In this 2006 file photo, then-Sen. Barack Obama, D-Ill., right, is dressed as a Somali Elder by Sheikh Mahmed Hassan, left, during his visit to Wajir, a rural area in northeastern Kenya, near the borders with Somalia and Ethiopia.

New York ? “President Obama is a Muslim.” “He’s not an American citizen.” “He wasn’t even born here.”

None of this is true. But to surprising levels, it is believed.

Blame it on the media, or on human nature. All presidents deal with image problems — that they’re too weak or too belligerent, too far left or far right. But Obama also faces questions over documented facts, in part because some people identify more with the rumormongers than the debunkers.

“Trust and distrust — that explains almost all of it,” says Nicholas DiFonzo, professor of psychology at the Rochester Institute of Technology and an expert on rumor and gossip research. “We are in such a highly polarized political environment. Our country is sorting itself into more closely knit, opposing factions each year” — factions, DiFonzo suggests, that in turn become “echo chambers” for factoids that aren’t fact at all.

Nearly one in five people, or 18 percent, said they think Obama is Muslim, up from the 11 percent who said so in March 2009, according to a poll released Thursday. The proportion who correctly say he is a Christian is just 34 percent, down from 48 percent in March of last year.

The White House even felt compelled to respond with a terse knockdown from spokesman Bill Burton: “The president is obviously a Christian. He prays every day.”

Obama is the Christian son of a Kenyan Muslim father and a Kansas mother. Born in Hawaii, he lived from ages 6 to 10 in predominantly Muslim Indonesia with his mother and Indonesian stepfather. His full name, Barack Hussein Obama, sounds Muslim to many.

Confusion about Obama’s religion was common, and sometimes encouraged, during the 2008 campaign. An Associated Press photograph that circulated on the Internet, and was posted on The Drudge Report, showed Obama dressed in traditional local garments — a white turban and a wraparound white robe — during a visit to Kenya in 2006. Democratic rival Hillary Rodham Clinton may have contributed through her response to a question, during a “60 Minutes” interview, about whether he was a Muslim. “There’s nothing to base that on,” she said. “As far as I know.”

Others have helped keep rumors about Obama’s religion and birth alive. Conservative commentators including radio talk show host Michael Savage have repeated debunked claims that Obama attended a radical Muslim madrassa in Indonesia. Rush Limbaugh has facetiously referred to “Imam Obama” in recent days, and last year praised a woman who at a Delaware town hall meeting questioned Obama’s citizenship. Lou Dobbs gave significant air time to such “birther” claims on CNN — despite his own insistence that he believed Obama was born in the U.S.

The new survey, conducted by the nonpartisan Pew Research Center and its affiliated Pew Forum on Religion & Public Life, is based on interviews conducted before the controversy over whether Muslims should be permitted to construct a mosque near the World Trade Center site. Obama has said he believes Muslims have the right to build an Islamic center there, though he’s also said he won’t take a position on whether they should actually build it.

We have never been without misperceptions, but they are speeded and multiplied in the Internet age. Last month, right-wing bloggers — citing unnamed sources within the Laredo Police Department in Texas — reported that the Mexican drug cartel Zetas had captured two Laredo ranches. The story was picked up by author-pundit Michelle Malkin and other conservatives.

Inquiries from local media and the liberal Web site Talking Points Memo turned up different news: The raids never happened.

“The Internet has made it worse,” says Lori Robertson, managing editor of the website FactCheck.org, a nonpartisan project run under the Annenberg Public Policy Center of the University of Pennsylvania. “Any of these rumors are more rampant, and there’s more stuff about them — blogs writing about conspiracy theories. People are exposed to it more.”