Net migration from Mexico to the United States has come to a statistical standstill, stalling one of the most significant demographic trends of the last four decades.

Amid an economic downturn and increased enforcement at the U.S.-Mexico border, the number of Mexicans coming to the United States dropped significantly, while the number of those returning home increased sharply over the last several years, according to a report by the Pew Hispanic Center.

“The largest wave of immigration in history from a single country to the United States has come to a standstill,” the report says.

Between 2005 and 2010, 1.4 million Mexicans immigrated to the United States, less than half the number that migrated between 1995 and 2000. At the same time, the number of Mexicans and their children who moved to Mexico in the same five-year period rose to 1.4 million, about double the number that did so between 1995 and 2000.

The estimates are based on data from the U.S. Census Bureau and on Mexican census data. The most recent data indicate that the historic flow of migrants into the U.S. might even have started to reverse.

“We’re fairly confident that by the end of the period we were seeing more people moving to Mexico than leaving” for the United States, said co-author Jeffrey S. Passel, a senior demographer at the Pew Hispanic Center.

Illegal immigrants from Mexico residing in the U.S. still outnumber their legal counterparts, but the proportion has shifted, the report found. The number of Mexican illegal immigrants residing in the U.S. fell from 7 million in 2007 to about 6.1 million in 2011. In the same period, the number of legal immigrants from Mexico residing here increased from 5.6 million to 5.8 million.

The report attributes the changes to several factors, including the weakened economy, increased border enforcement, a rise in deportations, growing dangers at the border and a long-term decline in Mexican birthrates. Which factor is dominant is still unknown.

“At this point, it’s very hard to say because all of them are kind of working in the same direction,” Passel said.

The report comes as the Supreme Court on Wednesday is set to take up elements of Arizona’s tough crackdown on illegal immigrants, known as SB 1070. Advocates for reduced immigration cited the report as proof that tightened enforcement works. Many immigrant advocates have long cited job prospects as a primary factor in decisions to migrate.

Several factors in Mexico contributed to the shift, including a drop in fertility rates there, according to the report’s authors. That rate dropped from 7.3 children per woman in 1960 to 2.4 in 2009, meaning there are fewer people entering the workforce. At the same time, indicators of development such as literacy, average years of education and healthcare have improved.

The Mexican demographic trends and the U.S. political shift toward increased enforcement mean that immigration from Mexico is probably down for the long run, said David Fitzgerald, a sociology professor with the Center for Comparative Immigration Studies at UC San Diego.

Even if the U.S. economy bounces back, he said, there’s strong reason to believe that illegal immigration from Mexico will never surge the way it did about 10 years ago, he said.

The Center for Comparative Immigration Studies does an annual survey of three communities in Mexico that are routine sources of immigrants to the U.S. All three show decreases in immigration, he said.

“This report is showing at the binational level what we’ve seen at the community level in the sense that new, unauthorized immigration to the U.S. has dropped off dramatically,” Fitzgerald said. “I think by far the most important factor is the downturn in the U.S. job market, and the second factor is the concentrated border enforcement activity.”

Fitzgerald said it’s too early to say whether tough state laws have caused illegal immigrants to return to Mexico, although there is evidence that Arizona’s law caused some people to leave that state, he said.

Advocates against illegal immigration said the report confirms that stricter enforcement combined with tougher job prospects work to curtail illegal immigration.

“If you dry up the job magnet because of the bad economy or increased work-site enforcement, you reduce illegal immigration,” said Bob Dane, a spokesman for the Federation for American Immigration Reform, which advocates for restricted immigration.

The federal government needs to continue to crack down on illegal immigration, regardless of the report’s conclusions, Dane said.

“The fact remains there’s still 7 million illegal aliens occupying jobs that should go to American citizens,” he said. “It’s nowhere near mission accomplished.”

Jessica Dominguez, a Los Angeles-based immigration attorney, said increasing numbers of deportees who have families in the U.S. are not returning because the trip is more dangerous and the consequences of being caught are more severe.

“Some people are staying away until they can come back legally,” Dominguez said. “They don’t want to be exposed to being detained again and losing the opportunity to come back legally later. The laws are very strict at this point.”

About 29% of the immigrant population is from Mexico, far surpassing the next largest group, Indians, who comprise about 4.5% of the 40-million immigrants living in the U.S., according to the report. The number of Mexicans who came to the U.S. over the last several decades surpasses that of any other nationality in U.S. history, but when measured as a share of the U.S. immigrant population at the time, the Irish and German influx in the 19th century equaled or exceeded the Mexican migration, the report noted.

paloma.esquivel@latimes.com

hector.becerra@latimes.com