The Senate debate over immigration reform, as in the House, increasingly hinges on a desire from Republicans to "tighten" border security, by which they mean the border with Mexico. To that end, a new version of the Senate proposal that's advancing today would nearly double the number of border patrol agents to 40,000. But in recent years, more agents have corresponded only with a decrease in apprehensions and removals.

Customs and Border Protection has a document that outlines the number of agents in each of three zones over the past twenty years. Over that time, the vast, vast majority of agents have been dedicated to the "Southwest Border Sectors" — the one in need of tightening. Even as the number of agents has risen dramatically, that ratio has been maintained. The graph below stacks up agents from all three sectors to get the total number of agents, but it's mostly just yellow, for the Southwest sector.

The agency of which Customs and Border Protection is a part compiles its own statistics which show how well those agents are doing. And the numbers aren't great. The most recent year for which data is available is 2011. From 1993 to that year, the number of apprehensions (people caught illegally crossing the border), removals (deportations), and returns (people sent back informally) have largely declined as the number of agents has risen.

The thick blue line below is the number of agents, which keeps rising. The red line, the number of people those agents caught. The dark green line is the number of people returned, which largely syncs with apprehensions. Removals, forced deportations, is the only number that has increased — for which Obama has faced criticism.

But those are overall numbers. How many of these people are attempting to enter from Mexico? DHS indicates that every year, nearly every apprehension is of someone from North or South America — meaning they are mostly entering from Mexico.

The removals data backs that up. Nearly three-quarters of all removals over the past decade were of people from Mexico — which doesn't include any other Central American or South American country. Just Mexico.

What this means is clear. Most apprehensions and removals are of people crossing the southern border — but those numbers have decreased substantially despite the increase in patrols. One would think that with more officers, there would be more apprehensions. It's possible that the increase in patrols acts as a deterrent, but that's not actually the main factor at play.

According to a report released last year by Pew Research, the drop-off is due far more to economic factors than any shift in law enforcement activity. Meaning that those additional border patrol agents over the past two decades haven't had much effect — nor would doubling the number yet again.

The data suggests that this might very well be exactly the sort of government inefficiency against which Republicans usually rail. Calls for increased border patrols, then, are either based on a lack of familiarity with effectiveness — or maybe something else.