The May 3 update includes the full methodology appendix and a statistical profile of Mexican immigrants in the United States.

The largest wave of immigration in history from a single country to the United States has come to a standstill. After four decades that brought 12 million current immigrants—most of whom came illegally—the net migration flow from Mexico to the United States has stopped and may have reversed, according to a new analysis of government data from both countries by the Pew Hispanic Center, a project of the Pew Research Center.

The standstill appears to be the result of many factors, including the weakened U.S. job and housing construction markets, heightened border enforcement, a rise in deportations, the growing dangers associated with illegal border crossings, the long-term decline in Mexico’s birth rates and broader economic conditions in Mexico.

It is possible that the Mexican immigration wave will resume as the U.S. economy recovers. Even if it doesn’t, it has already secured a place in the record books. The U.S. today has more immigrants from Mexico alone—12.0 million—than any other country in the world has from all countries of the world. Some 30% of all current U.S. immigrants were born in Mexico. The next largest sending country—China (including Hong Kong and Taiwan)—accounts for just 5% of the nation’s current stock of about 40 million immigrants.

Looking back over the entire span of U.S. history, no country has ever seen as many of its people immigrate to this country as Mexico has in the past four decades. However, when measured not in absolute numbers but as a share of the immigrant population at the time, immigration waves from Germany and Ireland in the late 19th century equaled or exceeded the modern wave from Mexico.

Beyond its size, the most distinctive feature of the modern Mexican wave has been the unprecedented share of immigrants who have come to the U.S. illegally. Just over half (51%) of all current Mexican immigrants are unauthorized, and some 58% of the estimated 11.2 million unauthorized immigrants in the U.S. are Mexican (Passel and Cohn, 2011).

The sharp downward trend in net migration from Mexico began about five years ago and has led to the first significant decrease in at least two decades in the unauthorized Mexican population. As of 2011, some 6.1 million unauthorized Mexican immigrants were living in the U.S., down from a peak of nearly 7 million in 2007, according to Pew Hispanic Center estimates based on data from the U.S. Census Bureau. Over the same period, the population of authorized immigrants from Mexico rose modestly, from 5.6 million in 2007 to 5.8 million in 2011.

The net standstill in Mexican-U.S. migration flows is the result of two opposite trend lines that have converged in recent years. During the five-year period from 2005 to 2010, a total of 1.4 million Mexicans immigrated to the United States, down by more than half from the 3 million who had done so in the five-year period of 1995 to 2000. Meantime, the number of Mexicans and their children who moved from the U.S. to Mexico between 2005 and 2010 rose to 1.4 million, roughly double the number who had done so in the five-year period a decade before. While it is not possible to say so with certainty, the trend lines within this latest five-year period suggest that return flow to Mexico probably exceeded the inflow from Mexico during the past year or two.

Of the 1.4 million people who migrated from the U.S. to Mexico since 2005, including about 300,000 U.S.-born children, most did so voluntarily, but a significant minority were deported and remained in Mexico. Firm data on this phenomenon are sketchy, but Pew Hispanic Center estimates based on government data from both countries suggest that 5% to 35% of these returnees may not have moved voluntarily.

In contrast to the decrease of the Mexican born, the U.S. immigrant population from all countries has continued to grow and numbered 39.6 million in 2011, according to the Census Bureau’s Current Population Survey.

In addition, the number of Mexican-Americans in the U.S.—both immigrants and U.S.-born residents of Mexican ancestry—is continuing to rise. The Mexican-American population numbered 33 million in 2010. As reported previously (Pew Hispanic Center, 2011), between 2000 and 2010 births surpassed immigration as the main reason for growth of the Mexican-American population.

The population of Mexican-born residents of the U.S. is larger than the population of most countries or states. Among Mexican-born people worldwide, one-in-ten lives in the United States.

This report has five additional sections. The next section analyzes statistics on migration between Mexico and the United States from data sources in both countries. The third uses mainly Mexican data to examine characteristics, experience and future intentions of Mexican migrants handed over to Mexican authorities by U.S. law enforcement agencies. The fourth, based on U.S. data, examines trends in border enforcement statistics. The fifth looks at changing conditions in Mexico that might affect migration trends. The report’s last section looks at characteristics of Mexican-born immigrants in the U.S., using U.S. Census Bureau data. The appendix explains the report’s methodology and data sources.

Among the report’s other main findings from these sections:

Changing Patterns of Border Enforcement

In spite of (and perhaps because of) increases in the number of U.S. Border Patrol agents, apprehensions of Mexicans trying to cross the border illegally have plummeted in recent years—from more than 1 million in 2005 to 286,000 in 2011—a likely indication that fewer unauthorized migrants are trying to cross. Border Patrol apprehensions of all unauthorized immigrants are now at their lowest level since 1971.

As apprehensions at the border have declined, deportations of unauthorized Mexican immigrants–some of them picked up at work sites or after being arrested for other criminal violations–have risen to record levels. In 2010, 282,000 unauthorized Mexican immigrants were repatriated by U.S. authorities, via deportation or the expedited removal process.

Changing Characteristics of Return Migrants

Although most unauthorized Mexican immigrants sent home by U.S. authorities say they plan to try to return, a growing share say they will not try to come back to the U.S. According to a survey by Mexican authorities of repatriated immigrants, 20% of labor migrants in 2010 said they would not return, compared with just 7% in 2005.

A growing share of unauthorized Mexican immigrants sent home by U.S. authorities had been in the United States for a year or more—27% in 2010, up from 6% in 2005. Also, 17% were apprehended at work or at home in 2010, compared with just 3% in 2005.

Demographic Trends Related to Mexican Migration

In Mexico, among the wide array of trends with potential impact on the decision to emigrate, the most significant demographic change is falling fertility: As of 2009, a typical Mexican woman was projected to have an average 2.4 children in her lifetime, compared with 7.3 for her 1960 counterpart.

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.