The current tide of labor-driven border crossings from Mexico has very real roots in the laws of supply and demand. Seasonal laborers, mostly single men, have been crossing the border for over a century now, pushed out by lack of jobs at home and pulled north by labor shortages and promises of high monetary rewards. When it has suited U.S. interests, we have opened our gates wide, and when the tides have changed, we''ve tried fruitlessly to seal the 3,000-mile-wide border we share with our southern neighbor.

As early as the 1880s, Mexicans began trekking northward to work as small-scale merchants, gold diggers and laborers in the railroads and agriculture. With the passage of exclusionary immigration acts against the Chinese nationals in the 1880s, California turned increasingly to Mexican workers to fuel the state''s growing agricultural economy, and by 1920, over 80 percent of farm laborers in Southern California were of Mexican descent.

The outbreak of World War I and the havoc wreaked by Mexico''s 10-year revolution combined to send more workers north to fill employment gaps. Powerful American employers lobbied to exclude the Western Hemisphere from increasingly stringent immigration standards in order to continue the constant supply of cheap labor. The Great Depression and waves of anti-immigrant sentiment initiated bouts of repatriation, but seasonal crossings to work the fields continued.

But it was the "Bracero Program," a wartime labor agreement between the U.S. and Mexico initiated in 1942, that opened the border wide for sorely needed agricultural workers. The U.S. government paid for the transport and housing for an average 350,000 Braceros (manual laborers) a year, who entered the country legally to work at the rate of 46 cents an hour. Although the Braceros worked under contract, poor housing conditions and labor abuses were commonplace, and advocates organized for the program''s closure. Racism and joblessness in the late 1950s led to "Operation Wetback," which summarily expulsed 3.8 million Mexicans.

The Bracero Program ended in 1964, but the need for workers in U.S. agriculture and the high unemployment and low wages in Mexico persisted, resulting in a rise of undocumented border crossings. As the U.S. increased militarization of the border, fueled by efforts such as "Operation Gatekeeper," illegal border crossings became more dangerous and more costly, leading many workers to move their families to the U.S. to avoid annual crossings. Many others became legal residents and naturalized citizens. The 1983 Migrant and Seasonal Agricultural Worker Protection Act officially affords all laborers minimum wages and livable conditions, regardless of immigration status, but the threat of deportation nevertheless leads to persistent abuses in labor and housing.

Nowadays, the agriculture industry is fighting for the reinstatement of the Bracero Program in Washington to reopen the legal stream of temporary workers. But many advocates believe the industry is saturated with workers already, and all that''s needed is an amnesty that would allow those already here to stay.