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 too far right. But Obama also faces questions over documented facts.

"Trust and distrust -- that explains almost all of it," said 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 1 in 5 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."

Confusion about Obama's religion was common, and sometimes encouraged, during the 2008 campaign. An Associated Press photograph that circulated on the Internet 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 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."

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 World Trade Center site mosque controversy.

"The Internet has made it worse," said 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."