What explains variation in immigration policy, especially policy regulating low-skill workers? A common argument invokes prejudice against foreigners as an explanation for why nations close their economies to immigrants. This prejudice has been ubiquitous throughout history even as immigration policies changed. Social theories of this sort may be descriptively true but are not helpful in predicting variation in policy. Other scholars have turned to the role that native labor plays in protecting its interests against immigration, but they have not explained why labor is able to restrict immigration when it has not been able to restrict trade, even though open trade has wreaked as much, if not more, havoc on labor. A third group of scholars focuses on states' concerns about the fiscal costs of immigrants as an explanation for the changes in policy over time. While fiscal costs are likely to play a role, this argument cannot explain exclusion prior to the creation of the modern welfare state in the early twentieth century. Finally, a fourth group of scholars has examined the power of immigrants themselves. While immigrants clearly affect immigration policy in democracies, they have never been a sufficiently large plurality of the polity to be able to change policy on their own, and they have less voice in autocracies where they can more easily be deported.