In 2013 America’s federal government allowed farmers to fill 99,000 jobs with temporary foreign workers, most of whom came from Mexico. This year it is on track to let in about 240,000. A Republican-sponsored bill would raise the limit to 450,000 a year and allow them to stay for up to three years.

Martin says that "Foreign workers are self-regulating. Because there are usually more willing migrants than farm jobs, and because workers tend to be hired in groups, they have a strong incentive to behave impeccably and ensure that others do, too."

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