Policies toward strangers who want to come to a nation's shores are shaped by demography, economics, politics and long term institutional structures. In the case of immigration, the long-term decline in birth rates and fertility of advanced industrialized countries have led to a need for immigrants whether they be guest workers, full-fledged immigrants, or illegal immigrants. Most of the demographic migratory transition is backed by economic needs, though military and political forces have also played a role. Many political forces lead to long-term institutional structures that reinforce or deter immigration, and they form four immigration regime types based on imperial or colonizing empires, former colonies that become settler countries, long-term countries of emigration, and social democratic countries. But many of the factors that lead to immigration do not provide much of the explanation of actually making immigrants citizens of the nation-state - naturalization. Immigration has little or no correlation with naturalization, though naturalization could not occur without immigration. Demographic forces are the basis for immigration but not naturalization. Instead, long term institutional forces, which are measured by a 'barrier to naturalization index' based on naturalization law are strongly connected to naturalization, and they embed themselves within the regime types just mentioned - empires, settler countries, emigrating, and social democratic countries. However, the ironies involved in immigration and naturalization are that countries that most opposed immigration and naturalization - Germany and Belgium - have reversed their laws and have recently accepted much more liberal regime types; while countries that most supported immigration and naturalization - the UK, France, and Denmark - have immigration and naturalization rates have recently tightened and do not really reflect their past history.

These explanations will be supported by comparative historical evidence, cross-sectional analyses of the post-WWII period, and a pooled time-series and cross-sectional analysis of 18 advanced industrialized countries.