PHOENIX -- The number of illegal immigrants in Arizona is down -- much more sharply than the rest of the country.

New figures from the Pew Hispanic Center estimate there were 375,000 people in this state last year who either entered the country illegally or overstayed their legal visas. That compares with Pew's estimate 475,000 for 2008.

That 21 percent drop compares with a decline of just 4.3 percent nationwide.

The new figures released Wednesday are a sharp departure in the trends which Pew has been tracking since 1990: This is the first time the numbers have dropped..

Jeffrey Passel, senior demographer for the organization which studies Hispanic population and employment trends, said states with healthier economies seem to have been less affected. For example, the estimates of the "unauthorized'' population of Texas between 2007 and 2008 are, within margins of error, virtually unchanged.

But Passel said he cannot explain the sharp drop in Arizona. And he specifically said it is impossible to determine whether that is the effect of a law approved in 2007 which allows judges to punish employers found guilty of knowingly hiring undocumented workers.

Pew researchers said that the numbers, based on estimates from March 2008 Census Bureau numbers, don't reflect more recent developments like the legislative debate and gubernatorial approval of SB 1070. That law includes several provisions designed to let state and local police arrest illegal immigrants. Several of the provisions have been put on hold by a federal judge. But the judge allowed other sections to take effect on July 29 including a new law aimed at day laborers and a prohibition against public officials from limiting their police officers from enforcing federal immigration laws.

Passel said it may be difficult to separate out the effects of the economy and things like new state laws or even stepped-up border enforcement.

"As somebody who's been doing estimates here for a long time, I have observed that the debates about illegal immigration get more heated when the economy is in trouble,'' he said.

There is no official count of illegal immigrants.

While the Census Bureau, in both its decennial count and its annual surveys, asks things like place of birth, the agency specifically does not inquire about legal status. But Passel said there are ways to extrapolate out some numbers.

He said federal agencies have numbers of people who are legally admitted to this country. That, Passel said, can be compared to the number who say they were born outside the United States.

Then, based on various estimates of undercounts, further adjustments are made. Passel said that adds anywhere from 10 to 15 percent to the totals.

He said the computations can be verified somewhat by looking at census surveys done by the Mexican government which can track people returning to the country.

So what happened to those migrants?

Within the country, some likely followed the jobs.

"They may well be going to different parts of the country,'' Passel said. He said that could be why Texas, where the economy is better, is not showing the same declines as other states.

On a nationwide basis, Passel stressed this is not a "static population.''

He said while there always have been illegal immigrants coming into this country, there also are people who leave. Others already here gain legal status. And some smaller percentage simply die.

Within the new report there are some identifiable trends.

Passel said most of the decline in new migrants was from Latin American countries other than Mexico. During the last two years, he said migration from those countries has dropped by 22 percent.

Mexican migrants represent about 60 percent of the total nationwide, a number that is likely larger in border states like Arizona.

"It's a very young population,'' Passel said. He estimated that 58 percent of illegal immigrants are between 18 and 39; for the U.S.-born population, that figure is just 28 percent.

And men outnumber women by a 3-2 margin.