By Jim Forsyth

SAN ANTONIO (Reuters) - Even as immigration remains a hot topic in the U.S. presidential campaign, the number of people emigrating from Mexico to the United States, legally and illegally, has dropped sharply in recent years, research published Wednesday shows.

Demographers at the University of Texas San Antonio and the University of New Hampshire say the number of immigrants coming from Mexico peaked in 2003, and has fallen by more than half since then.

Rogelio Saenz, dean of the College of Public Policy at UTSA and the study's lead author, said reasons include fewer construction jobs due to the 2007-2009 recession and a Mexico's growing economy and a falling birth rate.

A total of 819,000 people migrated from Mexico to the United States from 2008 to 2012, compared with 1.9 million from 2003 to 2007, for a drop of 57 percent, according to U.S. Census figures, which do not differentiate between legal and illegal immigration.

"Back in the 1960s and 1970s, the average Mexican woman was having about seven births. That created a very youthful population in Mexico, with 35 percent of the population a few decades ago less than 15 years of age," Saenz said.

Those demographics led to large numbers of young people who could not find jobs at home, but that has ended with the birth rate there now about the same as the U.S. rate, he said.

"There is no longer the excess labor force that Mexico had just a few decades ago," he said.

Mexican immigrants today are wealthier and more likely to arrive on special visa programs that often require them to make job-creating investments, Saenz said.

Many are coming not to find jobs in construction but to escape the crime and violence that continues to rage in Mexico. They are far more likely to be naturalized U.S. citizens coming back, English-speaking, better educated, older, and female, than in the past, Saenz said.

The research was published by the University of New Hampshire Casey School of Public Policy.

(Editing by Eric Walsh and Mohammad Zargham)