Why have so many fallen for nonsense about Obama's health care plan? About his birth? About ... ?

In the online edition of Newsweek, columnist Sharon Begley has devoted two articles to try to get to the bottom of the question. She cites fresh research by sociologist Steven Hoffman that a mental phenomenon known as"motivated reasoning" is to blame.

Begley:

His conclusions arise from a study he and six colleagues conducted. They were looking at the well-known phenomenon of Americans believing that Saddam Hussein was involved in the 9/11 attacks. Some people, mostly liberals, have blamed that on false information and innuendo spread by the Bush administration and its GOP allies (by former members of the Bush White House, too, as recently as this past March). (As Dick Cheney said in June, suspicion of a link"turned out not to be true.") But the researchers think another force is at work. In a paper to be published in the September issue of the journal Sociological Inquiry(you have to subscribe to the journal to read the full paper, but the authors kindly posted it on their Web site here), they argue that some Americans believe the Saddam-9/11 link because it"made sense of the administration's decision to go to war against Iraq . . . [T]he fact of the war led to a search for a justification for it, which led them to infer the existence of ties between Iraq and 9/11," they write.

For their study, the scientists whittled down surveys filled out by 246 voters, of whom 73 percent believed in a Saddam-9/11 link, to 49 believers who were willing to be interviewed at length in October 2004. Even after the 49 were shown newspaper articles reporting that the 9/11 Commission had not found any evidence linking Saddam and 9/11, and quoting President Bush himself denying it, 48 stuck to their guns: yup, Saddam Hussein, directly or indirectly, brought down the Twin Towers.

When the scientists asked the participants why they believed in the link, they offered many justifications. Five argued that Saddam supported terrorism generally, or that evidence of a link to 9/11 might yet emerge. These counterarguments are not entirely illogical. But almost everyone else offered some version of"I don't know; I don't know anything"—that is, outright confusion over the conflict between what they believed and what the facts showed—or switched subjects to the invasion of Iraq. As one put it, when asked about his Saddam-9/11 belief,"There is no doubt in my mind that if we did not deal with Saddam Hussein when we did, it was just a matter of time when we would have to deal with him." In other words, holding fast to the Saddam-9/11 belief helped people make sense of the decision to go to war against Iraq.

"We refer to this as 'inferred justification,'" says Hoffman. Inferred justification is a sort of backward chain of reasoning. You start with something you believe strongly (the invasion of Iraq was the right move) and work backward to find support for it (Saddam was behind 9/11)."For these voters," says Hoffman,"the sheer fact that we were engaged in war led to a post-hoc search for a justification for that war."