to contract with a criminal syndicate that offers smuggling services. Non-Mexicans may require assistance traveling illegally through the interior of Mexico, another service more likely to be provided by transnational cartels than by local small-scale smugglers in Mexican sending villages or at the U.S.–Mexico border.


The decision to engage in unauthorized migration is highly complex, as more formal models by Chang and colleagues (2012), Guzman and colleagues (2008), MITRE Corporation (2008), and Wein and colleagues (2009) portray. It is influenced by economic, social, and environmental conditions in sending and destination areas; immigration policies; and interior and border enforcement. Furthermore, the border itself can be seen as a “system” of its own, in which enforcement efforts and policy changes can have spillover effects and generate adaptive responses by unauthorized migrants and others. As can be seen in Figure 2-3, none of the existing survey and administrative data sources captures the entire migration process.

All of these factors have evolved over the past 50 years, as have the numbers and profiles of migrants. Crossing routes, modes, and the roles of smugglers (who are separate from, but may be increasingly connected to, organized crime and drug cartels) have also changed considerably in recent times. Survey designs and modeling approaches that may be well suited for capturing certain aspects of unauthorized migration flows today may not be able to do so with the same reliability and validity in the future. The methods and assumptions used to estimate flows should be accordingly flexible.

•   Conclusion 2.1: To understand migration flows in any one sector, it is important to view the entire border as a system; localized increases in border enforcement may simply change where migrants cross without reducing the overall flow in the long run.

•   Conclusion 2.2: The migration process is complex and dynamic. Undocumented migration is the outcome of many interrelated factors that can vary widely across people, space, and time; migrant characteristics and the geography of sending and destination areas are changing constantly. This complexity and dynamism should be incorporated into the analytical approaches and study designs used to estimate flows of unauthorized migrants.

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