Unions create barriers to immigration

See Do Interest Groups Affect U.S. Immigration Policy? IMF Working Paper by Giovanni Facchini, Anna Maria Mayda and Prachi Mishra.

"Abstract

While anecdotal evidence suggests that interest groups play a key role in shaping immigration policy, there is no systematic empirical analysis of this issue. In this paper, we construct an industry-level dataset for the United States, by combining information on the number of temporary work visas with data on lobbying activity associated with immigration. We find robust evidence that both pro- and anti-immigration interest groups play a statistically significant and economically relevant role in shaping migration across sectors. Barriers to migration are lower in sectors in which business interest groups incur larger lobby expenditures and higher in sectors where labor unions are more important."