Washington, United States | It is campaign season, and facts are taking it on the chin.

Republican Donald Trump's claim that Arab Americans cheered during the September 11, 2001 attacks is just the latest in a string of falsehoods from US presidential candidates.

Trump is not alone among the candidates in distorting the truth, according to fact-checkers.

Carly Fiorina falsely claimed the United States was preparing to accept 250,000 Syrian refugees; Marco Rubio said that welders earn more than philosophers; and Ben Carson stated that no signatories of the Declaration of Independence had elected office experience.

Democrats have stretched the truth as well -- Hillary Clinton by claiming that her handling of emails through a private server was "permitted" by the State Department. Bernie Sanders overstated the evidence by asserting that "climate change is directly related to the growth of terrorism."

Such claims, which have been debunked by fact checkers, are part of political life.

But the 2016 Republican campaign has been notable for incendiary claims, most notably by Trump.

"There is no rigorous way to quantify deception being better or worse over time," said Brendan Nyhan, a Dartmouth College political scientist who follows fact-checking and campaigns.

"But I do think it's fair to say Donald Trump is on the verge of melting down the fact-checking sites with what he is saying."

Trump earlier this year said the US unemployment rate was as high as 42 percent. More recently, he tweeted a graphic showing that 81 percent of white homicide victims were killed by blacks. The website PolitiFact said the correct figure from Department of Justice statistics was 15 percent.

Asked by Fox News about the mistake, Trump said, "I didn't tweet, I retweeted somebody that was supposedly an expert... am I gonna check every statistic?"

The New York Times said in an editorial Tuesday that the past week of the campaign had been "dominated by Donald Trump's racist lies."

- Standing by false claims -Politicians in many cases have stood by their claims -- including Trump arguing that he saw thousands cheering the 9/11 attacks -- after being confronted with facts.

Trump even fired back at the Washington Post which debunked his 9/11 claims, tweeting "I want an apology! Many people have tweeted that I am right!"

The stubbornness surprises even the fact-checkers.

"There have definitely been times when I scratch my head wondering how could they say something when it is so obviously false and then not acknowledge that it is false," said Bill Adair, a founder and contributing editor of PolitiFact and a journalism professor at Duke University.

"But I'm not a psychologist and don't try to figure out why people say these things."

Republican candidate Jeb Bush claimed for example that "Florida led the nation in job creation" while he was governor -- a statement given a "four Pinocchios" rating as false by the Washington Post's fact-checker.

Ted Cruz maintained that Hispanic unemployment and teen unemployment has gone up under President Barack Obama, even though FactCheck.org found statistics showing the contrary.

Carson -- who repeated Trump's claim about people cheering on 9/11 -- also erroneously claimed that US border patrols released many people attempting to enter the country who were from Iraq, and Somalia and Russia, when FactCheck placed the figure arriving from those countries at less than one percent of the total.

Fiorina ignited ire by claiming that the women's health provider Planned Parenthood was "butchering babies for body parts," claiming the existence of video evidence that has not been located.

- Lingering impacts -Boston College political scientist Emily Thorson said that "misinformation" can often have lingering effects even if a falsehood is quickly corrected. For example, if people are told a restaurant has an infestation, they might feel squeamish about the place even if they learn it was a mistake.

"It's hard to undo the initial effects," she told AFP. "Misinformation gets out there and gets repeated."

Thorson's research on the topic showed that when people hear negative things about a candidate which turn out to be false, a correction only "mutes" the impact.

Researchers also say that hard-core backers of candidates are rarely swayed by evidence from fact-checkers.

"There is a group of people who won't believe a correction if you have a subject that ties into their partisan identities," Thorson said.

Nyhan agreed, saying that fact-checking "can often be ineffective or counterproductive when it comes to the most controversial political issues."

When confronted with facts about such hot issues, "people tend to resist information that runs counter to their pre-existing beliefs and attitudes," he said.

Nonetheless, Nyhan said it would be wrong to diminish the value of fact-checking the political candidates.

"The threat of fact-checking can help constrain politicians from making misleading statements," he said. "And so it's reasonable to assume that if we didn't have fact checking things would be worse."

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