For too long, the policy debate over border enforcement has been split between those who believe the border can be sealed against illegal entry by force alone, and those who believe that any effort to do so is futile and without expanded legal work opportunities. And for too long, both sides have been able to muster evidence to make their cases — the enforcers pointing to targeted successes at sealing the border, and the critics pointing to continued illegal entry despite the billions spent on enforcement. Until recently it has been hard to referee the disputes with any confidence because the data was simply inadequate — both sides could muster their preferred measures to make their case. But improvements in both data and analysis are increasingly making it possible to offer answers to the critical question of the effectiveness of border enforcement in stopping and deterring illegal entry. The new evidence suggests that unauthorized migration across the southern border has plummeted, with successful illegal entries falling from roughly 1.8 million in 2000 to just 200,000 by 2015. Border enforcement has been a significant reason for the decline — in particular, the growing use of “consequences” such as jail time for illegal border crossers has had a powerful effect in deterring repeated border crossing efforts. The success of deterrence through enforcement has meant that attempted crossings have fallen dramatically even as the likelihood of a border crosser being apprehended by the Border Patrol has only risen slightly, to just over a 50–50 chance. These research advances should help to inform a more rational public debate over the incremental benefits of additional border enforcement expenditures. With Congress gearing up to consider budget proposals from the Trump administration that seek an additional $2.6 billion for border security, including construction of new physical barriers, the debate is long overdue. In particular, Congress should be taking a careful look at the incremental gains that might come from additional spending on border enforcement. The evidence suggests that deterrence through enforcement, despite its successes to date in reducing illegal entry across the border, is producing diminishing returns. There are three primary reasons. First, arrivals at the border are increasingly made up of asylum seekers from Central America rather than traditional economic migrants from Mexico; this is a population that is both harder to deter because of the dangers they face at home, and in many cases not appropriate to deter because the United States has legal obligations to consider serious requests for asylum. Second, the majority of additions to the US unauthorized population is now arriving on legal visas and then overstaying; enforcement at the southern border does nothing to respond to this challenge. And finally, among Mexican migrants, a growing percentage of the repeat border crossers are parents with children left behind in the United States, a population that is far harder to deter than young economic migrants. The administration could better inform this debate by releasing to scholars and the public the research it has sponsored in order to give Americans a fuller picture on border enforcement.
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