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For further reading, Andrew Selee discusses these phenomena in his book Vanishing Frontiers: The Forces Driving Mexico and the United States Together, available through Better World Books here.
With regard to the decline in Mexican migration, Selee attributes this shift to increased cross-border activity since the enactment of NAFTA in 1994 that has quadrupled trade among the U.S, Canada, and Mexico and significantly grown the economies of the three member nations on a macro level. He also notes that remittances of American Dollars from Mexicans in the United States to their families back home has helped develop many communities in Mexico and reduced the economic desperation that fueled migration in the latter half of the 20th Century.
See also:
https://cmsny.org/publications/warren-undocumented-2016/.
https://www.pewresearch.org/fact-tank/2019/06/12/us-unauthorized-immigrant-population-2017/
https://www.businessinsider.com/number-of-people-moving-from-us-to-mexico-2019-5