MEXICO CITY (AP) -- The number of migrants crossing illegally into the United States from Mexico appears to have risen some in the first half of 2012, while the number of migrants returning to Mexico decreased, a report by U.S. and Mexican researchers said Tuesday.

It was the first time the net outflow of migrants from Mexico has increased since the 2007 economic slump caused a sharp drop in both migration and the amount of money sent home by Mexicans working in the U.S. as migrants found it harder to find work north of the border.

The report by Mexico's Colegio de la Frontera Norte and the University of Southern California's Tomas Rivera Policy Institute said the number of Mexican-born people in the United States seemed to have stabilized at around 11.7 million and might grow slightly by year's end. The number included Mexicans who migrated legally and those who crossed over illegally.

"The recession-induced decline of undocumented migration from Mexico appears to have stopped in the first half of 2012 amid tentative signs of a renewed northbound flow," the study said.

The report is based on surveys done at Mexican border crossings, bus stations and airports and on U.S. deportation, repatriation and demographics data. It says heightened U.S. enforcement of immigration laws and state initiatives like one enacted in Arizona didn't appear to have persuaded illegal migrants already in the United States to leave.

"Despite evidence of growing psychological effects on the migrants who are removed, the available data suggest that these efforts have failed to have substantial, ongoing effects on the size of the Mexican migrant population," the report said. "Neither the border survey nor the other indicators examined here offers any evidence that those efforts have had any effect on the number of Mexican migrants leaving the country. On the contrary, fewer Mexican migrants have left the United States since those enforcement efforts went into effect."

The report added that "the available data for migration trends in 2012 suggest that the size of that (U.S. Mexican migrant) population might show a small increase across the entire year unless the U.S. economy flattens or declines in the third and fourth quarters."

Rodolfo Garcia Zamora, an immigration expert at the state university in Zacatecas, a Mexican state that is home to many migrants to the U.S., expressed caution about the report, which he was not involved in.

Garcia Zamora feels it is too early to say illegal migration is rebounding, even with a slight uptick in the number of migrants heading north. He said considerable evidence suggests many migrants continue to return to Mexico voluntarily in the face of difficult economic conditions in the United States.

"The evidence we have is that the flow of undocumented migrants to the United States continues stagnant and blocked, and that the number of migrants returning from the United States continues to increase," Garcia Zamora said.

He said about 1,000 families had returned to Zacatecas so far this year. He said returning migrants inform their fellow townspeople about difficult conditions north of the border, thus discouraging them from making the trip.

The report, "The Mexican Migration Monitor," said the prospect for getting a job remains the determining factor for would-be migrants. "Economic conditions in the United States have been, and remain, the primary determinants of the size of Mexican migration flows," it said.

Garcia Zamora said some evidence suggests there may have been "a very slight, very temporary rebound" in migration in the first half of 2012, "because of a rebound in some specific industries where Mexicans work" in the United States, like food service and janitorial service.

The report also cited an increase in the percentage of migrants crossing illegally into the U.S. In 2006, at the height of the migration boom, eight of 10 would-be Mexican migrants sought to enter the U.S. illegally. That dropped to less than half following the 2007 downturn, but now about 60 percent of migrants are crossing illegally, the report said.

Mexico's most recent national census, in 2010, said migration had fallen to about one-third of its peak level of about 450,000 Mexicans who left each year from 2000 through 2005.

Garcia Zamora said the U.S. no longer serves "an escape valve for our country, as it has for the last 50 years." The result, he said, is that Mexico will probably now see increased internal migration, with people moving from farm states to Mexico's industrial cities.