or by climbing over the double fence in more transited areas (López Castro, 1997). If migrants are spotted while still close to the line, they generally choose to return to the Mexican side in order to attempt another crossing. This is what USBP calls a “turn-back south.” USBP reportedly keeps records of the number of turn-backs south detected during a shift, but these have not been publicly released, nor does the panel know of any other data source that reports numbers of turn-backs south. In any case, such attempts are not individually identifiable and linkable to prior or subsequent attempts and apprehensions.

An undocumented crossing can result in three different outcomes (other than a turn-back south). First, some undocumented crossers succeed in eluding USBP after being detected; these are termed “got-aways” and become part of the unauthorized population, along with individuals who manage to cross undetected by USBP. If DHS could estimate the number of attempted crossings, they would then be able to indirectly estimate the number of migrants who cross undetected as the difference between the total number of attempts and the sum of turn-backs south, getaways, and apprehensions. The ratio of apprehensions to the total number of attempts is a proxy for the effectiveness of USBP in actually catching the migrants they encounter in the field, a measure DHS officials call the “interdiction rate.”26

Death is another possible outcome of an illicit border crossing, typically brought about by exposure to extreme hot or cold temperatures and dehydration. Deaths at the border have increased (Cornelius, 2001; Eschbach et al., 1999) as more migrants have been crossing through desolate parts of the Arizona desert and remote parts of Texas, typically walking for several days in order to circumvent USBP checkpoints on the highways.

Finally, a crossing attempt can be stopped by USBP (or other federal law enforcement body), at which point the migrant is fingerprinted and photographed and his identity is run through an FBI database to check for prior criminal convictions. After the apprehending agent reviews the individual’s migration history and checks for any prior apprehensions, the officer applies a “consequence” and the migrant is eventually returned to his homeland. Possible outcomes under consequence policies include voluntary return, a formal removal order, or criminal charges. The choice of the consequence depends on the migration and criminal history of the migrant, as well as on applicable USBP policies in effect in the vicinity of the apprehension.27 Depending on the consequence applied to the migrants, they


26 We did not have access to these measures or to data that would allow us to calculate these rates, and we know no published source that shows estimates of them.

27 This description is based on the discussion of enforcement pipelines in National Research Council (2011:Chapter 4).

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