It started as an off-the-cuff question to Maria del Carmen Ferrez, who came to clean my house twice a month. Did she plan to have more children? Carmen, always chatty, suddenly went silent. She started sobbing. She told me about four children she had left behind in Guatemala. Her husband had left her, and Carmen simply couldn't feed them more than once or twice a day. They would ask for food. She didn't have it. So she left them in Guatemala with their grandmother and came to work in El Norte. She hadn't seen them in 12 years. Her youngest daughter was a year old when she left.

Carmen's answer stunned me and sent me on a journey of my own. How could a mother leave her children and travel 2,000 miles away, not knowing when or if she would see them again? After nearly two years of research in the United States and in Latin America, I found some answers — and many more Carmens. Regardless of the law, regardless of the danger and pain, millions of women, often single mothers, come to the United States from Mexico and Central America and send dollars to the children they leave behind. And after years apart, their children, desperate to be with their mothers, often make their own harrowing journeys through Mexico to find them.

These mothers and children offer up almost certain proof that the legislative "solutions" that Congress is debating — and that brought thousands out into the streets in protest — won't make a difference.

First, some facts. Clearly, illegal immigration is out of control. The United States is experiencing the largest wave of immigration in its history. An estimated 850,000 people enter the United States illegally each year — more than double the number who came in the 1980s and early 1990s. Today, there are an estimated 12 million illegal immigrants here. Nearly 1 million people come here legally or become residents each year — more than twice the number as in the 1970s.

There are undeniable benefits to all this. Most people agree that U.S.-born workers won't do some of the backbreaking jobs that illegal immigrants take, especially for rock-bottom wages. Picking lettuce. Cutting sugarcane. Or, in the case of one woman I interviewed, cleaning houses where there had been a suicide or violent crime.

Immigrants' low wages keep some businesses from closing or going abroad to compete. A 1997 study by the National Research Council, still considered the most objective and authoritative on the effects of immigration, found that immigrant labor lowered the cost of food and clothing for all of us, and it put such things as childcare services within reach of far more Americans. Immigrants bring new ideas that drive creativity and spur advances.

Yet the downside is real, too. Because they have lower incomes, immigrants and their U.S.-born children qualify for and use more government services — including welfare — than the native-born. They have more children, and therefore more youngsters in public schools. Compared with native households, the NRC found that immigrants and their native-born children paid one-third less in taxes per capita than others in the United States.

According to a Harvard University study, immigrant pay scales have lowered wages for the least educated — and the neediest — among the native-born, mostly African Americans and Latino immigrants.

The cost-benefit calculation is just as troubling when it comes to the immigrants themselves. The mothers I talked to were able to send money to their children in their home countries so the kids could eat better and go to school past the third grade.

But after spending years apart from their mothers, these children often felt abandoned, and they resented — even hated — their mothers for leaving them. Many mothers ultimately lose what is most important to them: the love of their children.

Will the proposals roiling Congress end the problems of illegal immigration? It's not likely.

"Get tough" sums up one side in the debate, but it's a policy that has had little success to date. Starting in 1993, the number of agents patrolling the border and the amount of money spent on enforcement tripled, according to a 2002 Public Policy Institute of California study. Yet the number of illegal immigrants in the United States only grew more quickly. Why? More immigrants came and more stayed for good, knowing that entry and re-entry would be more difficult and costly in the future.

The other, less draconian approach is to "control" immigration via temporary guest-worker programs and promises of future green cards — perhaps even citizenship.

Unfortunately, a past guest-worker program, in which Mexican braceros filled agricultural jobs between 1942 and 1964, laid the groundwork for the massive illegal migration of workers from Mexico that followed. And the last time the United States offered illegal immigrants a path to a green card, in 1986, it resulted in about 2.7 million immigrants becoming legal, but it didn't stem the tide of newcomers.

If you travel the routes that feed Latin Americans into the United States, you'll come to believe that there is only one way to stem illegal immigration — at its source, in Mexico, Guatemala, Honduras and wherever people are desperately poor. That's because desperate people find ways around obstacles such as walls and temporary guest-worker rules.

One woman I met at a migrant shelter in southern Mexico was Leti Isabela Mejia Yanes. She had left Honduras, where 42 percent of the population is unemployed or underemployed. Mejia Yanes, a single mother, left three children in Honduras because she could feed each of them only two pieces of bread a day. The youngest, a 1-year-old boy, got breast milk and one piece of bread daily. Sometimes she quieted their cries of hunger with a dollop of tortilla dough mixed into a big glass of water. She had lost both legs trying to board a moving freight train that would take her north through Mexico. Months later, she would return to Honduras defeated.

Time and again, I met migrants willing to endure months of danger and misery to reach the United States. As long as they had any hope of success, they refused to go home.

Instead of arguing about green card rules and wall heights, the United States should be formulating a new foreign policy. It should be aiming resources and diplomacy at improving conditions in Mexico and the few Central American countries whose migrants make up more than two-thirds of those in the United States illegally. Trade policies could give preference to goods from immigrant-sending countries to spur job growth. More aid could be invested there for the same purpose.