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Study: The U.S. stops about half of illegal border crossings from Mexico

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There's no question that the United States has poured vast resources into border security over the past few decades. The government has built 650 miles of fence and hired 21,000 Border Patrol personnel. It spent $18 billion on enforcement in 2012 alone.

Yet for all those billions of dollars, we still don't have a great idea of how effective U.S. border security actually is. Many of the Obama administration's favorite metrics — like the declining number of apprehensions at the border — don't really tell us all that much.

So here comes a new report from the Council of Foreign Relations asking the key question: "How effective is enforcement?" The authors — Bryan Roberts, Edward Alden and John Whitley — come to a few big conclusions:

— The U.S. government doesn't provide good data on whether border security is actually working. Government agencies have plenty of stats like how many miles of fence they're building or how many guards they've hired. But they don't always report results and actual outcomes, such as the apprehension rate at the border.

As a result, little is known about the effectiveness of various enforcement measures. For example, some experts have argued that it's easier to prevent employers from hiring illegal immigrants than it is to guard the border. Yet, the report notes, "analytical work that attempts to understand the relative effectiveness of workplace enforcement versus border enforcement in increasing behind-the-border deterrence has been limited."

— The best outside estimate is that the U.S. government now stops about half of all illegal border-crossings from Mexico. The apprehension rate appears to have increased in recent years as a result of stepped-up border security:

To put this figure in context: The proposed Senate immigration bill would provide $3 billion for reducing traffic over the Mexican border through more drone surveillance and more patrol officers. If, by the fifth year, the "effectiveness rate" doesn't reach 90 percent, the bill would provide another $2 billion in security funding and launch a new commission. Currently, the government estimates its effectiveness rate at around 84 percent.*

— The flow of unauthorized immigrants has decreased in recent years. Part of that is due to better security, but most of it is due to the bad economy: "The best estimate available to date," the report notes, "is that enforcement increases explain approximately one-third of the recent reduction in the flow of undocumented migrants, and economic factors the remainder."

— But the incentive for illegal immigration isn't going to disappear anytime soon. Even though Mexico is getting older and wealthier, illegal immigration will still be a factor for years to come: "While demographic change in Mexico and Central America will reduce the pressure for illegal immigration in the long term, the wage gap that is one of the primary drivers of unauthorized migration has narrowed only slightly."

The big recommendation in the Council of Foreign Relations report is that Congress and the federal government need to develop much, much better metrics on how well its enforcement policies are working. The authors also urge the Department of Homeland Security to share its border security data with outside researchers.

*Update: See this post by Matt Graham on the difference between the "apprehension rate" (thought to be around 50 percent) and the "effectiveness rate" (thought to be around 84 percent). Note that the latter also includes "turn backs."