One popular argument against a legalization, or amnesty, of unlawful immigrants is that it will merely incentivize future unlawful immigration. Unlawful immigrants will be more likely to break immigration laws because they will eventually be legalized anyway, so why bother to attempt to enter legally (ignoring the fact that almost none of them could have entered legally)? This claim is taken at face value because the stock of unlawful immigration eventually increased in the decades after the 1986 Immigration Reform and Control Act (IRCA) that amnestied roughly 2.7 million.
However, that doesn’t prove that IRCA was responsible for the increase in the stock of unlawful immigrants. The stock of unlawful immigrants may have been increasing at a steady rate prior to the amnesty and that rate may have just continued after the amnesty. Measuring the flows of unlawful immigrants is the best way to gauge whether the 1986 Reagan amnesty incentivized further unlawful immigration. If the flows increased after IRCA, then the amnesty likely incentivized more unlawful immigration. The number of annual apprehensions of unlawful immigrants on the Southwest border is a good way to approximate for these cross-border flows.
It’s perfectly reasonable to think that an amnesty of unlawful immigrants could increase their numbers in the future. There are at least two ways this could occur. The first is through knowledge of an imminent amnesty. If foreigners thought Congress was about to grant legal status to large numbers of unlawful immigrants, then some of those foreigners may rush the border on the chance that they would be included. Legislators were aware of this problem, which was why IRCA did not apply to unlawful immigrants who entered on January 1st 1982 or after. IRCA had been debated for years before passage and Congress did not want to grant amnesty to unlawful immigrants who entered merely because they heard of the amnesty. To prevent such a rush, subsequent immigration reform bills have all had a cutoff date for legalization prior to Congressional debate on the matter.
Even with the cutoff date, some recent unlawful immigrants would still be able to legalize due to fraud or administrative oversights. An unlawful immigrant who rushes the border to take advantage of an imminent amnesty still has a greater chance of being legalized than he did before, so legalization might be the marginal benefit that convinces him to try. This theory of a rush of unlawful immigrants prior to an imminent amnesty is not controversial.