Border Patrol apprehensions may have dipped to the lowest rate in 35 years, but it has nothing to do with border security. Rates of illegal migration are governed by social and economic forces, not by expensive surveillance technology, walls and the Border Patrol. It thus makes no sense to continue to rely on an expensive and failed border fortification as a centerpiece of our immigration policy.

Proponents of more border fortification argue that added manpower, new technologies and a get-tough detention policy have combined to bring the number of border apprehensions down nearly 35 percent over the last three years. But border enforcement is not what's reducing the flow of illegal immigration.

Wayne Cornelius and researchers at the Center for Comparative Immigration Studies at the University of California-San Diego have been collecting data for years on thousands of people who have migrated between the U.S. and Mexico. Their data shows that added enforcement does not deter people from attempting to cross illegally.

Among veteran and first-time migrants interviewed in 2007-08, concerns about the difficulty and danger of crossing had no statistical effect on their plans to migrate. Most (63 percent) were worried primarily about Mexican police or bandits. Only 35 percent said fences, the Border Patrol or National Guard personnel were their greatest concern.

More striking, border fortification has been impotent in stopping them from making it successfully: 97 percent of migrants from the state of Yucatán interviewed this year successfully crossed the border. Yucatecan migrants have maintained this success rate for more than a decade, and migrants from other Mexican sending communities from the last four years show success rates near 100 percent.

Fortification makes crossing the border more difficult but not impossible. Migrants almost always enlist the help of smugglers, who charge from $2,500-$3,000 per head. They find ways to move their human cargo around fencing and past the detection of the Border Patrol.

What looks like the result of an effective border control policy is almost entirely the result of a souring U.S. economy. In 2006, when the economy was solid, 34 percent of economically active Yucatecan residents considered migrating north. That figure plummeted to just 8 percent this year.

It's easy to see why fewer might come. According to the Pew Hispanic Center, 31 percent of employed illegal immigrants work in service-sector jobs, but Americans are cutting back on the services attached to these jobs. An additional 19 percent of illegal immigrants work in the construction industry, where, from the first quarter of 2007 to the third quarter of 2008, there were 195,000 net fewer Latino-immigrant construction workers employed. In other words, the same economic forces that spurred illegal immigration in the first place are accounting for its decline.

A dire economy is succeeding at doing what border fortification is intended to accomplish. But it makes no more sense to depend on a bad economy than it does to continue to rely on border fortification as the focus of our immigration policy.

An effective policy manages immigration instead of trying to control it at the border. That means placing less emphasis on catching workers and more on preventing the flow of contraband and terrorist activity. It means creating a pathway to legal residency for most illegal immigrants, establishing stiff and enforceable penalties for employers who hire workers illegally and developing a system that allows for the efficient and legal flow of labor. Such a policy would be practical, sustainable, humane and long overdue.

Tomás R. Jiménez is an Irvine fellow at the New America Foundation and assistant professor of sociology at Stanford University. He wrote this article for the Mercury News.