Are border security measures working? Why the answer is elusive.

Some of the ingredients that go into American border security are clear: The US-Mexico line is now flanked by 651 miles of fence and guarded by some 18,000 Border Patrol officers.

But what, exactly, has this arms race on the border achieved?

That question is shockingly difficult to answer, argues a new report by the Council on Foreign Relations, because the government refuses to release almost any substantive data on border security.

Aside from the much-touted statistic of apprehensions at the border – which have decreased dramatically in recent years – immigration authorities have done little to demonstrate the results of their beefed up border security, the study’s authors argue.

“The problem is the number of apprehensions is not self explanatory,” says Edward Alden, one of the authors of the CFR study. “It could mean people are being deterred from crossing, but it could also mean a lot of other things. When you think about it, it could even mean Border Patrol is doing a worse job because they’re catching fewer people.”

That lack of other substantive data on what’s happening at the border – and why – has left lawmakers largely in the dark as they attempt to craft meaningful immigration reform, the authors say.

“Designing better policies for the future will be difficult unless lawmakers have a better grasp on the effectiveness of immigration enforcement in reducing illegal immigration to the United States,” the study concludes.  

Indeed, how the government tracks its own successes and failures on the border – or fails to – has not gone unnoticed in Congress as members have debated immigration policy in recent months.

Under a proposed Senate bill, the US will spend an additional $4.5 billion in border security measures over the next five years, much of it designed to ensure that 90 percent of illegal border crossers are turned back or apprehended along high traffic corridors of the US-Mexico border.

But at a hearing last week, several Republican lawmakers asked how immigration authorities would prove that they really knew what percentage of illegal crossers were being captured.

The CFR study provides new kindling for that debate by suggesting that the government may be overestimating the percentage of border crossers it catches.  

In 2011, the most recent year for which such statistics are available, the Border Patrol estimated it caught 61 percent of the people who attempted to illegally cross the US-Mexico border. But when the CFR analysts crunched numbers from migrant surveys and the Department of Homeland Security, their estimates were that only 40 to 55 percent had been caught.

Counting the people who elude immigration authorities is notoriously difficult. To measure getaways, the Border Patrol relies largely on footprints, snapped twigs, and other traces of human activity scattered along the border.

But in the absence of more robust public records on border security, their precarious estimates have taken on an outsized importance in the public debate.

“What we use now is so clearly and totally inadequate,” says Don Kerwin, executive director at the Center for Migration Studies in New York. “Where are the records about crime rates in border communities, or the rates of illegal drugs coming through ports of entries, or the deaths of attempted border crossers? These all have to be a part of the equation in measuring security.”

However, the CFR report also points to another blind spot in the current border security debate – evidence that the majority of the decline in undocumented immigration in recent years hasn’t had anything to do with how well the borders are guarded. It’s been driven by the economy. 

When the US falls into a recession, fewer Mexicans try to illegally cross the border in search of jobs. And that has nothing to do with fences or border security agents.