By failing to control immigration the U.S. imports poverty and low skills, placing a long-term drag on the economy

Since 1970, America's largest source of immigrants has been Latin America, especially Mexico. More than half of these Latino immigrants lack a high school diploma.

Compare the U.S. experience with Canada's. More than half of all immigrants to Canada possess a university degree. Half of all Canada's Ph.D.s are foreign-born.

Why does America choose poorly educated immigrants? The short answer: America does not choose them. They choose themselves.

In the last decade, half of all the immigrants to the United States arrived illegally. Even many of the legal arrivals gained entry courtesy of relatives who originally slipped into the country against the law, then somehow regularized themselves.

By contrast, Canada (a country of 1/10 the U.S. population that takes proportionately many more immigrants than the United States) allows almost no illegal immigration.

The result: While immigration has enhanced the average skill level of the Canadian population, it has detracted from the average skill level of the U.S. population.

Many Americans carry in their minds a family memory of upward mobility, from great-grandpa stepping off the boat at Ellis Island to a present generation of professionals and technology workers. This story no longer holds true for the largest single U.S. immigrant group, Mexican-Americans.

Stephen Trejo and Jeffrey Groger studied the intergenerational progress of Mexican-American immigrants in their scholarly work, "Falling Behind or Moving Up?"

They discovered that third-generation Mexican-Americans were no more likely to finish high school than second-generation Mexican-Americans. Fourth-generation Mexican-Americans did no better than third.

If these results continue to hold, the low skills of yesterday's illegal immigrant will negatively shape the U.S. work force into the 22nd century.

The failure to enforce the immigration laws in the 1990s and 2000s means that the U.S. today has more poorly skilled workers, more poverty and more workers without health insurance than it would have generated by itself.

Arizona's new law against illegal immigration can do only so much to address the problem: