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