There is surprisingly little overlap between plagiarists and fabulists. The New Republic's fabulous fabulist Stephen Glass didn't plagiarize. Historian Joseph Ellis, who concocted a fake Vietnam War record for himself, seems to do rock-solid scholarship. Some pants-seat speculation why the two groups differ: Plagiarism and fantasy stem from opposite psychopathologies.

Essentially, fabulists can't find anything in the real world that equals their imagination. That's why they make things up, because what they invent is more interesting than what they see around them.

Plagiarists, by contrast, find too much in the real world that equals their imagination. They steal because there is too much other writing around that tempts them—what they see around them is more interesting than what they write themselves.