In what the Pew Hispanic Center called a "notable reversal of the historic pattern," the number of Mexicans leaving the United States increased sharply in the five years after 2005, while the new flow of migrants coming from Mexico fell steeply.

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Mexican immigration to the United States, the largest wave of migrants from a single country in the nation’s history, has slowed to a halt after four decades of surging growth and may be declining, according to a report released Monday by the Pew Hispanic Center.

In what the report called a “notable reversal of the historic pattern,” the number of Mexicans leaving the United States increased sharply in the five years after 2005, while the new flow of migrants coming from Mexico fell steeply.

For the first time in at least two decades, the population of illegal immigrants from Mexico living in this country was significantly decreased, according to the report. In 2011, about 6.1 million Mexicans were living here illegally, down more than 12 percent from a peak of nearly 7 million in 2007, it said.

“We really haven’t seen anything like this in the last 30 or 40 years,” said Jeffrey Passel, senior demographer at the Pew Hispanic Center, who co-wrote the report with D’Vera Cohn and Ana Gonzalez-Barrera.

The center is a nonpartisan research organization that does not advocate for policy positions.

Overall, the report said, about 58 percent of an estimated 11.2 million illegal immigrants in the country are from Mexico.

The report provides material for all sides in the fierce debate over immigration policy. A major episode in that debate is scheduled Wednesday when the Supreme Court hears arguments over an Arizona law that expands powers of local and state police to enforce immigration laws.

Arizona officials say tough measures like the law they passed, known as SB 1070, are the most effective way to curb illegal immigration because they pressure migrants who lack legal status to return home. Immigrant advocates say those measures separate families or drive them into the shadows, and they note prolonged high U.S. unemployment as the primary reason for reduced migrant flows.

Passel said the data does not allow Pew demographers to say which factor was most important in reducing the population of illegal immigrants. The report cited unemployment, particularly in construction; heightened border enforcement and increased deportations by U.S. authorities; and a long-term decline in birthrates in Mexico.

The report also said the migration from Mexico that began after 1970 brought by far the largest numbers from one country in U.S. history, with about 30 percent of all immigrants today born in Mexico. The 12 million Mexican-born people who live in the United States — about one in every 10 Mexicans in the world — comprise more than all the immigrants in any other country, the Pew report said.

The report presents a striking change from earlier findings by the Pew center and other demographers on the number of Mexicans who have been returning to their country. While earlier studies said the data was unclear, the Pew report includes new data from the 2010 Mexican census revealing that about twice as many Mexicans returned home from 2005 to 2010 than in the previous five years. In all, about 1.4 million people moved from the United States in that period, the Mexican census showed.

While most of the returning migrants left the United States on their own, the new data registered for the first time a significant increase in the number of Mexicans who were deported. The Pew Hispanic Center also cited Mexican data showing a huge increase in the number of U.S.-citizen children who were living in Mexico with their parents — to about 500,000 in 2010 from about 240,000 in 2000. Some of those Americans are the children of deportees, the data suggested.

The report ends on a note of concern about the possible difficulties for Mexican-born people in this country, regardless of their immigration status.

“Compared with other immigrants to the U.S., Mexican-born immigrants are younger, poorer, less-educated, less likely to be fluent in English and less likely to be naturalized citizens,” it concluded.