Pew has been tracking flows for about 15 years, said Ana Gonzalez-Barrera, a research associate who wrote the report. For its report, Pew analyzed government data from both countries.
"This is the first time that we have the actual evidence and numbers of people going back," she said.
About half of all adults in Mexico believe those who moved to the United States lead better lives, but 33 percent say life is neither better nor worse north of the border, up from 23 percent in 2007, Pew said.
Other reasons for the decreased inflow include the slow recovery of the U.S. economy after the recession and stricter enforcement of U.S. immigration laws at the border.