Editor's note: David Frum writes a weekly column for CNN.com. A special assistant to President Bush in 2001-2, he is the author of six books, including "Comeback: Conservatism That Can Win Again" and the editor of FrumForum.

Washington (CNN) -- When Arizona police ask suspected illegal immigrants for IDs, they are protecting your grandchildren's economic future.

Three years ago, ETS -- the people who administer the SAT -- released an alarming study. It combined information on test scores with demographic trends to predict that the U.S. work force of 2030 would be less literate, less skilled and worse paid than the U.S. work force of 1990.

ETS reported: "[B]y 2030 the average levels of literacy and numeracy in the working-age population will have decreased by about 5 percent while inequality will have increased by about 7 percent. Put crudely, over the next 25 years or so, as better-educated individuals leave the work force they will be replaced by those who, on average, have lower levels of education and skill. Over this same period, nearly half of the projected job growth will be concentrated in occupations associated with higher education and skill levels. This means that tens of millions more of our students and adults will be less able to qualify for higher-paying jobs."

Why?

One word: Immigration.

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:

•The law does not reach into the workplace, which is where enforcement will have most effect.

• It is not sufficiently neutral: Much better to check the work eligibility of all job applicants than to screen suspicious people at traffic stops.

• It is easily evaded by crossing into another state for work.

Yet there is good reason to expect that effective reduction of illegal work opportunities will slow the illegal immigration flow. The Center for Immigration Studies estimates the recession has induced a decline of 1.7 million in the illegal population, as some illegals have opted to return home and potential illegals have decided against trying to enter. Effective enforcement could have further effect.

At a minimum, it seems irresponsible for political leaders in Washington to speculate about amnesty before enforcement has had its full effect. Amnesty talk invites more illegal immigration, by raising hopes that those who can slip into the country in the next months will be allowed to stay.

Arizona's law offsets that loose talk. The law may expose Arizona's leaders to criticism. But it is at least a beginning contribution to a stronger American work force and a wealthier American future.

The opinions expressed in this commentary are solely those of David Frum.