More immigrants from Mexico are leaving the United States than coming into the country, according to a report published Thursday by the Pew Research Center, a finding that indicates the end of the largest wave of immigration from a single country in American history.

The shift is the first time since immigration from Mexico began to rise in the 1970s that fewer Mexicans came into America than returned home, the Pew report found. The reversal is primarily the result of a steep drop in Mexicans coming into the country.

While the border debate, especially among the Republican candidates for the presidential nomination, has focused on extending walls and expanding other enforcement measures, the Pew report indicates that stepped-up border measures under the Obama administration have helped significantly in reducing illegal crossings by Mexicans.

“We know that crossings are definitely down and we also know it is much more difficult and costly to cross now than it used to be,” said Ana Gonzalez-Barrera, a Pew research associate who is the author of the report. The United States’ slow recovery after the 2008 recession, particularly in construction and other low-paying jobs where many Mexicans had worked, is also a factor, she said.