| Sumario: | A study was conducted to estimate the probability of apprehension among undocumented Mexican migrants attempting to cross into the U.S. between 1965 and 1989. An average apprehension probability of 0.35 was found, which confirmed earlier estimates. When these figures were combined with published apprehension statistics to estimate annual levels of gross undocumented migration between the two countries, it was found that this gross inflow grew by an average of 20 percent per year from 1965 to 1978. It then leveled off until the onset of Mexico's economic crisis resulted in another period of rapid expansion that peaked in 1986. Adjustment of these figures to account for return migration revealed a strong return flow that substantially offset the huge inflow of undocumented Mexican migrants. Over the period studied, 86 percent of all entries were offset by departures, leaving a net inflow of undocumented migrants of only 5.2 million.
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