Between 2004 and 2012, United States Border Patrol (USBP) apprehensions of Mexican nationals along the US-Mexico border declined to numbers not seen since the early 1970s. Scholars and immigration experts have attempted to explain the current declines by looking at the same three factors that have historically been associated with causing the rise in illegal Mexican migration: immigration policy changes in the United States, demographic changes in Mexico, and economic factors in Mexico and the United States. In addition, scholars and immigration experts have continued to dismiss border enforcement as a possible explanatory variable, despite specific findings in much of their own research suggesting otherwise. I analyze the three historic causes of illegal Mexican migration and argue that they in fact could not have been responsible for the decline in USBP Mexican apprehensions since the mid-2000s. I found that while there have been some changes within these three factors; the changes have not been significant enough to cause a 40-year low in Southwest Border Mexican apprehensions. More importantly, I found that the only other factor that could have reduced illegal Mexican migration has been border enforcement. In fact, it is the only factor that has seen a significant change. The exponential increases in Southwest Border enforcement efforts since the mid-2000s, and the simultaneous decline in Mexican apprehensions along the US-Mexico border, suggests that border enforcement is the principal factor responsible for the historic drop in illegal Mexican migration.
