Immigration barriers began being erected in the New World in the late 19th century. They were motivated by fears that the immigration of unskilled workers would increase inequality. Controlling for economic factors, there appears to have been little independent role for factors such as racism or xenophobia in driving the retreat from liberal migration policies. A statistical analysis of individual voter attitudes towards immigration in the late 20th century leads to somewhat different conclusions: nationalism is strongly associated with more hostile attitudes towards immigrants. Heckscher-Ohlin theory and the Borjas theory of immigrant self-selection also help explain individual voter attitudes.