The Mexican population in the United States grew dramatically during the 2000s due largely to births and less to immigration, according to a new study by the Pew Hispanic Center.

Between 2000 and 2010, the Mexican-American population grew by 7.2 million due to births and by 4.2 million due to immigration, according to the Pew Hispanic Center. The research group examined census data from the United States and Mexico.

The trend was a reversal from the 1990s and 1980s when immigration from Mexico either matched or exceeded Mexican-American births, according to the study released last week.

Overall, people of Mexican origin make up about two-thirds of the nation's 50 million Latinos. Of the 31.8 million Mexicans in the country, 12.4 million or 39 percent are immigrants.

More than half of these immigrants, about 6.5 million, are living in the country without permission, according to the study.

The slowdown in immigration from Mexico in the 2000s was largely the result of economic forces on both sides of the border, while the surge in births had to do with more Mexican women of child-bearing age living in the country, according to the study.

More Mexicans may have stayed in their native country because of its improving economy and stayed away from the United States because of its struggling economy as well as tougher border security.

"All three are contributing to the declining trend," said Mark Lopez, one of the report's authors.

"It's hard to tell which is more important," he said. "Is it the fact that there are fewer job opportunities in the U.S. or is it because it's more difficult to cross the border, the drug wars, greater enforcement? Or is it because Mexico's economy has been growing more quickly than the U.S. economy?"

According to the study, the number of Mexicans annually leaving Mexico for the United States declined from more than one million in 2006 to 404,000 last year, a 60 percent reduction.

"As a result there were fewer new immigrant arrivals to the U.S. from Mexico in the 2000s (4.2 million), than in the 1990s (4.7 million)," the report said.

And yet, it added: "The Mexican-American population continued to grow rapidly with births accounting for 63 percent of the 11.2 million increase from 2000 to 2010."

According to the report, Mexican women are more likely to be in their child-bearing years compared to other women in the United States. The median age of Mexicans in the country is 25 compared to 41 for whites, according to the report.

Also, a Mexican woman in her early 40s is more likely to have given birth to two or more children than a woman of similar age from another ethnic or racial group in the United States, according to the study.

The report did not discuss how the findings would affect the ongoing debate in the country whether to deny automatic U.S. citizenship to the children of undocumented immigrants.

However, it noted that Mexican immigrant women on the average had more children than U.S.-born women of Mexican descent but about the same number as women living in Mexico.

The report said births will continue to fuel future growth of the Latino population. Latinos make up 16.3 percent of the nation's population and their share will rise to 29 percent by the middle of this century.

Lopez said more U.S.-born Latinos will prove beneficial for the country and its economy.

"These are all U.S. citizens," Lopez said. "When they come of age, they'll be able to enter the work force and play a role as workers and consumers."

Latinos have not yet translated their sheer numbers into political clout because too many are still under 18 and not eligible to vote. But that will change, he said, as more Latinos reach adulthood each year for decades to come.

Leonel.sanchez@sandiegored.com