By Jose Serrano (staff@latinospost.com) | First Posted: Jul 24, 2015 06:44 PM EDT

An extensive decline in Mexicans migrating to the United States is being attributed to stronger immigration enforcement efforts and the stagnant U.S. job market.

Demographers with the University of New Hampshire and the University of Texas at San Antonio released a report Wednesday citing a 57 percent dip in immigrants - legal and undocumented - crossing the border between 2003 and 2012, including a four-year span when less than a million people traveled to and fro.

"Possible factors include a declining Mexican fertility rate, heightened enforcement of border security, and enhanced detentions and deportations of unauthorized migrants in the United States," said Rogelio Saenz, dean of the College of Public Policy at UTSA and the study's lead author.

Other contributing factors were the sluggish U.S. economy when compared to Mexico's and a declining U.S. construction industry

The study, entitled "A Transformation of Mexican Migration to the United States," suggests Mexican and Central American migrants aren't simply looking for jobs anymore. They're escaping violence associated with gangs and drug cartels.

"The disparate impact of violence in Mexico may also play a role: Though the drug wars have displaced and encouraged the migration of upper- and middle-class professionals and entrepreneurs, violence along the border has intimidated migrants with more limited socioeconomic resources from clandestinely crossing into the United States," Saenz said.

Saenz's findings come as immigration issues return to the forefront of potential voters' minds. Last month, Donald Trump called Mexicans criminals and rapists, prompting numerous people and companies to distance themselves from the Republican presidential hopeful. While GOP candidates didn't condone his behavior, their silence on Trump's incendiary comments alienated a Latino voter base they're trying to win.

A majority of U.S. citizens share Trump's sentiment, to a degree. According to a recent Gallup poll, 70 percent of Americans believe immigration is the country's most pressing issue; many cited it as the fourth-most-frequently mentioned problem, behind race relations, government, and the economy.

Saenz said he isn't sure if the ease in traffic represents a permanent migration change or just a temporary response to conditions in the U.S.

"It is still not clear whether the more favorable socioeconomic standing of the most recent cohort of Mexican migrants will persist into the future and, if so, whether it will change the way Mexican migrants are commonly viewed in the United States," he said.