Voluntary agreements between people now in Mexico and people now in the U.S.—my renting an apartment in California to someone currently living in Mexico, or hiring him to mow my lawn—benefit both parties. Standard economic arguments suggest that although there may be negative effects on third parties, such as someone else who wants the apartment or someone else who wants to cut my lawn, the net effect is positive. The standard arguments for freedom of association, contract, and trade apply to immigration as well.
Opponents of open immigration have an obvious counterargument: not all interactions are voluntary. A Mexican who comes here in order to mow lawns benefits us as well as himself. One who comes to collect welfare benefits himself, but at our cost. A common conclusion is that free immigration may be desirable in a completely laissez-faire system, even in the relatively laissez-faire America of a hundred years ago, but not in a modern mixed economy.
The strength of this argument is in part an empirical question. Immigrants may get things they do not pay for, but they may also pay for things they do not get. On past evidence immigrants tend to be predominantly young adults, a long way from collecting Social Security or Medicare. The question is a complicated one and I am far from sure that a correct answer would support the argument against immigration, but that is not the point I want to explore in this post.
What I want to explore instead is the flip side of the argument. The existence of a welfare state may indeed make open immigration less attractive. But the existence of open immigration also makes a welfare state less attractive—which, for those who disapprove of a welfare state, is an additional argument in favor of open immigration.
Consider the analogous argument applied intrastate. Supporters of higher levels of welfare generally want them to be provided at the federal level—for a good reason. If welfare is provided and paid for by the states, high levels of income redistribution tend to pull poor people into, and drive taxpayers out of, states that provide them. That provides a potent political incentive to hold down redistribution. This is one example of a more general principle: The more mobile taxpayers are, the more governments, like businesses in a competitive market, have to provide them value for their money, and thus the less able they are to tax A in order to buy the votes of B.
The same argument applies across national borders.