Four years after the terrorist attack, an expert on folklore traced one such rumor to its source.

Muslims say a prayer during the Islam on Capitol Hill 2009 event at the West Front Lawn of the U.S. Capitol on September 25, 2009, in Washington, D.C. (Photo: Alex Wong/Getty Images)

Is Donald Trump's bellicose insistence that thousands of New Jersey Muslims cheered while watching television coverage of the 9/11 attack: a) a repugnant attempt to play to the racially inspired fears of his core supporters; b) a memory error, formed when he conflated television images from overseas with unsubstantiated reports of domestic celebrations; or c) both.

Even if you (generously) choose to give him the benefit of the doubt, and assume he actually believes what he's saying, his assertion still raises two questions: How did such a story come to circulate? And why do so many people choose to accept it?

Trump may be referencing a rumor that Arab employees at three New Jersey Dunkin' Donuts franchises either cheered or desecrated the American flag as hijacked airplanes flew into the World Trade Center towers. This was subsequently debunked by snopes.com, which noted it was "just one of many similar snippets of gossip aired about (celebrations at) a number of commercial entities, both large and small, after the terrorist attacks."

Rumors can spread more quickly and widely than ever before in our wired world.

Janet Langlois, a now-retired associate professor of English at Wayne State University, documented the spread of one such rumor in a 2005 paper. Published in the Journal of American Folklore, it's a sad story of how rumor-mongering triumphed over reason.

Needless to say, rumors can spread more quickly and widely than ever before in our wired world. However, unlike whispers over the back fence, electronically transmitted texts are relatively easy to trace. Langlois reports that, in this case, the initial email was sent on September 12, 2001, the day after the attacks. Unfortunately, it was not signed.

The message read: "My son-in-law, Dr. (name redacted), called me this morning. A nurse from the Henry Ford Hospital where he works went to the Sheik on Orchard Lane Road and Pontiac Trail to pick up lunch yesterday—and all the people in their were cheering as they watched the TV footage of our American tragedy. Do not patronize this restaurant and please pass the word to everyone you know."

The Sheik was a quite successful, 135-seat restaurant that specialized in Middle Eastern dishes. Owner Dean Hachem, who emigrated from Lebanon in 1978 and subsequently became an American citizen, looked at security-camera footage and called customers who had placed take-out orders on September 11; he found no evidence of any celebration.

Given the fact that virtually everyone was scared and shaken during the first few days after the attack, the doctor may simply have misheard or misunderstood what the nurse told him. In any event, the physician called his mother-in-law the next morning, and she sent the above email that afternoon.

Hachem learned of the allegation "when customers telephoned the restaurant in the next few days, asking about its veracity," Langlois writes. One caller also forwarded him a copy that included the email addresses of the sender and of the first recipients, with the comment: "I find it hard to believe that you would allow such action in your restaurant, and therefore, instead of forwarding it, I called you."

Sadly, that person was in the minority: Many other people simply passed it along without questioning its veracity. Hachem filed a defamation lawsuit in 2002, naming the doctor and his mother-in-law as co-defendants. She was called "Jane Doe" in the legal document, but a subsequent Detroit News story identified her as "an active member of one of the largest Reform synagogues in the area."

This is relevant because members of the synagogue's "sisterhood" affiliate—a group of women who oversaw volunteer opportunities—are "believed to be the first recipients of the e-mail," according to the October 30, 2001, issue of the Detroit Jewish News. Langlois credits that paper with printing the first news story about the rumor (framing it as a cautionary tale), and notes that its restaurant critic "encouraged his readers to patronize the Sheik."

But by that point, many minds were already closed. "There is every indication that the e-mail spread through a network of sisterhood associates," Langlois writes, "and then through other associated list serves within the greater metro Detroit area."

"Jewish community leaders ... referred to the rumor and asked their community members to dismiss it and return to the Sheik in the name of social justice," Langlois notes. It was to no avail: Business plummeted, and never fully recovered. According to a 2009 blog post, the restaurant "unfortunately closed after losing much business following certainly false rumors of staff celebrating on 9/11."

So why was this unsubstantiated report seen as plausible by so many? While simple prejudice is no doubt part of the answer, Langlois points to a more complex dynamic. She notes that the synagogue that the initial sender (and the first receivers) belonged to "defines itself, in part, through its commitment to the state of Israel."

Given that CNN had been showing footage of "Palestinians celebrating in the streets of East Jerusalem," she writes, "one can see synagogue members sending the e-mail and boycotting the Sheik as one more concerted action to protect the Jewish state, as they see it."

That same footage may have gotten conflated with the aforementioned rumors in the minds of Trump and his supporters, creating false memories of watching Arab Americans celebrating the attack.

Since CNN gets a small part of the blame here, it's appropriate to also give the network some credit. The evening of March 20, 2002, it aired a thoughtful report on the rumor and its consequences.

Anchor Aaron Brown called the story of the Sheik "a reminder of how easy it is to do wrong."

Findings is a daily column by Pacific Standard staff writer Tom Jacobs, who scours the psychological-research journals to discover new insights into human behavior, ranging from the origins of our political beliefs to the cultivation of creativity.