The article states:
Its laws of motion are set out in a recent paper [‘The Economic Logic of Illegal Immigration‘] by Gordon Hanson of the University of California, San Diego. He points out that unskilled labour is increasingly scarce in America. Since 1960 the share of native-born workers with less than a high-school diploma has fallen from 50% to 12%. In response, illegal immigration has proved to be a fairly efficient system for matching willing workers with eager employers. Some 24% of farm workers, 17% of cleaners and 14% of construction workers are foreigners doing their jobs illicitly.
Dr. Hanson is one of the more perceptive professors with whom I have come in contact at UCSD, and its nice to see him get some positive publicity.