We only have to look at Mexican field workers crossing into the US to see this is true. Though the New York Times reported that the number of Mexicans entering the US illegally for the first time has fallen from 525,000 in 2000 to fewer than 100,000 in 2010. The ROI for working in the US is lower, given the US's economical and political environment and Mexico's improving work opportunities.

https://www.nytimes.com/interactive/2011/07/06/world/america...