A Washington-based research organization says the number of illegal aliens in the United States has tripled over the last 20 years and currently accounts for nearly five percent of the U.S. work force. The Pew Hispanic Center released a report Tuesday on the size and composition of what it terms America's "unauthorized migrant population."

The Pew Hispanic Center says some 12 million undocumented people live in the United States, up from four million in 1986. Senior research associate Jeffrey Passel says, despite efforts to limit illegal immigration, the size of America's undocumented population continues to grow.

"The rate of growth appears to be continuing at about the same level, we seem to be adding about half a million a year," he said.

An estimated 40 percent of undocumented migrants have been in the country for five years or less. Latin Americans constitute 78 percent of the undocumented population, with 56 percent coming from Mexico.

About 49 percent are undocumented males, with women and children comprising the other 51 percent. Passel says these figures point to a reality about the undocumented population that runs counter to a perception held by many in the United States.

"The picture of this population is that of young working families, in contrast to the usual stereotype of the adult man who is here by himself," he explained.  "And half of the adult men have no spouse or children here. But they represent only about a quarter of all the unauthorized migrants."

The Pew Hispanic Center reports that illegal immigration to the United States temporarily dipped in 2002 and 2003. One might note that those years followed the September 11, 2001 terrorist attacks, and assume that the episode made the United States less attractive as a destination for undocumented workers. But Passel has a different theory: 2002 and 2003 saw harsh economic conditions in the United States, with the country only beginning to emerge from a recession.

"When unemployment went up, unauthorized migration from Mexico went down," he noted.  "When unemployment went down, unauthorized migration from Mexico went up. This suggested to us that it was really the availability of jobs rather than the security issues that caused the downturn [in migration from 2002-2003]."

In its report, the Pew Hispanic Center opted for the neutral-sounding term "unauthorized migrants" rather than "illegal aliens" to describe foreigners who have entered the United States without permission. Researchers note that the center is not an advocacy group, and takes no position on the many emotionally charged, politically polarizing issues surrounding the immigration debate in the United States.

President Bush has added his voice to a growing chorus calling for immigration reform. In January, he urged Congress to enact legislation that would strengthen U.S. border enforcement while establishing a guest worker program and providing undocumented workers a legal means to remain in the United States. Critics, including some within his own Republican party, say the president's proposal would reward those who broke U.S. law to enter the country and, in so doing, encourage others to attempt to cross the border illegally.