During the 2008 presidential campaign, approximately 20 percent of Americans believed that then-Senator Barack Obama was a Muslim despite news stories that attempted to debunk the rumor, according to a study in The Journal of Media and Religion.

The study analyzed data from a survey of more than 2,400 people taken at three points during the campaign to see if exposure to news media helped correct misperceptions about Obama's faith.

The findings are consistent with psychological studies on selective memory that show that people tend to discount facts that are inconsistent with their preexisting beliefs. People are also less likely to remember information that conflicts with their beliefs.

Because the survey asked the same questions of the same people in October, September and November of 2008, the study was able to explore how people's beliefs changed over time. The percentage of respondents who believed that Obama was Muslim stayed roughly the same over the study period, shifting from 20.2 percent in September to 19.7 percent in November.

More than 60 percent of people who believed Obama was Muslim in September maintained that belief in November. Ninety percent of those who answered that Obama was Christian maintained their belief, but the remaining 10 percent who answered correctly in September shifted to an incorrect answer in November.

Respondents who were younger, less educated, less politically interested, politically conservative, and believed in a literal interpretation of the Bible were more likely to be among those who shifted from a perception of Obama as Christian to a perception of Obama as Muslim. For these groups of people, he found, exposure to news media did nothing to improve their political knowledge.

"These are groups of people who are generally distrustful of the mainstream media," said Journalism Professor Barry Hollander, who authored the study. "So therefore journalists telling them that this is not true could actually have the opposite effect and make them more likely to believe the rumor."

"With most forms of political knowledge, media should theoretically make you more accurate,"  Hollander said. "In this case, media exposure had no effect. Ultimately, the message here is that people believe what they want to believe."

Hollander said that similar cases of selective memory are likely to become more common in the future.

"As the audience for mainstream news shrinks and as TV news becomes more partisan, I suspect we're going to see more cases like this," Hollander said. "I think this is one of those great examples that demonstrate that the best efforts of journalists sometimes fail."