In a dramatic shift, the number of Mexican immigrants living illegally in the US has dropped significantly for the first time in decades. Many illegal workers, already in the US and seeing few job opportunities, are returning to Mexico.

An analysis of census data from the US and Mexican governments details movements to and from Mexico, a nation accounting for nearly 60 percent of the illegal immigrants in the US. It comes amid renewed debate over US immigration policy as the Supreme Court hears arguments this week on tough immigration law in Arizona, a border state.

Roughly 6.1 million unauthorized Mexican immigrants were living in the US last year, down from a peak of nearly 7 million in 2007, according to the Pew Hispanic Center study released on Monday. It was the biggest sustained drop in modern history, believed to be surpassed in scale only by losses in the Mexican-born US population during the Great Depression.

Much of the drop in illegal immigrants is due to the persistently weak US economy, which lost construction and service-sector jobs, attractive to Mexican workers, following the housing bust.

But increased deportations, heightened US patrols and violence along the border also have played a role, as well as demographic changes, such as Mexico's declining birthrate.

In all, the Mexican-born population in the US last year - legal and illegal - fell to 12 million, marking an end to an immigration boom dating back to the 1970s, when foreign-born residents from Mexico stood at 760,000. The 2007 peak was 12.6 million.

Christian Ballesteros, who has been at a shelter for immigrants in Matamoros, Mexico, across the border from Brownsville, Texas, pointed to stiffer US penalties for repeat offenders and brutal criminal groups that control the Mexican side of the border as reasons for the immigration decline. Ballesteros, who has been deported four times, was recently caught after hopping the border fence near Nogales, Arizona.

"The Mexican cartels are taking over, are actually being like the border patrols on this side," Ballesteros said. "They say: 'If you don't pay, we're going to cut your head off.' That's the worst part."

After his last apprehension by US authorities, Ballesteros was sent to a detention facility in Las Vegas for two months. He fears it could be six months, if he is caught again.

"You can lose money, but if you lose time, there's no way you can recover that time," Ballesteros said, noting that many immigrants have families to support.

Jeffrey Passel, a senior demographer at Pew who co-wrote the analysis, said Mexican immigration may never return to its height during the mid-decade housing and construction boom, even with the US economy recovering. He cited longer-term factors such as a shrinking Mexican workforce.

He noted that government statistics show a clear shift among Mexican workers already in the US who are returning home. He said the numbers are a sign that many immigrants are giving up on life in the US.

In a dramatic shift, the number of Mexican immigrants living illegally in the US has dropped significantly for the first time in decades. Many illegal workers, already in the US and seeing few job opportunities, are returning to Mexico.

An analysis of census data from the US and Mexican governments details movements to and from Mexico, a nation accounting for nearly 60 percent of the illegal immigrants in the US. It comes amid renewed debate over US immigration policy as the Supreme Court hears arguments this week on tough immigration law in Arizona, a border state.

Roughly 6.1 million unauthorized Mexican immigrants were living in the US last year, down from a peak of nearly 7 million in 2007, according to the Pew Hispanic Center study released on Monday. It was the biggest sustained drop in modern history, believed to be surpassed in scale only by losses in the Mexican-born US population during the Great Depression.

Much of the drop in illegal immigrants is due to the persistently weak US economy, which lost construction and service-sector jobs, attractive to Mexican workers, following the housing bust.

But increased deportations, heightened US patrols and violence along the border also have played a role, as well as demographic changes, such as Mexico's declining birthrate.

In all, the Mexican-born population in the US last year - legal and illegal - fell to 12 million, marking an end to an immigration boom dating back to the 1970s, when foreign-born residents from Mexico stood at 760,000. The 2007 peak was 12.6 million.

Christian Ballesteros, who has been at a shelter for immigrants in Matamoros, Mexico, across the border from Brownsville, Texas, pointed to stiffer US penalties for repeat offenders and brutal criminal groups that control the Mexican side of the border as reasons for the immigration decline. Ballesteros, who has been deported four times, was recently caught after hopping the border fence near Nogales, Arizona.

"The Mexican cartels are taking over, are actually being like the border patrols on this side," Ballesteros said. "They say: 'If you don't pay, we're going to cut your head off.' That's the worst part."

After his last apprehension by US authorities, Ballesteros was sent to a detention facility in Las Vegas for two months. He fears it could be six months, if he is caught again.

"You can lose money, but if you lose time, there's no way you can recover that time," Ballesteros said, noting that many immigrants have families to support.

Jeffrey Passel, a senior demographer at Pew who co-wrote the analysis, said Mexican immigration may never return to its height during the mid-decade housing and construction boom, even with the US economy recovering. He cited longer-term factors such as a shrinking Mexican workforce.

He noted that government statistics show a clear shift among Mexican workers already in the US who are returning home. He said the numbers are a sign that many immigrants are giving up on life in the US.