2007 to 2009 reveals that the probability of migrating to the United States during the next 12 months dropped in tandem with the intensification of the economic recession (Massey, Durand, and Pren, 2009; Cornelius et al., 2010; Fitzgerald et al., 2011; Massey, 2011). The research over many years suggests that high wages and persistent labor demand in the United States are the principal drivers of undocumented migration (Jenkins, 1977; Blejer et al., 1978; Bean et al., 1990; Espenshade, 1990; Hanson and Spilembergo, 1999; Davila et al., 2002), with the precise number of migrants being conditioned strongly by the size of the worker cohorts who are entering the Mexican labor market (Hanson and McIntosh, 2009, 2010). Economic conditions in Mexico are significant but secondary in their effects on outmigration (Frisbie, 1975; Jenkins, 1977; Taylor, 1987; Massey and Espinosa, 1997; Davila et al., 2002).

As U.S. economic conditions have deteriorated in the past 5 years, enforcement activities have increased; but rising enforcement does not seem to have played a significant role in lowering the likelihood of undocumented migration. Although Amuedo-Dorantes and Bansak (2012) found that increased time spent patrolling the border (“linewatch” hours) decreased the willingness of experienced migrants to cross again and increased the waiting time before the next attempted crossing, their study focused on intentions rather than behavior.

Studies of behavior generally show that rising enforcement has little deterrent effect on undocumented migration. Davila et al. (2002) found that although increased linewatch hours reduced apprehensions initially, the effect was short-lived as migrants adapted their behavior to avoid capture. In her analysis of Mexican migration, Gathman (2008) found that linewatch hours had no effect on the probability of taking an undocumented trip once other factors were held constant. Massey and Riosmena (2010) similarly found no significant effect of linewatch hours on the likelihood of undocumented migration from several Latin American nations, including Mexico. When they measured the enforcement effort using the probability of apprehension at the border, Massey and Espinosa (1997) found a positive effect on the likelihood of initiating undocumented migration. Massey and Riosmena (2010) found that rising deportations from the United States likewise increased the odds of undocumented migration.

Rather than acting as a deterrent, increased enforcement appears to have other effects on migrant behavior: it increases the duration of trips and reduces the likelihood of return migration (Kossoudji, 1992; Massey, Durand, and Malone, 2002; Reyes, 2004; Riosmena, 2004); it shifts border crossing away from areas of concentrated enforcement (Massey, Durand, and Malone, 2002; Orrenius, 2004; Carrion-Flores and Sorenson, 2006; Massey, 2007; Massey, Durand, and Pren, 2009); and it increases the like-

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