Despite vast differences, the House and Senate immigration reform bills share a common strategy on border security. Both would build fences (real and "virtual") and beef up border patrols to keep out migrants.

But sealing the borders will not, in and of itself, reduce the number of illegal aliens living in this country. Indeed, research suggests that the approach may well produce the opposite effect.

Admittedly, the experts are not unanimous. Some researchers conclude increased border enforcement makes little difference in reducing illegal immigrations. Others find it helps, while still others say it exacerbates the problem.

But a review of the entire body of research clearly suggests that enhanced border security will, at best, do little to reduce the overall number of illegal immigrants living in the U.S. Why? Because even as it pares the entry of illegals, it slows the flow of illegal immigrants leaving the United States

The most comprehensive investigation of this phenomenon, conducted by University of Arizona Professor Manuela Angelucci, finds that each additional border patrol agent hired:

• Stops roughly 771 to 1,621 individuals from entering the country illegally.
• Encourages roughly 831 to 1,966 illegals already living here to extend their stay, for fear of being caught trying to exit or re-enter the country.

The upshot of putting more manpower on the border, then, is unclear. Taking into account the likely length of "extended stays" by would-be illegal emigrants, it appears that this initiative may yield a net reduction of as many as 503 illegal residents for each additional agent hired, or a net increase that is nearly twice as large (995 illegal residents).

Three factors work to undermine the effectiveness of the Border Patrol:

First, the wage disparity between the U.S. and Latin American countries provides a powerful incentive to cross the border. The U.S. minimum hourly wage is $5.15, while the minimum daily wage in Mexico is $4.50. In Honduras, the minimum daily wage ranges between $3.24 and $5.17, depending on the worker's occupation. Given this economic reality, the potential reward of higher earnings in this country would still outweigh the higher risk of apprehension from beefed up border security. Research indicates that those determined to enter the United States illegally in search of a better life will likely keep trying to cross the border until they succeed.

Second, the deterrent effect of dramatically expanding the Border Patrol will be undercut if those who are caught are not strongly penalized. Currently, the federal government imposes virtually no sanctions (such as fines or detention) on the illegal immigrants it apprehends. Instead, nearly all detained illegal immigrants sign a voluntary departure form promising to return to their home countries. In 1998, the feds prosecuted only 1.25 percent of the 1.6 million illegals they detained. For the overwhelming majority of illegal immigrants, the cost of being apprehended is the loss of a day's wages.

Third, the rigor with which the Border Patrol enforces the law appears to vary according to domestic labor demands. While the agency's public mission is to guard the border, research indicates that it relaxes enforcement when the demand for migrant workers is high.

Professor Gordon H. Hanson of the University of California, San Diego, and Antonio Spilimbergo of the International Monetary Fund found that price increases in low-skilled sectors are associated with decreases in the amount of "linewatch" hours devoted to watching the U.S.-Mexico border by Border Patrol agents. Specifically, when the price of fruits, vegetables and livestock rises, the number of linewatch hours declines. Similarly, when these prices fall, linewatch hours increase. A similar relationship between housing starts in the West and linewatch hours exists as well.

Clearly, a crackdown in border enforcement is not enough to solve the problem of illegal immigration. Immigration reform must also:

• Address the "demand" side of the equation, by cracking down on employers who hire illegal migrants.
• Simplify procedures to accommodate an expanded guest-worker program, so that immigrants may legally fill temporary jobs and easily return their country of origin.
• Encourage economic reforms in countries of origin.

In short, we need to consider far more than the largely symbolic act of hiring thousands of new Border Patrol agents and building fences.

If America wishes to significantly reduce the number of people living here illegally, we must adopt an immigration strategy that encompasses border security, interior enforcement and a lawful temporary work program. Unfortunately, Congress appears enamored of a "border first" approach that sounds tough, but will likely do little to reduce illegal immigration.

David B. Muhlhausen, Ph.D., is a senior policy analyst in the Center for Data Analysis at The Heritage Foundation.