J. Edward Taylor's research referred in a recent article in the New York Review Books

California’s San Joaquin Valley is one of the highest-value stretches of farmland in the country and is dominated by large growers who preside over a labor force of migrant workers in a way that has not changed much since  1939.  Fruit and vegetable picking is a one-generation job—farmworkers I spoke to neither wanted nor would allow their children to follow them into the fields. The heat and physical toll, combined with the feudal power of the growers, make it preferable to work in an air-conditioned hotel or packing houses.

This means that a constant supply of impoverished Mexican immigrants willing to do the work is required. Taylor estimates that the number of potential immigrants from rural Mexico shrinks every year by 150,000. This can be partly explained by improved economic conditions in northern and central Mexico, which have dimmed the allure of minimum-wage labor in the US, and partly by the cost and danger of venturing across the border. If you do make it into the US, payments to a smuggler can keep a minimum-wage laborer in debt for life.

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