Our results indicate that this sort of journalistic fact-checking often fails to reduce misperceptions among ideological or partisan voters. In some cases, we found that corrections can even make misperceptions worse. For example, in one experiment we found that the proportion of conservatives who believed that President George W. Bush’s tax cuts actually increased federal revenue grew from 36 percent to 67 percent when they were provided with evidence against this claim. People seem to argue so vehemently against the corrective information that they end up strengthening the misperception in their own minds.

The debate over health care reform, which was marred by false and misleading claims about the plan’s contents, provides a case study in how difficult it is to correct widely held misperceptions. Democrats cite various reasons to think that public understanding of the plan will improve in the aftermath of its enactment, but none of them are particularly persuasive.

First, some Democrats have suggested that politicians and others responsible for the spread of false information will be discredited when their doomsday predictions fail to materialize. Senator Robert Menendez of New Jersey predicted on Monday that “when this bill goes into effect, and none of the things Republicans warned about begin to happen  none of the death panels, none of the government takeover, none of the socialism  Republicans will have no credibility.”

This is too optimistic. While some provisions of the plan will start before November, the most far-reaching changes won’t take effect until 2014. False claims about the contents of the bill will just morph into harder-to-debunk predictions about the consequences of reform. We’ve seen this happen already with Sarah Palin’s claim that her parents and baby would “have to stand in front of Obama’s ‘death panel.’” After this claim was widely discredited in the press, some conservative pundits retreated to claims that future rationing of health care would amount to “de facto death panels.”

In addition, some have suggested that personal experience will change Americans’ beliefs about health care reform. But that reality will also take a long time to arrive for most voters. It will be years before many people experience substantial changes in how their health care is paid for or delivered. Even after the insurance expansion is complete, it’s not clear that direct contact will correct the public’s mistaken beliefs  remember the town hall participant who told a Republican congressman last summer to “keep your government hands off my Medicare”?