pattern holdsin Australia as well. There, in 1991, 22 percent of the population were foreign born; this popula￾tion is concentrated in the cities and suburbs of Sydney and Melbourne. Of the 147 16. Olzak 1992. 17. In conformity with academic fashion, all of the authors cited provide a critique of the alternative theoretical frameworks while making the case for their own. 18. White 1993b, 52. 690 International Organization