or treatment. The application of this analytical framework is a significant contribution to the
literature on immigration policy evaluation and farm labor markets as it had not been previously
applied in this context. The study also assumed that essential heterogeneity existed, meaning
that workers would not only display different responses to treatment but would also select into
treatment based on idiosyncratic gains. The test of essential heterogeneity suggested by
Heckman, Urzua and Vytlacil (2006a) was used to show that this assumption was indeed
supported by the data.
The results show an overall positive impact of legalization on farm worker outcomes.
There is positive sorting on the gains from legalization, implying that foreign workers who
specifically sought legalization benefited more than the average worker and even more so than
their cohorts who had not been legalized. The magnitude of gain is sensitive to the method of
estimation used, with modest increases noted for the parametric method relative to the
nonparametric methods. The findings from the marginal treatment effects are not entirely clear,
and seem to conflict in most respects with the average treatment effect results. Given the stark
differences between the parametric and nonparametric methods, it would appear that the
assumption of joint normality (on which the parametric method is based) is not supported by the
data. However, this is not to imply that the nonparametric MTEs offer less ambiguous
interpretations. As they are presently, they are somewhat difficult to reconcile with the findings
suggested by the positive sorting gains suggested by the average treatment effect parameters. If
the MTEs are viewed in the context of willingness to pay measures however, the interpretations
seem reasonable. Clearly, these findings suggest a need for future research. A likely starting
point would be additional refinement of estimates based on the nonparametric methods as the
normality assumption appears problematic. Most importantly, the results show that unauthorized
18