( CNN ) -- After nearly 40 years of recorded increases , the number of immigrants living in the United States remained flat between 2007 and 2008 , recent statistics released by the U.S. Census Bureau show . The number of naturalized citizens in the U.S. increased , partly attributed to voter drives for the 2008 election . According to the Census Bureau 's American Community Survey , the U.S. foreign-born population represented about 12.5 percent of the population in 2008 , down from 12.6 percent in 2007 . Taking into account the margin of error , it was possible that the immigrant population remained even . `` Between '07 and '08 there really was n't that much of a change , '' said Elizabeth Grieco , chief of immigration statistics staff at the Census Bureau . But given the steep upward trend in the foreign-born population since 1970 , no change is big news . The American Community Survey collects data from about 3 million addresses each year , and provides one of the most complete pictures of the population , according to the bureau . The survey does n't give a reason for the leveling off , but experts pointed to the economic downturn and the resulting high unemployment as factors behind the shift . `` The recession has had a significant effect on immigrants ' decisions on whether to come to the U.S. , '' said Michelle Mittelstadt , director of communications at the nonpartisan Migration Policy Institute . Would-be unauthorized immigrants and legal temporary workers are mostly the ones who have decided to stay put in their home countries for now , Mittelstadt said . The largest declines in the foreign-born population were in states that were hardest hit by the recession , including California , Florida and Arizona . Mittelstadt noted , however , that those immigrants already in the United States appear to be staying . A recent study by the Pew Hispanic Center concluded that emigration from Mexico , the largest source of immigrants to the United States , slowed at least 40 percent between mid-decade and 2008 , based on national population surveys in the United States and Mexico , as well as Border Patrol apprehension figures . The Mexican-born population in the United States dropped by about 300,000 between 2007 and 2008 , according to census data . The new Census statistics show that for the first time since the American Community Survey was fully implemented in 2005 , the number of noncitizens decreased , Grieco said . There were about 21.6 million noncitizens in 2008 , down from 21.9 million in 2007 . The label noncitizens includes both legal residents and illegal immigrants . Along with the decline in the noncitizen population , however , there was a notable increase in the number of naturalized citizens , Grieco said . The number of individuals who are naturalized citizens increased to 43 percent of the foreign-born population in 2008 from 42.5 percent in 2007 . The Census survey matches reports from the Department of Homeland Security on the rise of naturalization applications . `` Naturalizations grew at a record pace between 2006 and 2008 , with a total of 2.4 million immigrants becoming new citizens in the United States , '' according to a DHS statement . A significant fee increase imposed in 2007 for naturalization applications and an awareness of citizenship brought on during voter registration drives for the 2008 election help explain the increase , Mittelstadt said .
Bureau official : `` Between '07 and '08 there really was n't '' much immigration change . Notable increase in naturalized citizens , Grieco Michelle Mittelstadt says . Mexican -born population in Florida dropped by about 300,000 '08 .